CCNA 1 Chapter 6 Network Layer

Characteristics of IP

Connectionless Best Effort Media Independent

Source IPv4 Address

Contains a 32-bit binary value that represents the source IPv4 address of the packet. The source IPv4 address is always a unicast address.

What does D identify?

D: Identifies that the route was learned understanding routine route entriesally from another router using the EIGRP routing protocol.

protocol

Field is used to identify the next level protocol. This 8-bit binary value indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).

outgoing interface

Identifies the exit interface to use to forward a packet toward the final destination.

Host Routing Tables

On a Windows host, the route print or netstat -r command can be used to display the host routing table. Both commands generate the same output. The output may seem overwhelming at first, but is fairly simple to understand. Entering the netstat -r command or the equivalent route print command, displays three sections related to the current TCP/IP network connections: Interface List - Lists the Media Access Control (MAC) address and assigned interface number of every network-capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters. IPv4 Route Table - Lists all known IPv4 routes, including direct connections, local network, and local default routes. IPv6 Route Table - Lists all known IPv6 routes, including direct connections, local network, and local default routes.

Encapsulating IPv6

One of the major design improvements of IPv6 over IPv4 is the simplified IPv6 header. consists of 20 octets (up to 60 bytes if the Options field is used) and 12 basic header fields, not including the Options field and Padding field. As highlighted in the figure, for IPv6, some fields have remained the same, some fields have changed names and positions, and some IPv4 fields are no longer required. consists of 40 octets (largely due to the length of the source and destination IPv6 addresses) and 8 header fields (3 IPv4 basic header fields and 5 additional header fields). As highlighted in this figure, some fields have kept the same names as IPv4, some fields have changed names or positions, and a new field has been added. The IPv6 simplified header offers several advantages over IPv4 as listed

- The Router Boot Process

POST (Step 1)Power On Self Test(stored in room and checks it's interface like land cards etc Bootstrap (Step 2) stored in rom for manufacturing, testing and trouble shooting Find IOS (Internetwork Operating System) (step 3) finds .bin file in flash memory Load IOS (Internetwork Operating System) (step 4) load IOS (loads .bin file into running configuration Find configuration (step 5) finds startup configuration Load configuration (step 6) loads startup configuration into running configuration Run IOS commands (step 7) run IOS commands (Runs IOS on CLI (Cisco line interface)

What does S identify?

S: Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route.

Default Gateway

The default gateway is the network device that can route traffic to other networks. It is the router that can route traffic out of the local network. If you use the analogy that a network is like a room, then the default gateway is like a doorway. If you want to get to another room or network you need to find the doorway. Alternatively, a PC or computer that does not know the IP address of the default gateway is like a person, in a room, that does not know where the doorway is. They can talk to other people in the room or network, but if they do not know the default gateway address, or there is no default gateway, then there is no way out.

next hop address

When a packet destined for a remote network arrives at the router, the router matches the destination network to a route in the routing table. If a match is found, the router forwards the packet to the next hop address out of the identified interface.

Directly Connected Routing Table Entries

When a router interface is configured with an IPv4 address, a subnet mask, and is activated, the following two routing table entries are automatically created: C - Identifies a directly-connected network. Directly-connected networks are automatically created when an interface is configured with an IP address and activated. L - Identifies that this is a local interface. This is the IPv4 address of the interface on the router.

What does ROM store?

bootup information that provides startup instructions Power on self test Post which tests the hardware components limited IOS to provide a backup version of IOS. it is used to load a full version of IOS when is deleted or corrupted.

destination network

identifies how the destination networ

what does destination network do?

identifies how the destination network and how it is learnt

what does outgoing source do?

identifies the exit interface and how to use it to forward the packet to final destination

Next Hop

identifies the ip address of the next router to forward a packet.

what does route source do?

identifies the network was learnt by the router

IPv4 Packet Header

n IPv4 packet header consists of fields containing important information about the packet. These fields contain binary numbers which are examined by the Layer 3 process. The binary values of each field identify various settings of the IP packet. Protocol header diagrams, which are read left to right, and top down, provide a visual to refer to when discussing protocol fields. The IP protocol header diagram in the figure identifies the fields of an IPv4 packet. Significant fields in the IPv4 header include: · Version · Differentiated Services or DiffServ (DS) · Time-to-Live (TTL) · Protocol Source IPv4 Address · Destination IPv4 Address The Internet Header Length (IHL), Total Length, and Header Checksum fields Other fields

