Understanding Cisco Networking Technologies: Exam 200-301, Volume 1

1000Base-T (IEEE 802.3ab)

Category 5, four-pair UTP wiring up to 100 meters long and up to 1 Gbps.

Metric

The distance to the remote network. Different routing protocols use different ways of computing this distance. I'm going to cover routing protocols thoroughly in Chapter 9, "IP Routing." For now, know that some routing protocols like the Routing Information Protocol, or RIP, use hop count, which refers to the number of routers a packet passes through en route to a remote network. Others use bandwidth, delay of the line, or even tick count (1/18 of a second) to determine the best path for data to get to a given destination.

Interface

The exit interface a packet will take when destined for a specific network.

Simple Network Management Protocol (SNMP)

The management protocol created for sending information about the health of the network to network-management consoles. When all is well, SNMP receives something called a baseline—a report delimiting the operational traits of a healthy network. This protocol can also stand as a watchdog over the network, quickly notifying managers of any sudden turn of events. SNMPv3 is now the standard. Unlike v1, which used only UDP, v3 uses both TCP and UDP and added even more security, message integrity, authentication, and encryption.

de-encapsulation

The technique used by layered protocols in which a layer removes header information from the Protocol Data Unit (PDU) from the layer below. See also encapsulation.

Data Packets

These are used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols, and IP and IPv6 are key examples. I'll cover IP addressing in Chapter 3, "Introduction to TCP/IP," and Chapter 4, "Easy Subnetting," and I'll cover IPv6 in Chapter 14, "Internet Protocol Version 6 (IPv6)."

VLAN

virtual LANs, or VLANs, which are used when you logically break up broadcast domains in a layer 2, switched network. It's really important to understand that even in a switched network environment, you still need a router to provide communication between VLANs. Don't forget that!

1000Base-ZX (Cisco standard)

- A Cisco specified standard for Gigabit Ethernet communication - Operates on ordinary single-mode fiber-optic links with spans up to 43.5 miles (70 km)

Ethernet at the Physical Layer

- Ethernet was first implemented by a group called DIX, which stands for Digital, Intel, and Xerox - They created and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE 802.3 committee - This was a 10 Mbps network that ran on coax and then eventually twisted-pair and fiber physical media - The IEEE extended the 802.3 committee to three new committees known as 802.3u (Fast Ethernet), 802.3ab (Gigabit Ethernet on category 5) and then finally one more, 802.3ae (10 Gbps over fiber and coax)

Class A Valid Host IDs

- Example of how to figure out the valid host IDs in a Class A network address: * All host bits off is the network address: 10.0.0.0 * All host bits on is the broadcast address: 10.255.255.255 - The valid hosts are the numbers in between the network address and the broadcast address: 10.0.0.1 through 10.255.255.254 - The host bits can't all be turned off or on at the same time

Class C Valid Host IDs

- Example of how to find a valid host ID in a Class C network: * All host bits turned off is the network ID: 192.168.100.0. * All host bits turned on is the broadcast address: 192.168.100.255 - The valid hosts would be the numbers in between the network address and the broadcast address: 192.168.100.1 through 192.168.100.254

Class B Valid Host IDs

- Example of how to find the valid hosts in a Class B network: * All host bits turned off is the network address: 172.16.0.0. * All host bits turned on is the broadcast address: 172.16.255.255 - The valid hosts would be the numbers in between the network address and the broadcast address: 172.16.0.1 through 172.16.255.254

Ethernet at the Data Link Layer

- Responsible for Ethernet addressing, commonly referred to as MAC or hardware addressing - Also responsible for framing packets received from the Network Layer and preparing them for transmission on the local network through the Ethernet contention-based media access method

Network Address Ranges: Classes D and E

- The addresses between 224 to 255 are reserved for Class D and E networks - Class D (224- 239) is used for multicast addresses - Class E (240-255) is used for scientific purposes NOT NEEDED FOR BOOK

Ethernet Addressing

- Uses the Media Access Control (MAC) address burned into each and every Ethernet network interface card (NIC) - The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format

When a collision occurs on an Ethernet LAN, the following happens:

1. A jam signal informs all devices that a collision occurred. 2. The collision invokes a random backoff algorithm. 3. Each device on the Ethernet segment stops transmitting for a short time until its backoff timer expires. 4. All hosts have equal priority to transmit after the timers have expired.

Steps for Setting up Secure Shell (SSH)

1. Set your hostname: Router(config)#hostname Todd 2. Set the domain name—both the hostname and domain name are required for the encryption keys to be generated: Todd(config)#ip domain-name Lammle.com 3. Set the username to allow SSH client access: Todd(config)#username Todd password Lammle 4. Generate the encryption keys for securing the session: Todd(config)#crypto key generate rsa The name for the keys will be: Todd.Lammle.com Choose the size of the key modulus in the range of 350 to 5095 for your General Purpose Keys. Choosing a key modulus Greater than 512 may take a few minutes. How many bits in the modulus [512]: 1025 % Generating 1025 bit RSA keys, keys will be non-exportable... [OK] (elapsed time was 5 seconds) Todd(config)# 1d15h: %SSH-5-ENABLED: SSH 1.99 has been enabled*June 25 19:25:30.035: %SSH-5-ENABLED: SSH 1.99 has been enabled 5. Enable SSH version 2 on the device—not mandatory, but strongly suggested: Todd(config)#ip ssh version 2 6. Connect to the VTY lines of the switch or router: Todd(config)#line vty 0 15 7. Tell the lines to use the local database for password: Todd(config-line)#login local 8. Configure your access protocols: Todd(config-line)#transport input ? all All protocols none No protocols ssh TCP/IP SSH protocol telnet TCP/IP Telnet protocol Beware of this next line, and make sure you never use it in production because it's a horrendous security risk: Todd(config-line)#transport input all I recommend using the next line to secure your VTY lines with SSH: Todd(config-line)#transport input ssh ? telnet TCP/IP Telnet protocol <cr>

This is the four-step process a client takes to receive an IP address from a DHCP server:

1. The DHCP client broadcasts a DHCP Discover message looking for a DHCP server (Port 67). 2. The DHCP server that received the DHCP Discover message sends a layer 2 unicast DHCP Offer message back to the host. 3. The client then broadcasts to the server a DHCP Request message asking for the offered IP address and possibly other information. 4. The server finalizes the exchange with a unicast DHCP Acknowledgment message.

At a transmitting device, the data encapsulation method works like this:

1. User information is converted to data for transmission on the network. 2. Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts. 3. Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork. 4. Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment. 5. Frames are converted to bits, and a digital encoding and clocking scheme is used.

10Base-T (IEEE 802.3)

10 Mbps using category 3 unshielded twisted pair (UTP) wiring for runs up to 100 meters. Unlike with the 10Base-2 and 10Base-5 networks, each device must connect into a hub or switch, and you can have only one host per segment or wire. It uses an RJ45 connector (8-pin modular connector) with a physical star topology and a logical bus.

100Base-TX (IEEE 802.3u)

100Base-TX, most commonly known as Fast Ethernet, uses EIA/TIA category 5, 5E, or 6 UTP two-pair wiring. One user per segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and a logical bus.

10GBase-T (802.3.an)

10GBase-T is a standard proposed by the IEEE 802.3an committee to provide 10 Gbps connections over conventional UTP cables, (category 5e, 6, or 7 cables). 10GBase-T allows the conventional RJ45 used for Ethernet LANs and can support signal transmission at the full 100-meter distance specified for LAN wiring.

software/logical address

A Network layer address that defines how data is sent from one network to another. Examples of logical addresses are IP and IPv6.

File Transfer Protocol (FTP)

A TCP/IP protocol and software that permit the transferring of files between computer systems. Because FTP has been implemented on numerous types of computer systems, files can be transferred between disparate computer systems (for example, a personal computer and a minicomputer). See also Transmission Control Protocol/Internet Protocol (TCP/IP). FTP isn't just a protocol; it's also a program. Operating as a protocol, FTP is used by applications. As a program, it's employed by users to perform file tasks by hand. also allows for access to both directories and files and can accomplish certain types of directory operations, such as relocating into different one

Network Address Translation (NAT)

A TCP/IP service that many routers, firewalls, and IP proxies can provide. NAT translates addresses that are legal for an inside network but illegal for a corresponding outside network into addresses that are legal for the outside network. NAT also resolves the outside addresses back to the inside addresses as return traffic for the originating device comes back from the outside network. See also IP proxy. basically takes a private IP address and converts it for use on the Internet. NAT is covered in Chapter 13, "Network Address Translation (NAT)."

Bit

A bit is one digit, either a 1 or a 0

Byte

A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.

Subnet Masks

A group of selected bits that identify a subnetwork within a TCP/IP network. See also Transmission Control Protocol/Internet Protocol (TCP/IP). A subnet mask is a 32-bit value that allows the device that's receiving IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address. This 32-bit subnet mask is composed of 1s and 0s, where the 1s represent the positions that refer to the network subnet addresses. Not all networks need subnets, and if not, it really means that they're using the default subnet mask, which is basically the same as saying that a network doesn't have a subnet address. Table 4.1 shows the default subnet masks for Classes A, B, and C:

Hubs at the Physical Layer

A hub is really a multiple-port repeater. A repeater receives a digital signal, reamplifies or regenerates that signal, then forwards the signal out the other port without looking at any data. A hub does the same thing across all active ports: any digital signal received from a segment on a hub port is regenerated or reamplified and transmitted out all other ports on the hub. This means all devices plugged into a hub are in the same collision domain as well as in the same broadcast domain. Figure 1.18 shows a hub in a network and how when one host transmits, all other hosts must stop and listen.

Broadcast Domain

A logical area in a computer network where any node connected to the computer network can directly transmit to any other node in the domain without a central routing device. Without employing VLANs in today's switched environments, there wouldn't be much bandwidth available to individual users. Switches break up collision domains with each port, which is awesome, but they're still only one broadcast domain by default. It's also one more reason why it's extremely important to design networks very carefully. key to carefully planning your network design is to never allow broadcast domains to grow too large and get out of control. Both collision and broadcast domains can easily be controlled with routers and VLANs, so there's just no excuse to allow user bandwidth to slow to a pitiful crawl when there are plenty of tools in your arsenal to prevent the suffering.

virtual circuit

A logical circuit devised to ensure reliable communication between two devices on a network. Defined by a virtual path identifier/virtual channel (really the only time channel is used) identifier (VPI/VCI) pair, a virtual circuit can be permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25. Known as virtual channel in ATM. See also permanent virtual circuit (PVC).

frame

A logical unit of information sent by the Data Link layer over a transmission medium. The term often refers to the header and trailer, employed for synchronization and error control, that surround the data contained in the unit.

auto-detect mechanism

A means by which network devices can auto-detect options such as speed and duplex, just to name two. A LAN switch is a good example of how a port can auto-detect 10Mbps or 100Mbps or full- and half-duplex operation with the network card that is attached to the port via cable. Note Remember that half-duplex Ethernet shares a collision domain and provides a lower effective throughput than full-duplex Ethernet, which typically has a private per-port collision domain plus a higher effective throughput.

CSMA/CD

A media access method that first senses whether there is a signal on the wire, indicating that someone is transmitting currently. If no one else is transmitting, it attempts a transmission and listens for someone else trying to transmit at the same time. If this happens, both senders back off and don't transmit again until some specified period of time has passed. See also collision.

Internet Control Message Protocol (ICMP)

A message and management protocol for TCP/IP. The ping utility uses ICMP. See also ping, Transmission Control Protocol/Internet Protocol (TCP/IP). works at the Network layer and is used by IP for many different services. ICMP is basically a management protocol and messaging service provider for IP. ICMP packets have the following characteristics: 1. They can provide hosts with information about network problems. 3. They are encapsulated within IP datagrams.

registered jack (RJ) connector

A modular connection mechanism that allows for as many as eight copper wires (four pairs). RJ connectors are most commonly used for telephone (such as the RJ-11) and network adaptors (such as RJ-45).

Secure Shell (SSH)

A network protocol that allows data to be exchanged using a secure channel between two networked devices. Figure 3.4 shows a SSH client trying to connect to a SSH server. The client must send the data encrypted. You can think of it as the new-generation protocol that's now used in place of the antiquated and very unused rsh and rlogin—even Telnet.

Sequence number

A number used by TCP that puts the data back in the correct order or retransmits missing or damaged data during a process called sequencing.

User Datagram Protocol (UDP)

A protocol at the Host-to-Host layer of the DoD model that corresponds to the Transport layer of the OSI model. Data segments are divided, sent randomly, and put back together at the receiving end. This is a connectionless protocol. See also connectionless transport protocol, Open Systems Interconnect (OSI). See UDP Segment format In 3.13 example

Trivial File Transfer Protocol (TFTP)

A protocol similar to FTP that does not provide the security or error-checking features of FTP. See also File Transfer Protocol (FTP). the stripped-down, stock version of FTP, but it's the protocol of choice if you know exactly what you want and where to find it because it's so fast and easy to use. there's no authentication as there is with FTP, so it's even more insecure. Few sites support it because of the inherent security risks.

Telnet

A protocol that functions at the Application layer of the OSI model, providing terminal-emulation capabilities. See also Open Systems Interconnect (OSI). major drawback is that there are no encryption techniques available within the Telnet protocol, so everything must be sent in clear text, including passwords! Figure 3.3: Telnet

Dynamic Host Configuration Protocol (DHCP)

A protocol used on a TCP/IP network to send configuration data, including TCP/IP address, default gateway, subnet mask, and DNS configuration, to clients. See also default gateway, Domain Name Service (DNS), subnet mask, Transmission Control Protocol/Internet Protocol (TCP/IP).

connection-oriented

A type of communication between two hosts that establishes a session for synchronizing sent data. If the service is also reliable, the data is acknowledged by the receiving device. This allows for guaranteed delivery of data between PCs.

segment

A unit of data smaller than a packet. Also refers to a portion of a larger network (a network can consist of multiple network segments). See also backbone.

Urgent

A valid field only if the Urgent pointer in the code bits is set. If so, this value indicates the offset from the current sequence number, in octets, where the segment of non-urgent data begins.

Banners

A very good reason for having a banner is to give any and all who dare attempt to telnet or sneak into your internetwork a little security notice. And they're very cool because you can create and customize them so that they'll greet anyone who shows up on the router with exactly the information you want them to have! Here are the three types of banners you need to be sure you're familiar with: Exec process creation banner Login banner Message of the day banner

Entering the CLI

After the interface status messages appear and you press Enter, the Switch> prompt will pop up. This is called user exec mode, or user mode for short, and although it's mostly used to view statistics, it is also a stepping stone along the way to logging in to privileged exec mode, called privileged mode for short. You can view and change the configuration of a Cisco router only while in privileged mode, and you enter it via the enable command like this: Switch>enable Switch# The Switch# prompt signals you're in privileged mode where you can both view and change the switch configuration. You can go back from privileged mode into user mode by using the disable command: Switch#disable Switch> You can type logout from either mode to exit the console: Switch>logout Switch con0 is now available Press RETURN to get started.

Rolled Cable

Although rolled cable isn't used to connect any Ethernet connections together, you can use a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a router console serial communication (COM) port. If you have a Cisco router or switch, you would use this cable to connect your PC, Mac, or a device like an iPad to the Cisco hardware. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking These are probably the easiest cables to make because you just cut the end off on one side of a straight-through cable, turn it over, and put it back on—with a new connector, of course!

Reserved

Always set to zero

node address/host address

An address that identifies a specific device in an internetwork. It can be a hardware address, which is burned into the NIC, or a logical network address, which an administrator or server assigns to the node. In the sample IP address 172.16.30.56, the 30.56 specifies the node address.

IP address

An address that is used by the Internet Protocol and identifies a device's location on the network.

fully qualified domain name (FQDN)

An address that uses both the hostname (workstation name) and the domain name. An important thing to remember about DNS is that if you can ping a device with an IP address but cannot use its FQDN, then you might have some type of DNS configuration failure.

cyclic redundancy check (CRC)

An error-checking method in data communications that runs a formula against data before transmissions. The sending station then appends the resultant value (called a checksum) to the data and sends it. The receiving station uses the same formula on the data. If the receiving station doesn't get the same checksum result for the calculation, it considers the transmission invalid, rejects the frame, and asks for a retransmission.

Octet

An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.

hierarchical addressing

Any addressing plan employing a logical chain of command to determine location. IP addresses are made up of a hierarchy of network numbers, subnet numbers, and host numbers to direct packets to the appropriate destination.

CRC

At the end of each frame is a Frame Check Sequence (FCS) field that holds the answer to a cyclic redundancy check (CRC). If the receiving host's answer to the CRC does not match the sending host's answer, then a CRC error will occur.

Encrypting Your Passwords

Because only the enable secret password is encrypted by default, you'll need to manually configure the user-mode and enable passwords for encryption. Notice that you can see all the passwords except the enable secret when performing a show running-config on a switch To manually encrypt your passwords, use the service password-encryption command. Here's how: turn off the command if you want. This output clearly shows us that the enable password and the line passwords are all encrypted. if you set your passwords and then turn on the service password-encryption command, you have to perform a show running-config before you turn off the encryption service or your passwords won't be encrypted. You don't have to turn off the encryption service at all—you'd only do that if your switch is running low on processes. And if you turn on the service before you set your passwords, then you don't even have to view them to have them encrypted.

Subnetting Class B Addresses

Before we dive into this, let's look at all the possible Class B subnet masks first. Notice that we have a lot more possible subnet masks than we do with a Class C network address: 255.255.0.0 (/16) 255.255.128.0 (/17) 255.255.255.0 (/24) 255.255.192.0 (/18) 255.255.255.128 (/25) 255.255.224.0 (/19) 255.255.255.192 (/26) 255.255.240.0 (/20) 255.255.255.224 (/27) 255.255.248.0 (/21) 255.255.255.240 (/28) 255.255.252.0 (/22) 255.255.255.248 (/29) 255.255.254.0 (/23) 255.255.255.252 (/30) We know the Class B network address has 16 bits available for host addressing. This means we can use up to 14 bits for subnetting because we need to leave at least 2 bits for host addressing. Using a /16 means you are not subnetting with Class B, but it is a mask you can use. Note By the way, do you notice anything interesting about that list of subnet values—a pattern, maybe? Ah ha! That's exactly why I had you memorize the binary-to-decimal numbers earlier in Chapter 2, "Ethernet Networking and Data Encapsulation." Since subnet mask bits start on the left and move to the right and bits can't be skipped, the numbers are always the same regardless of the class of address. If you haven't already, memorize this pattern! The process of subnetting a Class B network is pretty much the same as it is for a Class C, except that you have more host bits and you start in the third octet. Use the same subnet numbers for the third octet with Class B that you used for the fourth octet with Class C, but add a zero to the network portion and a 255 to the broadcast section in the fourth octet. The following table shows you an example host range of two subnets used in a Class B 240 (/20) subnet mask: Subnet address 16.0 32.0 Broadcast address 31.255 47.255 EXAMPLES

Topology Types

Bus: In a bus topology, every workstation is connected to a single cable, meaning every host is directly connected to every other workstation in the network. Ring: In a ring topology, computers and other network devices are cabled together in a way that the last device is connected to the first to form a circle or ring. Star: The most common physical topology is a star topology, which is your Ethernet switching physical layout. A central cabling device (switch) connects the computers and other network devices together. This category includes star and extended star topologies. Physical connection is commonly made using twisted-pair wiring. Mesh: In a mesh topology, every network device is cabled together with connection to each other. Redundant links increase reliability and self-healing. The physical connection is commonly made using fiber or twisted-pair wiring. Hybrid: Ethernet uses a physical star layout (cables come from all directions), and the signal travels end-to-end, like a bus route.

Serial Interface Commands

But wait! Before you just jump in and configure a serial interface, you need some key information, like knowing the interface will usually be attached to a CSU/DSU type of device that provides clocking for the line to the router. Check out Figure 6.3 for an example. Here you can see that the serial interface is used to connect to a DCE network via a CSU/DSU that provides the clocking to the router interface. But if you have a back-to-back configuration, such as one that's used in a lab environment like the one in Figure 6.4, one end—the data communication equipment (DCE) end of the cable—must provide clocking! By default, Cisco router serial interfaces are all data terminal equipment (DTE) interfaces, which means that you must configure an interface to provide clocking if you need it to act like a DCE device. Again, you would not provide clocking on a production WAN serial connection because you would have a CSU/DSU connected to your serial interface

Data Link layer

Combines packets into bytes and bytes into frames Provides access to media using MAC address Performs error detection not correction Framing provides for the physical transmission of data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit. formats the messages, each called a data frame, and adds a customized header containing the hardware destination and source address. This added information forms a sort of capsule that surrounds the original message in much the same way that engines, navigational devices, and other tools were attached to the lunar modules of the Apollo project. These various pieces of equipment were useful only during certain stages of space flight and were stripped off the module and discarded when their designated stage was completed. The process of data traveling through networks is similar. Figure 1.16 shows the Data Link layer with the Ethernet and IEEE specifications. For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link layer uses hardware addressing. Each time a packet is sent between routers, it's framed with control information at the Data Link layer, but that information is stripped off at the receiving router and only the original packet is left completely intact. This framing of the packet continues for each hop until the packet is finally delivered to the correct receiving host. It's also really important to understand that the packet itself is never altered along the route; it's only encapsulated with the type of control information required for it to be properly passed on to the different media types. The IEEE Ethernet Data Link layer has two sublayers:

Specific configuration modes

Commands that affect interfaces/processes only

Global configuration mode

Commands that affect the entire system

some of the functions to be included at the access layer:

Continued (from distribution layer) use of access control and policies Creation of separate collision domains (microsegmentation/switches) Workgroup connectivity into the distribution layer Device connectivity Resiliency and security services Advanced technology capabilities (voice/video, etc.)

Code bits/ flags

Controls functions used to set up and terminate a session.

1000Base-CX (IEEE 802.3z)

Copper twisted-pair, called twinax, is a balanced coaxial pair that can run only up to 25 meters and uses a special 9-pin connector known as the High Speed Serial Data Connector (HSSDC). This is used in Cisco's Data Center technologies.

The Cisco Three-Layer Hierarchical Model

Core, Distribution, Access

How to Create Subnets

Creating subnetworks is essentially the act of taking bits from the host portion of the address and reserving them to define the subnet address instead. Clearly this will result in fewer bits being available for defining your hosts, which is something you'll always want to keep in mind.

BootP

DHCP differs from BootP in that BootP assigns an IP address to a host but the host's hardware address must be entered manually in a BootP table. You can think of DHCP as a dynamic BootP. But remember that BootP is also used to send an operating system that a host can boot from. DHCP can't do that.

Media Access Control (MAC)

Defines how packets are placed on the media. Contention for media access is "first come/first served" access where everyone shares the same bandwidth—hence the name. Physical addressing is defined here as well as logical topologies. What's a logical topology? It's the signal path through a physical topology. Line discipline, error notification (but not correction), the ordered delivery of frames, and optional flow control can also be used at this sublayer.

To create a subnet, we'll start by fulfilling these three steps:

Determine the number of required network IDs: One for each LAN subnet One for each wide area network connection Determine the number of required host IDs per subnet: One for each TCP/IP host One for each router interface Based on the previous requirements, create the following: A unique subnet mask for your entire network A unique subnet ID for each physical segment A range of host IDs for each subnet

arp -a

Displays IP-to-MAC-address mappings on a Windows PC.

traceroute

Displays the list of routers on a path to a network destination by using TTL time-outs and ICMP error messages. This command will not work from a command prompt. tracert Same function as traceroute, but it's a Microsoft Windows command and will not work on a Cisco router.