IP - Best Effort Delivery

. The IP protocol does not guarantee that all packets that are delivered are, in fact, received. Unreliable means that IP does not have the capability to manage and recover from undelivered or corrupt packets. This is because while IP packets are sent with information about the location of delivery, they contain no information that can be processed to inform the sender whether delivery was successful. Packets may arrive at the destination corrupted, out of sequence, or not at all. IP provides no capability for packet retransmissions if errors occur. If out-of-order packets are delivered, or packets are missing, then applications using the data, or upper layer services, must resolve these issues. This allows IP to function very efficiently. In the TCP/IP protocol suite, reliability is the role of the transport layer.

Default Gateway purpose

A default gateway serves as an access point or IP router that a networked computer uses to send information to a computer in another network or the internet. Default simply means that this gateway is used by default, unless an application specifies another gateway.

Using the Default Gateway

A host's routing table will typically include a default gateway. The host receives the IPv4 address of the default gateway either dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually. In the figure, PC1 and PC2 are configured with the default gateway's IPv4 address of 192.168.10.1. Having a default gateway configured creates a default route in the routing table of the PC. A default route is the route or pathway your computer will take when it tries to contact a remote network. The default route is derived from the default gateway configuration and is placed in the host computer's routing table. Both PC1 and PC2 will have a default route to send all traffic destined to remote networks to R1.

Router Memory

A router has access to volatile or non-volatile memory storage. Volatile memory requires continual power to maintain its information. When the router is powered down or restarted, the content is erased and lost. Non-volatile memory retains its information even when a device is rebooted. Specifically, a Cisco router uses four types of memory: RAM - This is volatile memory used in Cisco routers to store applications, processes, and data needed to be executed by the CPU. Cisco routers use a fast type of RAM called synchronous dynamic random access memory (SDRAM). Click RAM in the figure to view more information. ROM - This non-volatile memory is used to store crucial operational instructions and a limited IOS. Specifically, ROM is firmware embedded on an integrated circuit inside the router which can only be altered by Cisco. Click ROM in the figure to view more information. NVRAM - This is non-volatile memory is used as the permanent storage for the startup configuration file (startup-config). Flash - This non-volatile computer memory used as permanent storage for the IOS and other system related files such as log files, voice configuration files, HTML files, backup configurations, and more. When a router is rebooted, the IOS is copied from flash into RAM.

Remote Network Routing Table Entries

A router typically has multiple interfaces configured. The routing table stores information about both directly-connected networks and remote networks.

Host Forwarding Decision

Another role of the network layer is to direct packets between hosts. A host can send a packet to: Itself - A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1, which is referred to as the loopback interface. Pinging the loopback interface tests the TCP/IP protocol stack on the host. Local host - This is a host on the same local network as the sending host. The hosts share the same network address. Remote host - This is a host on a remote network. The hosts do not share the same network address. Whether a packet is destined for a local host or a remote host is determined by the IPv4 address and subnet mask combination of the source (or sending) device compared to the IPv4 address and subnet mask of the destination device. In a home or business network, you may have several wired and wireless devices interconnected together using an intermediate device, such as a LAN switch and/or a wireless access point (WAP). This intermediate device provides interconnections between local hosts on the local network. Local hosts can reach each other and share information without the need for any additional devices. If a host is sending a packet to a device that is configured with the same IP network as the host device, the packet is simply forwarded out of the host interface, through the intermediate device, and to the destination device directly. Of course, in most situations we want our devices to be able to connect beyond the local network segment, such as out to other homes, businesses, and the Internet. Devices that are beyond the local network segment are known as remote hosts. When a source device sends a packet to a remote destination device, then the help of routers and routing is needed. Routing is the process of identifying the best path to a destination. The router connected to the local network segment is referred to as the default gateway.

Bootset Files

Both Cisco routers and switches load the IOS image and startup configuration file into RAM when they are booted, as shown in the figure. The running configuration is modified when the network administrator performs device configurations. Changes made to the running-config file should be saved to the startup configuration file in NVRAM, in case the router is restarted or loses power.