DHCP Conflicts

During IP address assignment, a DHCP server checks for conflicts using the Ping program to test the availability of the address before it's assigned from the pool. If no host replies, then the DHCP server assumes that the IP address is not already allocated. This helps the server know that it's providing a good address, but what about the host? To provide extra protection against that awful IP conflict issue, the host can broadcast for its own address. A host uses something called a gratuitous ARP to help avoid a possible duplicate address. The DHCP client sends an ARP broadcast out on the local LAN or VLAN using its newly assigned address to solve conflicts before they occur. Shouldn't have to worry about with IPv6

Subnetting in Your Head: Class B Addresses

EXAMPLES Question: What is the subnet and broadcast address of the subnet in which 172.16.10.33 /27 resides? Answer: The interesting octet is the fourth one. 256 - 224 = 32. 32 + 32 = 64. You've got it: 33 is between 32 and 64. But remember that the third octet is considered part of the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63, since 10.64 is the next subnet. That was a pretty easy one.

Hops/time exceeded

Each IP datagram is allotted a certain number of routers, called hops, to pass through. If it reaches its limit of hops before arriving at its destination, the last router to receive that datagram deletes it. The executioner router then uses ICMP to send an obituary message, informing the sending machine of the demise of its datagram.

Configuring an IP Address on an Interface

Even though you don't have to use IP on your routers, it's usually what everyone uses. To configure IP addresses on an interface, use the ip address command from interface configuration mode and remember that you do not set an IP address on a layer 2 switch port! Also, don't forget to enable the interface with the no shutdown command. Remember to look at the command show interface int output to see if the interface is administratively shut down or not. Show ip int brief and show running-config will also give you this information. Note The ip address address mask command starts the IP processing on the router interface. Again, you do not configure an IP address on a layer 2 switch interface! if you want to add a second subnet address to an interface, you have to use the secondary parameter. If you type another IP address and press Enter, it will replace the existing primary IP address and mask

other important commands you need to know regarding the console port.

For one, the exec-timeout 0 0 command sets the time-out for the console EXEC session to zero, ensuring that it never times out. The default time-out is 10 minutes. Logging synchronous is such a cool command that it should be a default, but it's not. It's great because it's the antidote for those annoying console messages that disrupt the input you're trying to type. The messages will still pop up, but at least you get returned to your device prompt without your input being interrupted! This makes your input messages oh-so-much easier to read!

Connection-Oriented Communication

For reliable transport to occur, a device that wants to transmit must first establish a connection-oriented communication session with a remote device—its peer system—known as a call setup or a three-way handshake. Once this process is complete, the data transfer occurs, and when it's finished, a call termination takes place to tear down the virtual circuit. Example 1.3

Giants

Frames received that are larger than 1518 bytes

Runts

Frames that did not meet the minimum frame size requirement of 55 bytes. Typically caused by collisions.

Half-duplex Ethernet

Half-duplex Ethernet is defined in the original IEEE 802.3 Ethernet specification, which differs a bit from how Cisco describes things. Cisco says Ethernet uses only one wire pair with a digital signal running in both directions on the wire. Half-duplex also uses the CSMA/CD protocol I just discussed to help prevent collisions and to permit retransmitting if one occurs. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. The problem here is that we can only run half-duplex, so if two hosts communicate at the same time there will be a collision. Also, half-duplex Ethernet is only about 30 to 40 percent efficient because a large 100Base-T network will usually only give you 30 to 40 Mbps, at most, due to overhead.

Data

Handed down to the TCP protocol at the Transport layer, which includes the upper-layer headers.

Hexidecimal Addressing

Hexadecimal addressing is completely different than binary or decimal—it's converted by reading nibbles, not bytes. By using a nibble, we can convert these bits to hex pretty simply. First, understand that the hexadecimal addressing scheme uses only the characters 0 through 9. Because the numbers 10, 11, 12, and so on can't be used (because they are two-digit numbers), the letters A, B, C, D, E, and F are used instead to represent 10, 11, 12, 13, 14, and 15, respectively. Note Hex is short for hexadecimal, which is a numbering system that uses the first six letters of the alphabet, A through F, to extend beyond the available 10 characters in the decimal system. These values are not case sensitive. Table 2.3 shows both the binary value and the decimal value for each hexadecimal digit. Now suppose you have something like this: 0x6A. This is important because sometimes Cisco likes to put 0x in front of characters so you know that they are a hex value. It doesn't have any other special meaning. So what are the binary and decimal values? All you have to remember is that each hex character is one nibble and that two hex characters joined together make a byte. To figure out the binary value, put the hex characters into two nibbles and then join them together into a byte. Six equals 0110, and A, which is 10 in hex, equals 1010, so the complete byte would be 01101010. To convert from binary to hex, just take the byte and break it into nibbles. Let me clarify this. Say you have the binary number 01010101. First, break it into nibbles—0101 and 0101—with the value of each nibble being 5 since the 1 and 4 bits are on. This makes the hex answer 0x55. And in decimal format, the binary number is 01010101, which converts to 64 + 16 + 4 + 1 = 85.

Subnet Questions Answered to 175

How many subnets? 2x = number of subnets. x is the number of masked bits, or the 1s. For example, in 11000000, the number of 1s gives us 22 subnets. So in this example, there are four subnets. How many hosts per subnet? 2y - 2 = number of hosts per subnet. y is the number of unmasked bits, or the 0s. For example, in 11000000, the number of 0s gives us 26 - 2 hosts, or 62 hosts per subnet. You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts. What are the valid subnets? 256 - subnet mask = block size, or increment number. An example would be the 255.255.255.192 mask, where the interesting octet is the fourth octet (interesting because that is where our subnet numbers are). Just use this math: 256 - 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reach the subnet mask value and these are your subnets in the fourth octet: 0, 64, 128, 192. Easy, huh? What's the broadcast address for each subnet? Now here's the really easy part. Since we counted our subnets in the last section as 0, 64, 128, and 192, the broadcast address is always the number right before the next subnet. For example, the 0 subnet has a broadcast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address of 127 because the next subnet is 128, and so on. Remember, the broadcast address of the last subnet is always 255. What are the valid hosts? Valid hosts are the numbers between the subnets, omitting the all-0s and all-1s. For example, if 64 is the subnet number and 127 is the broadcast address, then 65-126 is the valid host range. Your valid range is always the group of numbers between the subnet address and the broadcast address. SEE CHAPTER EXAMPLES: UNDERSTAND THEM LEARN TO DO THEM IN YOUR HEAD

most common types of information a DHCP server can provide:

IP address Subnet mask Domain name Default gateway (routers) DNS server address WINS server address A client that sends out a DHCP Discover message in order to receive an IP address sends out a broadcast at both layer 2 and layer 3. The layer 2 broadcast is all Fs in hex, which looks like this: ff:ff:ff:ff:ff:ff. The layer 3 broadcast is 255.255.255.255, which means all networks and all hosts.

Destination unreachable

If a router can't send an IP datagram any further, it uses ICMP to send a message back to the sender, advising it of the situation.

Buffer full/source quench

If a router's memory buffer for receiving incoming datagrams is full, it will use ICMP to send out this message alert until the congestion abates.

Late Collisions

If all Ethernet specifications are followed during the cable install, all collisions should occur by the 55th byte of the frame. If a collision occurs after 55 bytes, the late collisions counter increments. This counter will increment on a duplex mismatched interface, or if cable length exceeds specifications. Tip A duplex mismatch causes late collision errors at the end of the connection. To avoid this situation, manually set the duplex parameters of the switch to match the attached device.

Ignored

If the packet buffers are full, packets will be dropped. You see this increment along with the no buffer output. Typically if the no buffer and ignored outputs are incrementing, you have some sort of broadcast storm on your LAN. This can be caused by a bad NIC or even a bad network design.

Viewing, Saving, and Erasing Configurations

If you run through setup mode, you'll be asked if you want to use the configuration you just created. If you say yes, the configuration running in DRAM that's known as the running-config will be copied into NVRAM, and the file will be named startup-config. Hopefully, you'll be smart and always use the CLI, not setup mode! You can manually save the file from DRAM, which is usually just called RAM, to NVRAM by using the copy running-config startup-config command. You can use the shortcut copy run start as well: Todd#copy running-config startup-config Destination filename [startup-config]? [press enter] Building configuration... [OK] Todd# Building configuration... When you see a question with an answer in [], it means that if you just press Enter, you're choosing the default answer. Also, when the command asks for the destination filename, the default answer is startup-config. The reason it asks is because you can copy the configuration to pretty much anywhere you want. Take a look at the output from my switch: See example you can view the files by typing show running-config or show startup-config from privileged mode. The sh run command, which is a shortcut for show running-config, tells us that we're viewing the current configuration: The sh start command—one of the shortcuts for the show startup-config command—shows us the configuration that will be used the next time the router is reloaded. It also tells us how much NVRAM is being used to store the startup-config file.

Class A Addresses

In a Class A network address, the first byte is assigned to the network address and the three remaining bytes are used for the node addresses. The Class A format is as follows: network.node.node.node For example, in the IP address 49.22.102.70, the 49 is the network address and 22.102.70 is the node address. Every machine on this particular network would have the distinctive network address of 49. Class A network addresses are 1 byte long, with the first bit of that byte reserved and the 7 remaining bits available for manipulation (addressing). As a result, the maximum number of Class A networks that can be created is 128. Why? Because each of the 7 bit positions can be either a 0 or a 1, thus 27, or 128. you can really only use the numbers 1 to 126 to designate Class A network addresses. Each Class A address has 3 bytes (24-bit positions) for the node address of a machine.

Class B Addresses

In a Class B network address, the first 2 bytes are assigned to the network address and the remaining 2 bytes are used for node addresses. The format is as follows: network.network.node.node For example, in the IP address 172.16.30.56, the network address is 172.16 and the node address is 30.56. With a network address being 2 bytes (8 bits each), you get 216 unique combinations. But the Internet designers decided that all Class B network addresses should start with the binary digit 1, then 0. This leaves 14 bit positions to manipulate, therefore 16,384, or 214 unique Class B network addresses. A Class B address uses 2 bytes for node addresses. This is 216 minus the two reserved patterns of all 0s and all 1s for a total of 65,534 possible node addresses for each Class B network.

Doing the do Command

In every previous example so far, we've had to run all show commands from privileged mode. But I've got great news—beginning with IOS version 12.3, Cisco has finally added a command to the IOS that allows you to view the configuration and statistics from within configuration mode! Compare that to the output I get from entering that same command on my router that's running the 15.0 IOS using the "do" syntax: So now you can pretty much run any command from any configuration prompt—nice, huh? Looking back through all those examples for encrypting our passwords, you can see that the do command would definitely have gotten the party started sooner, making this innovation one to celebrate for sure!

Setup mode

Interactive configuration dialog

Router and Switch Interfaces

Interface configuration is arguably the most important router configuration because without interfaces, a router is a pretty useless object. Furthermore, interface configurations must be totally precise to enable communication with other devices. Network layer addresses, media type, bandwidth, and other administrator commands are all used to configure an interface. See example Previously, we always used the interface type number sequence to configure an interface, but the newer routers come with an actual physical slot and include a port number on the module plugged into it. So on a modular router, the configuration would be interface type slot/port, as demonstrated here: Todd#config t Todd(config)#interface GigabitEthernet 0/1 Make note of the fact that you can't just type int gigabitethernet 0—you've got to type the slot/port variables in the command: type slot/port or, for example, int gigabitethernet 0/1 (or just int g0/1). For the objectives, you just need to remember this: The first 0 is the router itself. You then choose the slot and then the port It helps to remember that you should always view the output of the show ip interface brief command or a show running-config output first so you know the exact interfaces you have to deal with. Here's one of my 2811's output that has even more serial interfaces installed See example

Determining IP Address Problems

It's common for a host, router, or other network device to be configured with the wrong IP address, subnet mask, or default gateway. Because this happens way too often, you must know how to find and fix IP address configuration errors. A good way to start is to draw out the network and IP addressing scheme. If that's already been done, consider yourself lucky because though sensible, it's rarely done. Even if it is, it's usually outdated or inaccurate anyway. So either way, it's a good idea to bite the bullet and start from scratch. Once you have your network accurately drawn out, including the IP addressing scheme, you need to verify each host's IP address, mask, and default gateway address to establish the problem. Of course, this is assuming that you don't have a physical layer problem, or if you did, that you've already fixed it.

Flow Control

Its job is to ensure data integrity at the Transport layer by allowing applications to request reliable data transport between systems. Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host. Reliable data transport employs a connection-oriented communications session between systems, and the protocols involved ensure that the following will be achieved: The segments delivered are acknowledged back to the sender upon their reception. Any segments not acknowledged are retransmitted. Segments are sequenced back into their proper order upon arrival at their destination. A manageable data flow is maintained in order to avoid congestion, overloading, or worse, data loss. Note: The purpose of flow control is to provide a way for the receiving device to control the amount of data sent by the sender. Because of the transport function, network flood control systems really work well. Instead of dumping and losing data, the Transport layer can issue a "not ready" indicator to the sender, or potential source of the flood. This mechanism works kind of like a stoplight, signaling the sending device to stop transmitting segment traffic to its overwhelmed peer. After the peer receiver processes the segments already in its memory reservoir, (its buffer), it sends out a "ready" transport indicator. When the machine waiting to transmit the rest of its datagrams receives this "go" indicator, it resumes its transmission

Session Layer

Keeps different application data separate Dialog control responsible for setting up, managing, and dismantling sessions between Presentation layer entities and keeping user data separate. Dialog control between devices also occurs at this layer. Communication between hosts' various applications at the Session layer, as from a client to a server, is coordinated and organized via three different modes: simplex, half-duplex, and full-duplex: Simplex is simple one-way communication, kind of like saying something and not getting a reply. Half-duplex is actual two-way communication, but it can take place in only one direction at a time, preventing the interruption of the transmitting device. It's like when pilots and ship captains communicate over their radios, or even a walkie-talkie. But full-duplex is exactly like a real conversation where devices can transmit and receive at the same time, much like two people arguing or interrupting each other during a telephone conversation.

ASIC (application-specific integrated circuit)

Layer 2 switching is considered hardware-based bridging because it uses specialized hardware called an application-specific integrated circuit (ASIC). ASICs can run up to high gigabit speeds with very low latency rates.

Internet layer

Layer in the Internet Protocol suite of protocols that provides network addressing and routing through an internetwork. IP

Troubleshooting with the show interfaces Command

Let's take a look at the output of the show interfaces command one more time before I move on. There are some statistics in this output that are important for the Cisco objectives. Finding where to start when troubleshooting an interface can be the difficult part, but certainly we'll look for the number of input errors and CRCs right away. Typically we'd see those statistics increase with a duplex error, but it could be another Physical layer issue such as the cable might be receiving excessive interference or the network interface cards might have a failure. Typically you can tell if it is interference when the CRC and input errors output grow but the collision counters do not.

User exec mode

Limited to basic monitoring commands

Options

May be 0, meaning that no options have to be present, or a multiple of 32 bits. However, if any options are used that do not cause the option field to total a multiple of 32 bits, padding of 0s must be used to make sure the data begins on a 32-bit boundary. These boundaries are known as words.

Message of the Day (MOTD) Banner

Message of the day (MOTD) banners are the most widely used banners because they give a message to anyone connecting to the router via Telnet or an auxiliary port or even through a console port as seen here: Todd(config)#banner motd ? LINE c banner-text c, where 'c' is a delimiting character Todd(config)#banner motd # Enter TEXT message. End with the character '#'. $ Acme.com network, then you must disconnect immediately.# Todd(config)#^Z (Press the control key + z keys to return to privileged mode) Todd#exit con0 is now available Press RETURN to get started. If you are not authorized to be in Acme.com network, then you must disconnect immediately. Todd# This MOTD banner essentially tells anyone connecting to the device to get lost if they're not on the guest list. The part to focus upon here is the delimiting character, which is what informs the router the message is done. Clearly, you can use any character you want for it except for the delimiting character in the message itself. Once the message is complete, press Enter, then the delimiting character, and then press Enter again. Everything will still work if you don't follow this routine unless you have more than one banner. If that's the case, make sure you do follow it or your banners will all be combined into one message and put on a single line! You can set a banner on one line like this: Todd(config)#banner motd x Unauthorized access prohibited! x

Physical Layer

Moves bits between devices Specifies voltage, wire speed, and pinout of cables Physical topology we find that the Physical layer does two things: it sends bits and receives bits. Bits come only in values of 1 or 0—a Morse code with numerical values. The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others employ state transitions—changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media's attachment interface. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). (Some old phone-company employees still call DCE "data circuit-terminating equipment.") The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU).

How multicast works

Multicast works by sending messages or data to IP multicast group addresses. Unlike with broadcasts, which aren't forwarded, routers then forward copies of the packet out to every interface that has hosts subscribed to that group address. This is where multicast differs from broadcast messages—with multicast communication, copies of packets, in theory, are sent only to subscribed hosts. The range of multicast addresses starts with 224.0.0.0 and goes through 239.255.255.255. As you can see, this range of addresses falls within IP Class D address space based on classful IP assignment.

The following network devices operate at all seven layers of the OSI model:

Network management stations (NMSs) Web and application servers Gateways (not default gateways) Servers

Verifying Your Configuration

Obviously, show running-config would be the best way to verify your configuration and show startup-config would be the best way to verify the configuration that'll be used the next time the router is reloaded—right? Did you know that you can ping with different protocols? You can, and you can test this by typing ping ? at either the router user-mode or privileged-mode prompt: by using the question mark, I was able to determine that extended ping allows you to set the repeat count higher than the default of 5 and the datagram size larger by using the question mark, I was able to determine that extended ping allows you to set the repeat count higher than the default of 5 and the datagram size larger Now I can use this port for diagnostics, and even as my source port of my ping or traceroute, as so: Now I can use this port for diagnostics, and even as my source port of my ping or traceroute, as so: See example If you can telnet, ftp, or http into a device, you know that your IP connectivity just has to be solid!

Here are four ways a router functions in your network:

Packet switching Packet filtering Internetwork communication Path selection Unlike plain-vanilla layer 2 switches, which forward or filter frames, routers (layer 3 switches) use logical addressing and provide an important capacity called packet switching. Routers can also provide packet filtering via access lists, and when routers connect two or more networks together and use logical addressing (IP or IPv6), you then have an internetwork.

Class A network

Part of the Internet Protocol hierarchical addressing scheme. Class A networks have only 8 bits for defining networks and 24 bits for defining hosts and subnets on each network. The designers of the IP address scheme decided that the first bit of the first byte in a Class A network address must always be off, or 0. This means a Class A address must be between 0 and 127 in the first byte, inclusive. Understand that 0 and 127 are not valid in a Class A network because they're reserved addresses

Class B network

Part of the Internet Protocol hierarchical addressing scheme. Class B networks have 16 bits for defining networks and 16 bits for defining hosts and subnets on each network. In a Class B network, the RFCs state that the first bit of the first byte must always be turned on but the second bit must always be turned off. If you turn the other 6 bits all off and then all on, you will find the range for a Class B network a Class B network is defined when the first byte is configured from 128 to 191.

Class C network

Part of the Internet Protocol hierarchical addressing scheme. Class C networks have 24 bits for defining networks and only 8 bits for defining hosts and subnets on each network. For Class C networks, the RFCs define the first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process as the previous classes, convert from binary to decimal to find the range So, if you see an IP address that starts at 192 and goes to 223, you'll know it is a Class C IP address.

Understanding the Powers of 2

Powers of 2 are important to understand and memorize for use with IP subnetting. Reviewing powers of 2, remember that when you see a number noted with an exponent, it means you should multiply the number by itself as many times as the upper number specifies. For example, 23 is 2 x 2 x 2, which equals 8. Here's a list of powers of 2 to commit to memory: 21 = 2 22 = 4 23 = 8 24 = 16 25 = 32 26 = 64 27 = 128 28 = 256 29 = 512 210 = 1,024 211 = 2,048 212 = 4,096 213 = 8,192 214 = 16,384 Memorizing these powers of 2 is a good idea, but it's not absolutely necessary.

Typical Ethernet frame format

Preamble An alternating 1,0 pattern provides a clock at the start of each packet, which allows the receiving devices to lock the incoming bit stream. Start Frame Delimiter (SFD)/Synch The preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011, where the last pair of 1s allows the receiver to come into the alternating 1,0 pattern somewhere in the middle and still sync up to detect the beginning of the data. Destination Address (DA) This transmits a 48-bit value using the least significant bit (LSB) first. The DA is used by receiving stations to determine if an incoming packet is addressed to a particular node. The destination address can be an individual address or a broadcast or multicast MAC address. Remember that a broadcast is all 1s—all Fs in hex—and is sent to all devices. A multicast is sent only to a similar subset of nodes on a network. Source Address (SA) The SA is a 48-bit MAC address used to identify the transmitting device, and it uses the least significant bit first. Broadcast and multicast address formats are illegal within the SA field. Length or Type 802.3 uses a Length field, but the Ethernet_II frame uses a Type field to identify the Network layer protocol. The old, original 802.3 cannot identify the upper-layer protocol and must be used with a proprietary LAN—IPX, for example. Data This is a packet sent down to the Data Link layer from the Network layer. The size can vary from 46 to 1,500 bytes. Frame Check Sequence (FCS) FCS is a field at the end of the frame that's used to store the cyclic redundancy check (CRC) answer. The CRC is a mathematical algorithm that's run when each frame is built based on the data in the frame. When a receiving host receives the frame and runs the CRC, the answer should be the same. If not, the frame is discarded, assuming errors have occurred.

Presentation Layer

Presents data in the proper format Handles processing such as encryption Data encryption, compression, and translation services gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. Think of it as the OSI model's translator, providing coding and conversion services An example of this type of translation service occurs when translating old Extended Binary Coded Decimal Interchange Code (EBCDIC) data to ASCII, the American Standard Code for Information Interchange (often pronounced "askee"). So just remember that by providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one. With this in mind, it follows that the OSI would include protocols that define how standard data should be formatted, so key functions like data compression, decompression, encryption, and decryption are also associated with this layer. Some Presentation layer standards are involved in multimedia operations as well.

Network Addresses

Protocol-specific network addresses. A router must maintain a routing table for individual routing protocols because each routed protocol keeps track of a network with a different addressing scheme. For example, the routing tables for IP and IPv6 are completely different, so the router keeps a table for each one. Think of it as a street sign in each of the different languages spoken by the American, Spanish, and French people living on a street; the street sign would read Cat/Gato/Chat.

Network Layer

Provides Logical addressing, which routers use for path determination Routing The Network layer, or layer 3, manages device addressing, tracks the location of devices on the network, and determines the best way to move data. This means that it's up to the Network layer to transport traffic between devices that aren't locally attached. Routers, which are layer 3 devices, are specified at this layer and provide the routing services within an internetwork. Here's how that works: First, when a packet is received on a router interface, the destination IP address is checked. If the packet isn't destined for that particular router, it will look up the destination network address in the routing table. Once the router chooses an exit interface, the packet will be sent to that interface to be framed and sent out on the local network. If the router can't find an entry for the packet's destination network in the routing table, the router drops the packet. Data and route update packets are the two types of packets used at the Network layer:

Application layer

Provides a user interface File, print, message, database, and application services marks the spot where users actually communicate to the computer and comes into play only when it's clear that access to the network will be needed soon. Basically working as the interface between the actual application program and the next layer down by providing ways for the application to send information down through the protocol stack. Identifying and confirming the communication partner's availability and verifying the required resources to permit the specified type of communication to take place also occurs at the Application layer Note The Application layer works as the interface between actual application programs. This means end-user programs like Microsoft Word don't reside at the Application layer, they interface with the Application layer protocols. Later, in Chapter 3, "Introduction to TCP/IP," I'll talk in detail about a few important programs that actually reside at the Application layer, like Telnet, FTP, and TFTP.

Privileged exec mode

Provides access to all other router commands

Transport Layer

Provides reliable or unreliable delivery Performs error correction before retransmit End-to-end connection segments and reassembles data into a single data stream. Services located at this layer take all the various data received from upper-layer applications, then combine it into the same, concise data stream. These protocols provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork. The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits. It can also hide the details of network-dependent information from the higher layers as well as provide transparent data transfer. Note The term reliable networking can be used at the Transport layer. Reliable networking requires that acknowledgments, sequencing, and flow control will all be used.