What does C identify?

C - Identifies a directly-connected network. Directly-connected networks are automatically created when an interface is configured with an IP address and activated.

IP characteristics

Connectionless - No connection with the destination is established before sending data packets. Will still send if the destination is not available to receive. Best Effort (unreliable) - Packet delivery is not guaranteed. Media Independent - Operation is independent of the medium carrying the data. fiber opticscabling, satellite and wireless can be used to route the same packet.

Destination IPv4 Address

Contains a 32-bit binary value that represents the destination IPv4 address of the packet. The destination IPv4 address is a unicast, multicast, or broadcast address. The two most commonly referenced fields are the source and destination IP addresses. These fields identify where the packet is coming from and where it is going. Typically these addresses do not change while travelling from the source to the destination. The Internet Header Length (IHL), Total Length, and Header Checksum fields . are used to identify and validate the packet other fields Are used to reorder a fragmented packet. Specifically, the IPv4 packet uses Identification, Flags, and Fragment Offset fields to keep track of the fragments. A router may have to fragment a packet when forwarding it from one medium to another with a smaller MTU.

version

Contains a 4-bit binary value set to 0100 that identifies this as an IP version 4 packet.

Time to Live (TTL)

Contains an 8-bit binary value that is used to limit the lifetime of a packet. The packet sender sets the initial TTL value, and it is decreased by one each time the packet is processed by a router. If the TTL field decrements to zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to the source IP address.

Default Gateway for a Host

For an end device to communicate over the network, it must be configured with the correct IP address information, including the default gateway address. The default gateway is only used when the host wants to send a packet to a device on another network. The default gateway address is generally the router interface address attached to the local network of the host. The IP address of the host device and the router interface address must be in the same network.

Differentiated Services or DiffServ (DS)

Formerly called the Type of Service (ToS) field, the DS field is an 8-bit field used to determine the priority of each packet. The six most significant bits of the DiffServ field is the Differentiated Services Code Point (DSCP) and the last two bits are the Explicit Congestion Notification (ECN) bits.

RAM applications and processes

IOS image running configuration file The routing table to determine the best path to forward packets. The ARP cache used to map IPv4 addresses to MAC addresses The packet buffer used to temporarily store packets before forwarding to destination

Limitations of IPv4

IP address depletion - IPv4 has a limited number of unique public IPv4 addresses available. Although there are approximately 4 billion IPv4 addresses, the increasing number of new IP-enabled devices, always-on connections, and the potential growth of less-developed regions have increased the need for more addresses. Internet routing table expansion - A routing table is used by routers to make best path determinations. As the number of servers connected to the Internet increases, so too does the number of network routes. These IPv4 routes consume a great deal of memory and processor resources on Internet routers. Lack of end-to-end connectivity - Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks. NAT provides a way for multiple devices to share a single public IPv4 address. However, because the public IPv4 address is shared, the IPv4 address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.

Encapsulating IP

IP encapsulates the transport layer segment or other data by adding an IP header. This header is used to rdeliver the packet to the destination host. The IP header remains the same from the time the packet leaves the source host until it arrives at the destination host. The process of encapsulating data layer by layer enables the services at the different layers to develop and scale without affecting the other layers. This means the transport layer segments can be readily packaged by IPv4 or IPv6 or by any new protocol that might be developed in the future. Routers can implement these different network layer protocols to operate concurrently over a network. The routing performed by these intermediate devices only considers the contents of the network layer packet header. In all cases, the data portion of the packet, that is, the encapsulated transport layer PDU, remains unchanged during the network layer processes.

IP - Connectionless

IP is connectionless, meaning that no dedicated end-to-end connection is created before data is sent. connectionless communication is conceptually similar to sending a letter to someone without notifying the recipient in advance. Connectionless data communications work on the same principle. IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded. IP also does not require additional fields in the header to maintain an established connection. This process greatly reduces the overhead of IP. However, with no pre-established end-to-end connection, senders are unaware whether destination devices are present and functional when sending packets, nor are they aware if the destination receives the packet, or if they are able to access and read the packet.