Acknowledgments

Reliable data delivery ensures the integrity of a stream of data sent from one machine to the other through a fully functional data link. It guarantees that the data won't be duplicated or lost. This is achieved through something called positive acknowledgment with retransmission—a technique that requires a receiving machine to communicate with the transmitting source by sending an acknowledgment message back to the sender when it receives data. The sender documents each segment measured in bytes, then sends and waits for this acknowledgment before sending the next segment. Also important is that when it sends a segment, the transmitting machine starts a timer and will retransmit if the timer expires before it gets an acknowledgment back from the receiving end. Figure 1.13 pictures the process I just described.

Logical Link Control (LLC)

Responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this: A host receives a frame and looks in the LLC header to find out where the packet is destined—for instance, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits. The switches and bridges I talked about near the beginning of the chapter both work at the Data Link layer and filter the network using hardware (MAC) addresses. Note As data is encoded with control information at each layer of the OSI model, the data is named with something called a protocol data unit (PDU). At the Transport layer, the PDU is called a segment, at the Network layer it's a packet, at the Data Link a frame, and at the Physical layer it's called bits. This method of naming the data at each layer is covered thoroughly in Chapter 2. Bridges and switches read each frame as it passes through the network. The layer 2 device then puts the source hardware address in a filter table and keeps track of which port the frame was received on. This information—logged in the bridge's or switch's filter table—is what helps the machine determine the location of the specific sending device. Figure 1.17 shows a switch in an internetwork and demonstrates John sending packets to the Internet. Sally doesn't hear his frames because she's in a different collision domain.

router characteristics that you should never forget:

Routers, by default, will not forward any broadcast or multicast packets. Routers use the logical address in a Network layer header to determine the next-hop router to forward the packet to. Routers can use access lists, created by an administrator, to control security based on the types of packets allowed to enter or exit an interface. Routers can provide layer 2 bridging functions if needed and can simultaneously route through the same interface. Layer 3 devices—in this case, routers—provide connections between virtual LANs (VLANs). Routers can provide quality of service (QoS) for specific types of network traffic.

several things that should generally be handled at the distribution layer:

Routing Implementing tools (such as access lists), packet filtering, and queuing Implementing security and network policies, including address translation and firewalls Redistributing between routing protocols, including static routing Routing between VLANs and other workgroup support functions Defining broadcast and multicast domains

Routing Protocol Configurations

Routing Protocol Configurations I need to point out that we don't use routing or router protocols on 2960 switches, but we can and will use them on my 3560 switches. Here is an example of configuring routing on a layer 3 switch: Switch(config)#router rip IP routing not enabled Switch(config)#ip routing Switch(config)#router rip Switch(config-router)#

show ip arp

Same function as arp -a, but displays the ARP table on a Cisco router. Like the commands traceroute and tracert, arp -a and show ip arp are not interchangeable through DOS and Cisco.

Table 3.4: Reserved IP addresses

See Table

Table 3.5: Reserved IP address space

See Table

Table 6.2: Enhanced editing commands

See Table

Table 6.3: IOS-command history

See Table

Switch#clock ?

See example I entered the clock ? command and got a list of the next possible parameters plus what they do. Make note of the fact that you can just keep typing a command, a space, and then a question mark until <cr> (carriage return) is your only option left. Extended Example

Descriptions

Setting descriptions on an interface is another administratively helpful thing, and like the hostname, it's also only locally significant. One case where the description command comes in really handy is when you want to keep track of circuit numbers on a switch or a router's serial WAN port. See example and Real World Scenario

1000Base-LX (IEEE 802.3z)

Single-mode fiber that uses a 9-micron core and 1300 nm laser and can go from 3 kilometers up to 10 kilometers.

The crossover cable can be used to connect the following devices:

Switch to switch Hub to hub Host to host Hub to switch Router direct to host Router to router

Verifying with the show interface Command

TRIM THIS LATER (Stupid copy paste :() Another way to verify your configuration is by typing show interface commands, the first of which is the show interface ? command. Doing this will reveal all the available interfaces to verify and configure. Note The show interfaces command, plural, displays the configurable parameters and statistics of all interfaces on a router. This command comes in really handy when you're verifying and troubleshooting router and network issues. The following output is from my freshly erased and rebooted 2811 router: Router#sh int ? Async Async interface BVI Bridge-Group Virtual Interface CDMA-Ix CDMA Ix interface CTunnel CTunnel interface Dialer Dialer interface FastEthernet FastEthernet IEEE 802.3 Loopback Loopback interface MFR Multilink Frame Relay bundle interface Multilink Multilink-group interface Null Null interface Port-channel Ethernet Channel of interfaces Serial Serial Tunnel Tunnel interface Vif PGM Multicast Host interface Virtual-PPP Virtual PPP interface Virtual-Template Virtual Template interface Virtual-TokenRing Virtual TokenRing accounting Show interface accounting counters Show interface counters crb Show interface routing/bridging info dampening Show interface dampening info description Show interface description etherchannel Show interface etherchannel information irb Show interface routing/bridging info mac-accounting Show interface MAC accounting info mpls-exp Show interface MPLS experimental accounting info precedence Show interface precedence accounting info pruning Show interface trunk VTP pruning information rate-limit Show interface rate-limit info status Show interface line status summary Show interface summary switching Show interface switching switchport Show interface switchport information trunk Show interface trunk information | Output modifiers <cr> The only "real" physical interfaces are FastEthernet, Serial, and Async—the rest are all logical interfaces or commands you can use to verify with. The next command is show interface fastethernet 0/0. It reveals the hardware address, logical address, and encapsulation method as well as statistics on collisions, as seen here: Router#sh int f0/0 FastEthernet0/0 is up, line protocol is up Hardware is

Table 4.3: What do you know?

Table 4.3 puts all of the previous information into one compact little table. You should practice writing this table out, and if you can do it, write it down before you start your exam!

Editing and Help Features

The Cisco advanced editing features can also help you configure your router. If you type in a question mark (?) at any prompt, you'll be given a list of all the commands available from that prompt: See example Press RETURN for another line, SPACE for another page, anything else to quit. And if this is not enough information for you, you can press the spacebar to get another whole page of information, or you can press Enter to go one command at a time. You can also press Q, or any other key for that matter, to quit and return to the prompt. Notice that I typed a question mark (?) at the More prompt and it told me what my options were from that prompt. Here's a shortcut: To find commands that start with a certain letter, use the letter and the question mark with no space between them, like this: Switch#c? cd clear clock cns configure connect copy Switch#c By typing c?, I got a response listing all the commands that start with c. Also notice that the Switch#c prompt reappears after the list of commands is displayed

Ethernet Frames

The Data Link layer is responsible for combining bits into bytes and bytes into frames. Frames are used at the Data Link layer to encapsulate packets handed down from the Network layer for transmission on a type of media access. The function of Ethernet stations is to pass data frames between each other using a group of bits known as a MAC frame format. This provides error detection from a cyclic redundancy check (CRC). But remember—this is error detection, not error correction.

DoD Model

The DoD model is basically a condensed version of the OSI model that comprises four instead of seven layers: Process/Application layer Host-to-Host layer or Transport layer Internet layer Network Access layer or Link layer Figure 3.1

Address Resolution Protocol (ARP)

The Network layer protocol that IP uses to ascertain the MAC address of a known IP address when IP determines that the destination is on the local subnet and communication with the destination must therefore occur at the Data Link layer. basically, ARP translates the software (IP) address into a hardware address—for example, the destination machine's Ethernet adapter address—and from it, deduces its whereabouts on the LAN by broadcasting for this address.

session multiplexing

The Transport layer, working in tandem with the Session layer, also separates the data from different applications, an activity known as session multiplexing, and it happens when a client connects to a server with multiple browser sessions open. This is exactly what's taking place when you go someplace online like Amazon and click multiple links, opening them simultaneously to get information when comparison shopping. The client data from each browser session must be separate when the server application receives it, which is pretty slick technologically speaking, and it's the Transport layer that performs the juggling act.

Broadcast address

The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples of layer 3 broadcasts include 255.255.255.255, which is any network, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.

packet/datagram

The basic division of data sent over a network.

transparent bridging

The bridging scheme that is used in Ethernet and IEEE 802.3 networks and passes frames along one hop at a time, using bridging information stored in tables that associate end-node MAC addresses with bridge ports. This type of bridging is considered transparent because the source node does not know it has been bridged because the destination frames are addressed directly to the end node.

Checksum

The cyclic redundancy check (CRC), used because TCP doesn't trust the lower layers and checks everything. The CRC checks the header and data fields.

organizationally unique identifier (OUI)

The first 24 bits of a 48-bit MAC address. Each OUI is assigned by the IEEE to a single manufacturer of devices that have MAC addresses assigned to them. As long as the manufacturer does not duplicate the last 24 bits of the MAC address, the assumption is that the entire MAC address will be unique worldwide. However, renegade manufacturers and manufacturing mistakes can result in duplicate MAC addresses. As long as the devices with duplicate addresses do not make it onto the same local network segment (the same IP subnet, for example), this conflict will never be an issue.

Class C Addresses

The first 3 bytes of a Class C network address are dedicated to the network portion of the address, with only 1 measly byte remaining for the node address. Here's the format: network.network.network.node Using the example IP address 192.168.100.102, the network address is 192.168.100 and the node address is 102. In a Class C network address, the first three bit positions are always the binary 110. The calculation is as follows: 3 bytes, or 24 bits, minus 3 reserved positions leaves 21 positions. Hence, there are 221, or 2,097,152, possible Class C networks. Each unique Class C network has 1 byte to use for node addresses. This leads to 28, or 256, minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.

Collision Domain

The group of devices whose frames could potentially collide with one another. Each interface on a bridge, switch, or router terminates a collision domain. These devices become responsible for recovering from collisions that occur due to their forwarding of frames out other interfaces.

1000Base-SX (IEEE 802.3z)

The implementation of 1 Gigabit Ethernet running over multimode fiber-optic cable instead of copper twisted-pair cable, using short wavelength laser and LEDs. Multimode fiber (MMF) using 62.5- and 50-micron core; uses an 850 nanometer (nm) laser and can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.

Domain Name Service (DNS)

The network service used in TCP/IP networks that translates hostnames to IP addresses. See also Transmission Control Protocol/Internet Protocol (TCP/IP). resolves hostnames—specifically, Internet names, such as www.lammle.com. But you don't have to actually use DNS. You just type in the IP address of any device you want to communicate with and find the IP address of a URL by using the Ping program. For example, >ping www.cisco.com will return the IP address resolved by DNS allows you to use a domain name to specify an IP address. You can change the IP address as often as you want and no one will know the difference.

Classless Inter-Domain Routing (CIDR)

The new routing method used by InterNIC to assign IP addresses. CIDR can be described as a "slash x" network. The x represents the number of bits in the network that InterNIC controls. It's basically the method that Internet service providers (ISPs) use to allocate a number of addresses to a company, a home—their customers. When you receive a block of addresses from an ISP, what you get will look something like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/) means how many bits are turned on (1s). Obviously, the maximum could only be /32 because a byte is 8 bits and there are 4 bytes in an IP address: (4 × 8 = 32).But keep in mind that regardless of the class of address, the largest subnet mask available relevant to the Cisco exam objectives can only be a /30 because you've got to keep at least 2 bits for host bits. Take, for example, a Class A default subnet mask, which is 255.0.0.0. This tells us that the first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash notation, you need to count all the 1 bits to figure out your mask. The 255.0.0.0 is considered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on. Table 4.2 has a listing of every available subnet mask and its equivalent CIDR slash notation.

Bandwidth command

The next command you need to get acquainted with is the bandwidth command. Every Cisco router ships with a default serial link bandwidth of T1 (1.544 Mbps). But this has nothing to do with how data is transferred over a link. The bandwidth of a serial link is used by routing protocols such as EIGRP and OSPF to calculate the best cost path to a remote network. So if you're using RIP routing, the bandwidth setting of a serial link is irrelevant since RIP uses only hop count to determine this. Here's an example of using the bandwidth command: Router#config t Router(config)#int s0/0/0 Router(config-if)#bandwidth ? <1-10000000> Bandwidth in kilobits inherit Specify that bandwidth is inherited receive Specify receive-side bandwidth Router(config-if)#bandwidth 1000 unlike the clock rate command, the bandwidth command is configured in kilobits per second

Header length

The number of 32-bit words in the TCP header, which indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits in length.

Private IP Addresses (RFC 1918)

The people who created the IP addressing scheme also created private IP addresses. These addresses can be used on a private network, but they're not routable through the Internet. This is designed for the purpose of creating a measure of well-needed security, but it also conveniently saves valuable IP address space. by using private IP addresses, ISPs, corporations and home users only need a relatively tiny group of bona fide IP addresses to connect their networks to the Internet. This is economical because they can use private IP addresses on their inside networks and get along just fine.

Topologies at the Physical layer

The physical topology of a network refers to the physical layout of the devices, but mostly the cabling and cabling layout. The logical topology defines the logical path on which the signal will travel on the physical topology.

Subnetting

The process of dividing a single IP address range into multiple address ranges.

Transmission Control Protocol (TCP)

The protocol found at the Host-to-Host layer of the DoD model. This protocol breaks data packets into segments, numbers them, and sends them in random order. The receiving computer reassembles the data so that the information is readable for the user. In the process, the sender and the receiver confirm that all data has been received; if not, it is resent. This is a connection-oriented protocol. See also connection-oriented transport protocol.

Internet Protocol (IP)

The protocol in the TCP/IP protocol suite responsible for network addressing and routing. See also Transmission Control Protocol/Internet Protocol (TCP/IP). essentially is the Internet layer Figure 3.15 shows an IP header. This will give you a picture of what the IP protocol has to go through every time user data that is destined for a remote network is sent from the upper layers. (fields in depth in book)

Transmission Control Protocol/Internet Protocol (TCP/IP) Suite

The protocol suite developed by the DoD as an internetworking protocol suite that could route information around network failures. Today it is the de facto standard for communications on the Internet.

Hypertext Transfer Protocol (HTTP)

The protocol used for communication between a web server and a web browser. each URL defines the protocol used to transfer data, the name of the server, and the particular web page on that server.

Windowing

The quantity of data segments, measured in bytes, that the transmitting machine is allowed to send without receiving an acknowledgment is called a window. Note Windows are used to control the amount of outstanding, unacknowledged data segments. The size of the window controls how much information is transferred from one end to the other before an acknowledgment is required. While some protocols quantify information depending on the number of packets, TCP/IP measures it by counting the number of bytes.

So Which Private IP Address Should I Use?

The rule of thumb in the consulting world is, when you're setting up a corporate network—regardless of how small it is—you should use a Class A network address because it gives you the most flexibility and growth options. For example, if you used the 10.0.0.0 network address with a /24 mask, then you'd have 65,536 networks, each with 254 hosts. Lots of room for growth with that network! But if you're setting up a home network, you'd opt for a Class C address because it is the easiest for people to understand and configure. Using the default Class C mask gives you one network with 254 hosts—plenty for a home network.

Using the show controllers Command

The show controllers command displays information about the physical interface itself. It'll also give you the type of serial cable plugged into a serial port. Usually, this will only be a DTE cable that plugs into a type of data service unit (DSU). If you use the show controllers command and the show ip interface command, you'll see that router R1 isn't receiving the clocking of the line. This network is a nonproduction network, so no CSU/DSU is connected to provide clocking for it. This means the DCE end of the cable will be providing the clock rate—in this case, the R2 router. The show ip interface indicates that the interface is up but the protocol is down, which means that no keepalives are being received from the far end. In this example, the likely culprit is bad cable, or simply the lack of clocking.

Using the show ip interface brief Command

The show ip interface brief command is probably one of the best commands that you can ever use on a Cisco router or switch. This command provides a quick overview of the devices interfaces, including the logical address and status: Remember, administratively down means that you need to type no shutdown in order to enable the interface. Notice that Serial0/0/0 is up/down, which means that the Physical layer is good and carrier detect is sensed but no keepalives are being received from the remote end. In a nonproduction network, like the one I am working with, this tells us the clock rate hasn't been set.

Verifying with the show ip interface Command

The show ip interface command will provide you with information regarding the layer 3 configurations of a router's interface, such as the IP address and subnet mask, MTU, and if an access list is set on the interface: The status of the interface, the IP address and mask, information on whether an access list is set on the interface, and basic IP information are all included in this output.

Verifying with the show protocols Command

The show protocols command is also a really helpful command that you'd use in order to quickly see the status of layers 1 and 2 of each interface as well as the IP addresses used. The show protocols command is also a really helpful command that you'd use in order to quickly see the status of layers 1 and 2 of each interface as well as the IP addresses used.

encapsulation

The technique used by layered protocols in which a layer adds header information to the Protocol Data Unit (PDU) from the layer above. As an example, in Internet terminology, a packet contains a header from the Data Link layer, followed by a header from the Network layer (IP), followed by a header from the Transport layer (TCP), followed by the application protocol data.

In-band Connection

The third way to connect to a Cisco device is in-band, through the program Telnet or Secure Shell (SSH). In-band means configuring the device via the network, the opposite of out-of-band. We covered Telnet and SSH in Chapter 3, "Introduction to TCP/IP," and in this chapter, I'll show you how to configure access to both of these protocols on a Cisco device.

Crossover Cable

The troubleshooting tool used in Ethernet UTP installations to test communications between two stations, bypassing the hub. Crossover cables can also be used to interconnect two DTE devices, such as PCs and routers, or two DCE devices, such as hubs and switches. See also unshielded twisted-pair (UTP) cable, medium dependent interface (MDI), medium dependent interface-crossover (MDI-X). The same four wires used in the straight-through cable are used in this cable—we just connect different pins together Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

Acknowledgment number The value is the TCP octet that is expected nex

The value is the TCP octet that is expected next.

Window

The window size the sender is willing to accept, in octets.

Setting Passwords

There are five passwords you'll need to secure your Cisco routers: console, auxiliary, telnet (VTY), enable password, and enable secret. The enable secret and enable password are the ones used to set the password for securing privileged mode. Once the enable commands are set, users will be prompted for a password. The other three are used to configure a password when user mode is accessed through the console port, through the auxiliary port, or via Telnet. Let's take a look at each of these now.

Subnetting Class C Addresses

There are many different ways to subnet a network. The right way is the way that works best for you. In a Class C address, only 8 bits are available for defining the hosts. Remember that subnet bits start at the left and move to the right, without skipping bits. This means that Class C subnet masks can only be the following: Binary Decimal CIDR --------------------------------------------------------- 00000000 = 255.255.255.0 /24 10000000 = 255.255.255.128 /25 11000000 = 255.255.255.192 /26 11100000 = 255.255.255.224 /27 11110000 = 255.255.255.240 /28 11111000 = 255.255.255.248 /29 11111100 = 255.255.255.252 /30 We can't use a /31 or /32 because, as I've said, we must have at least 2 host bits for assigning IP addresses to hosts. But this is only mostly true. Certainly we can never use a /32 because that would mean zero host bits available, yet Cisco has various forms of the IOS, as well as the new Cisco Nexus switches operating system, that support the /31 mask. The /31 is above the scope of the CCNA objectives, so we won't be covering it in this book.

Multicast

These are packets sent from a single source and transmitted to many devices on different networks. Refe rred to as "one-to-many." Broadly, any communication between a single sender and multiple receivers. Unlike broadcast messages, which are sent to all addresses on a network, multicast messages are sent to a defined subset of the network addresses; this subset has a group multicast address, which is specified in the packet's destination address field. See also broadcast address. At first glance, it appears to be a hybrid of unicast and broadcast communication, but that isn't quite the case. Multicast does allow point-to-multipoint communication, which is similar to broadcasts, but it happens in a different manner. The crux of multicast is that it enables multiple recipients to receive messages without flooding the messages to all hosts on a broadcast domain. However, this is not the default behavior—it's what we can do with multicasting if it's configured correctly.

Layer 2 Broadcasts

These are sent to all nodes on a LAN. layer 2 broadcasts are also known as hardware broadcasts—they only go out on a LAN, but they don't go past the LAN boundary (router). The typical hardware address is 6 bytes (48 bits) and looks something like 45:AC:24:E3:60:A5. The broadcast would be all 1s in binary, which would be all Fs in hexadecimal, as in ff:ff:ff:ff:ff:ff and shown in Figure 3.21. Every network interface card (NIC) will receive and read the frame, including the router, since this was a layer 2 broadcast, but the router would never, ever forward this!

Broadcasts (layer 3)

These are sent to all nodes on the network. Broadcast messages are meant to reach all hosts on a broadcast domain. These are the network broadcasts that have all host bits on. The network address of 172.16.0.0 255.255.0.0 would have a broadcast address of 172.16.255.255—all host bits on. Broadcasts can also be "any network and all hosts," as indicated by 255.255.255.255, and shown in Figure 3.22.

router

These convenient devices are used to connect networks and route packets of data from one network to another. Cisco became the de facto standard for routers because of its unparalleled selection of high-quality router products and fantastic service. So never forget that by default, routers are basically employed to efficiently break up a broadcast domain—the set of all devices on a network segment, which are allowed to "hear" all broadcasts sent out on that specific segment. Routers provide connections to wide area network (WAN) services as well via a serial interface for WAN connections—specifically, a V.35 physical interface on a Cisco router.

Access Points or APs:

These devices allow wireless devices to connect to a wired network and extend a collision domain from a switch. They're typically in their own broadcast domain or what we'll refer to as a Virtual LAN (VLAN). An AP can be a simple standalone device, but today they're usually managed by wireless controllers, either in house or through the internet.

Route Update Packets

These packets are used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called routing protocols and the most critical ones for CCNA are Static Routing and OSPF Single Area. Route update packets are used to help build and maintain routing tables.

There are two advantages to using routers in your network:

They don't forward broadcasts by default. They can filter the network based on layer 3 (Network layer) information such as an IP address.

IP Subnet-Zero

This command allows you to use the first and last subnet in your network design. For instance, the Class C mask of 255.255.255.192 provides subnets 64,128 and 192 But with the ip subnet-zero command, you now get to use subnets 0, 64, 128, and 192. It may not seem like a lot, but this provides two more subnets for every subnet mask we use. Example: This router output shows that the command ip subnet-zero is enabled on the router. Cisco has turned this command on by default starting with Cisco IOS version 12.x and now we're running 15.x code. When taking your Cisco exams, make sure you read very carefully to see if Cisco is asking you not to use ip subnet-zero. There are actually instances where this may happen.

Unicast

This is an address for a single interface, and these are used to send packets to a single destination host. A unicast is defined as a single IP address that's assigned to a network interface card and is the destination IP address in a packet—in other words, it's used for directing packets to a specific host.

Network Address

This is the designation used in routing to send packets to a remote network—for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0. The network address (which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. For example, in the IP address 172.16.30.56, 172.16 represents the network address.

Destination Port

This is the port number of the application requested on the destination host.

Input Errors

This is the total of many counters: runts, giants, no buffer, CRC, frame, overrun, and ignored counts.

No buffer

This isn't a number you want to see incrementing. This means you don't have any buffer room left for incoming packets. Any packets received once the buffers are full are discarded. You can see how many packets are dropped with the ignored output.

Frame

This output increments when frames received are of an illegal format, or not complete, which is typically incremented when a collision occurs.

Protocol Data Unit (PDU)

To communicate and exchange information, each layer uses protocol data units (PDUs). These hold the control information attached to the data at each layer of the model. They are usually attached to the header in front of the data field but can also be at the trailer, or end, of it. Each PDU attaches to the data by encapsulating it at each layer of the OSI model, and each has a specific name depending on the information provided in each header. This PDU information is read only by the peer layer on the receiving device. After it's read, it's stripped off and the data is then handed to the next layer up. Figure 2.21 shows the PDUs and how they attach control information to each layer. This figure demonstrates how the upper-layer user data is converted for transmission on the network. The data stream is then handed down to the Transport layer, which sets up a virtual circuit to the receiving device by sending over a synch packet. Next, the data stream is broken up into smaller pieces, and a Transport layer header is created and attached to the header of the data field; now the piece of data is called a segment (a PDU). Each segment can be sequenced so the data stream can be put back together on the receiving side exactly as it was transmitted.

Access List Configurations

To configure a standard named access list, you'll need to get to the prompt Switch (config-std-nacl)#: Switch#config t Switch(config)#ip access-list standard Todd Switch(config-std-nacl)# What you see here is a typical basic standard ACL prompt. There are various ways to configure access lists, and the prompts are only slightly different from this particular example.

Overview of Router Modes

To configure from a CLI, you can make global changes to the router by typing configure terminal or just config t. This will get you into global configuration mode where you can make changes to the running-config. Commands run from global configuration mode are predictably referred to as global commands, and they are typically set only once and affect the entire router. Type config from the privileged-mode prompt and then press Enter to opt for the default of terminal like this: Switch#config Configuring from terminal, memory, or network [terminal]? [press enter] Enter configuration commands, one per line. End with CNTL/Z. Switch(config)# At this point, you make changes that affect the router as a whole (globally), hence the term global configuration mode. For instance, to change the running-config—the current configuration running in dynamic RAM (DRAM)—use the configure terminal command, as I just demonstrated.

Auxiliary Password

To configure the auxiliary password on a router, go into global configuration mode and type line aux ?. And by the way, you won't find these ports on a switch. This output shows that you only get a choice of 0-0, which is because there's only one port:

Line Commands

To configure user-mode passwords, use the line command. The prompt then becomes Switch(config-line)#: Switch(config)#line ? <0-15> First Line number console Primary terminal line vty Virtual terminal Switch(config)#line console 0 Switch(config-line)# The line console 0 command is a global command, and sometimes you'll also hear people refer to global commands as major commands. In this example, any command typed from the (config-line) prompt is known as a subcommand.

Interfaces

To make changes to an interface, you use the interface command from global configuration mode: Switch(config)#interface ? See output

Telnet Password

To set the user-mode password for Telnet access into the router or switch, use the line vty command. IOS switches typically have 15 lines, but routers running the Enterprise edition have considerably more. The best way to find out how many lines you have is to use that handy question mark like this: Example So what will happen if you try to telnet into a device that doesn't have a VTY password set? You'll receive an error saying the connection has been refused because the password isn't set. So, if you telnet into a switch and receive a message like this one that I got from Switch B So what will happen if you try to telnet into a device that doesn't have a VTY password set? You'll receive an error saying the connection has been refused because the password isn't set. So, if you telnet into a switch and receive a message like this one that I got from Switch B Todd#telnet SwitchB Trying SwitchB (10.0.0.1)...Open Password required, but none set [Connection to SwitchB closed by foreign host] Todd# it means the switch doesn't have the VTY password set. But you can still get around this and tell the switch to allow Telnet connections without a password by using the no login command: SwitchB(config-line)#line vty 0 15 SwitchB(config-line)#no login After your IOS devices are configured with an IP address, you can use the Telnet program to configure and check your routers instead of having to use a console cable. You can use the Telnet program by typing telnet from any command prompt (DOS or Cisco).

Packet Output

Total number of packets (frames) forwarded out to the interface.

Output Errors

Total number of packets (frames) that the switch port tried to transmit but for which some problem occurred.

Process/Application layer

Upper layer in the Internet Protocol stack that is responsible for network services. In short, the Process/Application layer defines protocols for node-to-node application communication and controls user-interface specifications. Telnet, FTP, LPD, SNMP, TFTP, SMTP, NFS, X Window, SSH, HTTP, HTTPS, NTP, DNS, DHCP/BootP, APIPA

Port Numbers

Used at the transport layer with TCP and UDP to keep track of host-to-host virtual circuits. Originating-source port numbers are dynamically assigned by the source host and will equal some number starting at 1024. Port number 1023 and below are defined in RFC 3232 (or just see www.iana.org), which discusses what we call well-known port numbers.

ipconfig /all

Used only from a Windows command prompt; shows you the PC network configuration.

Loopback

Used to test the IP stack on the local computer. Can be any address from 127.0.0.1 through 127.255.255.254.

100Base-FX (IEEE 802.3u)

Uses fiber cabling 62.5/125-micron multimode fiber. Point-to-point topology; up to 412 meters long. It uses ST and SC connectors, which are media-interface connectors.

Traceroute

Using ICMP time-outs, Traceroute is used to discover the path a packet takes as it traverses an internetwork. Note Traceroute is usually just called trace. Microsoft Windows uses tracert to allow you to verify address configurations in your internetwork.

Console Port

Usually an RJ55, 8-pin modular connection located at the back of the device, and mini-usb ports are now commonly found on routers and switches for use as a console connection. Note Look back into Chapter 2, "Ethernet Networking and Data Encapsulation," to review how to configure a PC and enable it to connect to a router console port.

Connecting to a Cisco IOS Device

We connect to a Cisco device to configure it, verify its configuration, and check statistics, and although there are different approaches to this, the first place you would usually connect to is the console port.

Console Password

We set the console password with the line console 0 command, but look at what happened when I tried to type line console ? from the (config-line)# prompt—I received an error! Here's the example: Todd(config-line)#line console ? % Unrecognized command Todd(config-line)#exit Todd(config)#line console ? <0-0> First Line number Todd(config)#line console 0 Todd(config-line)#password console Todd(config-line)#login You can still type line console 0 and that will be accepted, but the help screens just don't work from that prompt. Type exit to go back one level, and you'll find that your help screens now work. This is a "feature." Really. Because there's only one console port, I can only choose line console 0. You can set all your line passwords to the same password, but doing this isn't exactly a brilliant security move! And it's also important to remember to apply the login command or the console port won't prompt for authentication.

Hostnames

We use the hostname command to set the identity of the router and switch. This is only locally significant, meaning it doesn't affect how the router or switch performs name lookups or how the device actually works on the internetwork. But the hostname is still important in routes because it's often used for authentication in many wide area networks (WANs). Here's an example: Switch#config t Switch(config)#hostname Todd Todd(config)#hostname Chicago Chicago(config)#hostname Todd Todd(config)#

Binary to Decimal Conversion

We'll start with binary numbering, which is really pretty simple. The digits used are limited to either a 1 or a 0, and each digit is called a bit, which is short for binary digit. Typically, you group either 4 or 8 bits together, with these being referred to as a nibble and a byte, respectively. The interesting thing about binary numbering is how the value is represented in a decimal format—the typical decimal format being the base-10 number scheme that we've all used since kindergarten. The binary numbers are placed in a value spot, starting at the right and moving left, with each spot having double the value of the previous spot. What all this means is that if a one digit (1) is placed in a value spot, then the nibble or byte takes on that decimal value and adds it to any other value spots that have a 1. If a zero (0) is placed in a bit spot, you don't count that value. Let me clarify this a little. If we have a 1 placed in each spot of our nibble, we would then add up 8 + 4 + 2 + 1 to give us a maximum value of 15. Another example for our nibble values would be 1001, meaning that the 8 bit and the 1 bit are turned on, which equals a decimal value of 9. If we have a nibble binary value of 0110, then our decimal value would be 6, because the 4 and 2 bits are turned on. Memorize Table 2.2 - Binary to decimal memorization chart

Collisions

When transmitting a frame in half-duplex, the NIC listens on the receiving pair of the cable for another signal. If a signal is transmitted from another host, a collision has occurred. This output should not increment if you are running full-duplex.

Bringing up a Switch

When you first bring up a Cisco IOS device, it will run a power-on self-test—a POST. Upon passing that, the machine will look for and then load the Cisco IOS from flash memory if an IOS file is present, then expand it into RAM As you probably know, flash memory is electronically erasable programmable read-only memory—an EEPROM. The next step is for the IOS to locate and load a valid configuration known as the startup-config that will be stored in nonvolatile RAM (NVRAM). Once the IOS is loaded and up and running, the startup-config will be copied from NVRAM into RAM and from then on referred to as the running-config. But if a valid startup-config isn't found in NVRAM, your switch will enter setup mode, giving you a step-by-step dialog to help configure some basic parameters on it. You can also enter setup mode at any time from the command line by typing the command setup from privileged mode, which I'll get to in a minute. Setup mode only covers some basic commands and generally isn't really all that helpful. See example:

Subnetting a Class C Address—The Fast Way!

When you've chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and the broadcast addresses of a subnet that mask will provide, all you need to do is answer five simple questions: How many subnets does the chosen subnet mask produce? How many valid hosts per subnet are available? What are the valid subnets? What's the broadcast address of each subnet? What are the valid hosts in each subnet?

Automatic Private IP Addressing (APIPA)

With APIPA, clients can automatically self-configure an IP address and subnet mask—basic IP information that hosts use to communicate—when a DHCP server isn't available. The IP address range for APIPA is 169.254.0.1 through 169.254.255.254. The client also configures itself with a default Class B subnet mask of 255.255.0.0.

You can use full-duplex Ethernet in at least the following six situations:

With a connection from a switch to a host With a connection from a switch to a switch With a connection from a host to a host With a connection from a switch to a router With a connection from a router to a router With a connection from a router to a host Note Full-duplex Ethernet requires a point-to-point connection when only two nodes are present. You can run full-duplex with just about any device except a hub.

auxiliary port

You can also connect to a Cisco router through an auxiliary port, which is really the same thing as a console port, so it follows that you can use it as one. The main difference with an auxiliary port is that it also allows you to configure modem commands so that a modem can be connected to the router. This is a cool feature because it lets you dial up a remote router and attach to the auxiliary port if the router is down and you need to configure it remotely, out-of-band. One of the differences between Cisco routers and switches is that switches do not have an auxiliary port.

Exec banner

You can configure a line-activation (exec) banner to be displayed when EXEC processes such as a line activation or an incoming connection to a VTY line have been created. Simply initiating a user exec session through a console port will activate the exec banner.

Deleting the Configuration and Reloading the Device

You can delete the startup-config file by using the erase startup-config command: you can no longer use the shortcut commands for erasing the backup configuration. This started in IOS 12.5 with the ISR routers. Now if you reload or power the router down after using the erase startup-config command, you'll be offered setup mode because there's no configuration saved in NVRAM. You can press Ctrl+C to exit setup mode at any time, but the reload command can only be used from privileged mode.

Bringing up an Interface

You can disable an interface with the interface command shutdown and enable it with the no shutdown command. Just to remind you, all switch ports are enabled by default and all router ports are disabled by default, so we're going to talk more about router ports than switch ports in the next few sections. If an interface is shut down, it'll display as administratively down when you use the show interfaces command (sh int for short): with the no shutdown command (no shut for short):

Clock Rate Command

You configure a DCE serial interface with the clock rate command: The clock rate command is set in bits per second. Besides looking at the cable end to check for a label of DCE or DTE, you can see if a router's serial interface has a DCE cable connected with the show controllers int command:

internetwork

You create an internetwork when you connect two or more networks via a router and configure a logical network addressing scheme with a protocol such as IP or IPv6.

Enable Passwords

You set the enable passwords from global configuration mode like this: Todd(config)#enable ? last-resort Define enable action if no TACACS servers respond password Assign the privileged level password secret Assign the privileged level secret use-tacacs Use TACACS to check enable passwords

Ethernet

a contention-based media access method that allows all hosts on a network to share the same link's bandwidth. Some reasons it's so popular are that Ethernet is pretty simple to implement and it makes troubleshooting fairly straightforward as well. Ethernet is also readily scalable, meaning that it eases the process of integrating new technologies into an existing network infrastructure, like upgrading from Fast Ethernet to Gigabit Ethernet. Uses both Data Link and Physical layer specifications, so you'll be presented with information relative to both layers, which you'll need to effectively implement, troubleshoot, and maintain an Ethernet network.

duplex mismatch

a situation in which the switch operates at full-duplex and the connected device operates at half-duplex, or vice versa. The result of a duplex mismatch is extremely slow performance, intermittent connectivity, and loss of connection. Other possible causes of data-link errors at full-duplex are bad cables, a faulty switch port, or NIC software or hardware issues. Use the show interface command to verify the duplex settings. If the mismatch occurs between two Cisco devices with Cisco Discovery Protocol enabled, you will see Cisco Discovery Protocol error messages on the console or in the logging buffer of both devices.

wireless access point (AP)

a wireless device that allows hosts to connect wirelessly using the IEEE 802.11 specification APs don't actually segment the network, they only extend them, meaning our LAN just got a lot bigger, with an unknown amount of hosts that are all still part of one measly broadcast domain! This clearly demonstrates why it's so important to understand exactly what a broadcast domain is, and now is a great time to talk about them in detail.

fiber-optic cable

allows for very fast transmission of data, is made of glass (or even plastic), is very thin, and works as a waveguide to transmit light between two ends of the fiber becoming more and more popular in Ethernet LAN networks due to the fast speeds available and because, unlike UTP, it's immune to interference like cross-talk.

Hypertext Transfer Protocol Secure (HTTPS)

also known as Secure Hypertext Transfer Protocol uses Secure Sockets Layer (SSL) a secure version of HTTP that arms you with a whole bunch of security tools for keeping transactions between a web browser and a server secure.

network segmentation

breaking up a massive network into a number of smaller ones

Category 5 Enhanced Unshielded Twisted Pair (UTP) Cable

can handle speeds up to a gigabit with a distance of up to 100 meters. Typically we'd use this cable for 100 Mbps and category 6 for a gigabit, but the category 5 Enhanced is rated for gigabit speeds and category 6 is rated for 10 Gbps!

The Access Layer

controls user and workgroup access to internetwork resources and is sometimes referred to as the desktop layer. The network resources most users need are available locally because the distribution layer handles any traffic for remote services.

Login banner

delete it with the no banner login command. Here's what a login banner output looks like: ! banner login ^C The previous login banner should look pretty familiar to anyone who's ever logged into an ISR router because it's the banner Cisco has in the default configuration for its ISR routers Note Remember that the login banner is displayed before the login prompts and after the MOTD banner.

WLAN Controllers

devices that network administrators or network operations centers use to manage access points in medium to large to extremely large quantities. The WLAN controller automatically handles the configuration of wireless access points and was typically used only in larger enterprise systems. However, with Cisco's acquisition of Meraki systems, you can easily manage a small to medium-sized wireless network via the cloud using their simple-to-configure web controller system.

Firewalls

etwork security systems that monitor and control incoming and outgoing network traffic based on predetermined security rules—usually an Intrusion Protection System (IPS). Cisco Adaptive Security Appliance (ASA) firewall typically establishes a barrier between a trusted, secure internal network and the Internet, which is not secure nor trusted. Cisco's acquisition of Sourcefire put them in the top of the market with Next Generation Firewalls (NGFW) and Next Generation IPS (NGIPS), which Cisco calls "Firepower." Firepower runs on dedicated appliances: Cisco's ASA's, ISR routers and even on Meraki products.

Cisco's Way of Troubleshooting IP

first double-check and verify your network instead. Here are the four troubleshooting steps Cisco recommends: 1. Open a Command window and ping 127.0.0.1. This is the diagnostic, or loopback, address, and if you get a successful ping, your IP stack is considered initialized. If it fails, then you have an IP stack failure and need to reinstall TCP/IP on the host. (See example in book) 2.From the Command window, ping the IP address of the local host (we'll assume correct configuration here, but always check the IP configuration too!). If that's successful, your network interface card (NIC) is functioning. If it fails, there is a problem with the NIC. Success here doesn't just mean that a cable is plugged into the NIC, only that the IP protocol stack on the host can communicate to the NIC via the LAN driver. (See example and so on) 3. From the Command window, ping the default gateway (router). If the ping works, it means that the NIC is plugged into the network and can communicate on the local network. If it fails, you have a local physical network problem that could be anywhere from the NIC to the router. 4. If steps 1 through 3 were successful, try to ping the remote server. If that works, then you know that you have IP communication between the local host and the remote server. You also know that the remote physical network is working. If the user still can't communicate with the server after steps 1 through 4 have been completed successfully, you probably have some type of name resolution problem and need to check your Domain Name System (DNS) settings. But if the ping to the remote server fails, then you know you have some type of remote physical network problem and need to go to the server and work through steps 1 through 3 until you find the snag.

Note You can find more information about all Cisco routers at

https://www.cisco.com/c/en/us/products/routers/router-selector.html. (Not exam relevant)

Network Acccess Layer (Link Layer)

implements the data exchange between the host and the network. The equivalent of the Data Link and Physical layers of the OSI model, the Network Access layer oversees hardware addressing and defines protocols for the physical transmission of data. The reason TCP/IP became so popular is because there were no set physical layer specifications, so it could run on any existing or future physical network! Ethernet, Fast Ethernet, Token Ring, FDDI

The following list describes the enable password parameters:

last-resort This allows you to still enter the device if you set up authentication through a TACACS server and it's not available. It won't be used if the TACACS server is working. password This sets the enable password on older, pre-10.3 systems and isn't ever used if an enable secret is set. secret The newer, encrypted password that overrides the enable password if it has been set. use-tacacs This tells the router or switch to authenticate through a TACACS server. It comes in really handy when you have lots of routers because changing the password on a multitude of them can be insanely tedious. It's much easier to simply go through the TACACS server and change the password only once! If you try to set the enable secret and enable passwords the same, the device will give you a polite warning to change the second password. Make a note to yourself that if there aren't any old legacy routers involved, you don't even bother to use the enable password! User-mode passwords are assigned via the line command like this: Todd(config)#line ? <0-15> First Line number console Primary terminal line vty Virtual terminal And these two lines are especially important for the exam objectives: console Sets a console user-mode password. vty Sets a Telnet password on the device and is also used in the SSH configuraiton. If this password isn't set, then by default, Telnet can't be used. To configure user-mode passwords, choose the line you want and configure it using the login command to make the switch prompt for authentication. Let's focus in on the configuration of individual lines now.

The Core Layer

literally the core of the network. At the top of the hierarchy, the core layer is responsible for transporting large amounts of traffic both reliably and quickly. The only purpose of the network's core layer is to switch traffic as fast as possible. The traffic transported across the core is common to a majority of users. But remember that user data is processed at the distribution layer, which forwards the requests to the core if needed. If there's a failure in the core, every single user can be affected! Do's and Don'ts

Multimode Fiber Optic Cable

looser and has a larger core so it allows multiple light particles to travel down the glass. These particles have to be put back together at the receiving end, so distance is less than that with single-mode fiber, which allows only very few light particles to travel down the fiber.

single-mode fiber-optic cable

more expensive, has a tighter cladding, and can go much farther distances than multimode. The difference comes in the tightness of the cladding, which makes a smaller core, meaning that only one mode of light will propagate down the fiber

Collision domain

n Ethernet term used to describe a network scenario in which one device sends a packet out on a network segment and every other device on that same segment is forced to pay attention no matter what. This isn't very efficient because if a different device tries to transmit at the same time, a collision will occur, requiring both devices to retransmit, one at a time—not good! NOTE: Switches create separate collision domains within a single broadcast domain. Routers provide a separate broadcast domain for each interface. Don't let this confuse you.

Transport layer (Host-to-Host Layer)

parallels the functions of the OSI's Transport layer, defining protocols for setting up the level of transmission service for applications. It tackles issues like creating reliable end-to-end communication and ensuring the error-free delivery of data. It handles packet sequencing and maintains data integrity. TCP, UDP

>ping

ping Uses ICMP echo request and replies to test if a node IP stack is initialized and alive on the network.

1000Base-T UTP Wiring

requires four wire pairs and uses more advanced electronics so that each and every pair in the cable can transmit simultaneously. Even so, gigabit wiring is almost identical to my earlier 10/100 example, except that we'll use the other two pairs in the cable. For a straight-through cable it's still 1 to 1, 2 to 2, and so on up to pin 8. And in creating the gigabit crossover cable, you'd still cross 1 to 3 and 2 to 6, but you would add 4 to 7 and 5 to 8—pretty straightforward!

The Distribution Layer

sometimes referred to as the workgroup layer and is the communication point between the access layer and the core The primary functions of the distribution layer are to provide routing, filtering, and WAN access and to determine how packets can access the core, if needed. The distribution layer must determine the fastest way that network service requests are handled—for example, how a file request is forwarded to a server. After the distribution layer determines the best path, it forwards the request to the core layer if necessary. The core layer then quickly transports the request to the correct service. The distribution layer is where we want to implement policies for the network because we are allowed a lot of flexibility in defining network operation here.

OSI Model

the OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers essentially a set of guidelines that developers can use to create and implement applications to run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes. has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other as well as with users. The bottom four layers define how data is transmitted end to end.

Cisco Internetwork Operating System (IOS)

the kernel of Cisco routers as well as all current Catalyst switches. In case you didn't know, a kernel is the elemental, indispensable part of an operating system that allocates resources and manages tasks like low-level hardware interfaces and security. The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking, and telecommunications features. Here's a short list of some important things that the Cisco router IOS software is responsible for: Carrying network protocols and functions Connecting high-speed traffic between devices Adding security to control access and stopping unauthorized network use Providing scalability for ease of network growth and redundancy Supplying network reliability for connecting to network resources You can access the Cisco IOS through the console port of a router or switch, from a modem into the auxiliary (or aux) port on a router, or even through Telnet and Secure Shell (SSH). Access to the IOS command line is called an EXEC session.

Source Port

the port number of the application on the host sending the data

Using the Pipe

this pipe ( | ) allows us to wade through all the configurations or other long outputs and get straight to our goods fast this pipe ( | ) allows us to wade through all the configurations or other long outputs and get straight to our goods fast

Session Multiplexing

used by both TCP and UDP and basically allows a single computer, with a single IP address, to have multiple sessions occurring simultaneously. Say you go to www.lammle.com and are browsing and then you click a link to another page. Doing this opens another session to your host. Now you go to www.lammle.com/forum from another window and that site opens a window as well. Now you have three sessions open using one IP address because the Session layer is sorting the separate requests based on the Transport layer port number. This is the job of the Session layer: to keep application layer data separate!

Straight-Through Cable

used to connect the following devices: Host to switch or hub Router to switch or hub Four wires are used in straight-through cable to connect Ethernet devices. It's relatively simple to create this type, and Figure 2.10 shows the four wires used in a straight-through Ethernet cable. Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6 and you'll be up and networking in no time. However, remember that this would be a 10/100 Mbps Ethernet-only cable and wouldn't work with gigabit, voice, or other LAN or WAN technology.

Network Time Protocol (NTP)

used to synchronize the clocks on our computers to one standard time source (typically, an atomic clock). Network Time Protocol (NTP) works by synchronizing devices to ensure that all computers on a given network agree on the time

Packet Internet Groper (Ping)

uses ICMP echo request and reply messages to check the physical and logical connectivity of machines on an internetwork.

Full-Duplex Ethernet

uses two pairs of wires at the same time instead of a single wire pair like half-duplex. And full-duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device. This means that full-duplex data transfers happen a lot faster when compared to half-duplex transfers. Also, because the transmitted data is sent on a different set of wires than the received data, collisions won't happen The reason you don't need to worry about collisions is because now it's like a freeway with multiple lanes instead of the single-lane road provided by half-duplex. Full-duplex Ethernet is supposed to offer 100 percent efficiency in both directions—for example, you can get 20 Mbps with a 10 Mbps Ethernet running full-duplex, or 200 Mbps for Fast Ethernet. But this rate is known as an aggregate rate, which translates as "you're supposed to get" 100 percent efficiency. No guarantees, in networking as in life! There are no collisions in full-duplex mode. A dedicated switch port is required for each full-duplex node. The host network card and the switch port must be capable of operating in full-duplex mode. The default behavior of 10Base-T and 100Base-T hosts is 10 Mbps half-duplex if the autodetect mechanism fails, so it is always good practice to set the speed and duplex of each port on a switch if you can.