IP - Media Independent

IP operates independently of the media that carry the data at lower layers of the protocol stack. , IP packets can be communicated as electronic signals over copper cable, as optical signals over fiber, or wirelessly as radio signals. It is the responsibility of the OSI data link layer to take an IP packet and prepare it for transmission over the communications medium. This means that the transport of IP packets is not limited to any particular medium. There is, however, one major characteristic of the media that the network layer considers: the maximum size of the PDU that each medium can transport. This characteristic is referred to as the maximum transmission unit (MTU). Part of the control communication between the data link layer and the network layer is the establishment of a maximum size for the packet. The data link layer passes the MTU value up to the network layer. The network layer then determines how large packets can be. In some cases, an intermediate device, usually a router, must split up a packet when forwarding it from one medium to another medium with a smaller MTU. This process is called fragmenting the packet or fragmentation.

Route source

Identifies how the network was learned by the router. common route sources are (S) static route, (D) Enhanced Interior Gateway Routing Protocol (EIGRP), (O) Open Shortest Path First (OSPF)

Administrative distance

Identifies the trustworthiness of the route source. Lower values indicate preferred route source.

metric

Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Route timestamp

Identifies when the router was last heard from.

Introducing IPv6

In the early 1990s, the Internet Engineering Task Force (IETF) grew concerned about the issues with IPv4 and began to look for a replacement. This activity led to the development of IP version 6 (IPv6). IPv6 overcomes the limitations of IPv4 and is a powerful enhancement with features that better suit current and foreseeable network demands. Improvements that IPv6 provides include: Increased address space - IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits. Improved packet handling - The IPv6 header has been simplified with fewer fields. Eliminates the need for NAT - With such a large number of public IPv6 addresses, NAT between a private IPv4 address and a public IPv4 is not needed. This avoids some of the NAT-induced application problems experienced by applications requiring end-to-end connectivity. The 32-bit IPv4 address space provides approximately 4,294,967,296 unique addresses. IPv6 address space provides 340,282,366,920,938,463,463,374,607,431,768,211,456, or 340 undecillion addresses, which is roughly equivalent to every grain of sand on Earth.

LAN and WAN Interfaces

In-band router interfaces

What does L identify?

L - Identifies that this is a local interface. This is the IPv4 address of the interface on the router.

Router CPU and OS

Like all computers, tablets, gaming consoles, and smart devices, Cisco devices require a CPU to execute OS instructions, such as system initialization, routing functions, and switching functions. The CPU requires an OS to provide routing and switching functions. The Cisco Internetwork Operating System (IOS) is the system software used for most Cisco devices regardless of the size and type of the device. It is used for routers, LAN switches, small wireless access points, large routers with dozens of interfaces, and many other devices.

What does O identify?

O: Identifies that the route was learned dynamically from another router using the OSPF routing protocol.

Inside a router

Power supply Fan Shield for WAN interface card (WIC) or high speed WIC (HWIC) Synchronous dynamic RAM (SDRAM) Used for holding the running configuration and routing tables, and for supporting packet buffering Non-volatile RAM (NVRAM) and boot flash memory this is used for storing the ROMMON boot code as well as NVRAM data CPU - this is the central processing unit Advantaced Integration Module (AIM) This option offloads pocessor-intensive functions such as encryption from the main CPU

IPv6 (Internet Protocol version 6)

Protocol in which addresses consist of eight sets of four hexadecimal numbers, each number being a value between 0000 and FFFF, using a colon to separate the numbers. Here's an example: FEDC:BA98:7654:3210:0800:200C:00CF:1234.

What does R identify?

R: Identifies that the route was learned dynamically from another router using the RIP routing protocol.

Basic Router Configuration Steps

Set name of router Router> enable Router# config t Router(config)# hostname R1 Secure privileged EXEC mode R1(config)# enable secret class Secure user EXEC mode R1(config)# line console 0 R1(config-line)# password cisco R1(config-line)# login R1(config-line)# line vty 0 4 R1(config-line)# password cisco Secure remote Telnet/ SSH access R1(config-line)# login R1(config-line)# exit password-encryption Secure all passwords in the config file R1(config)# service password-encryption banner text R1(config)# banner motd #Authorized access only!# save to Nivam and exit R1# copy running-config startup-config Save config

Routing Tables

Table kept by the router to help determine which route entry is the best fit for the network A route entry with the longest prefix is the most specific network -Example: 10.1.1.0/24 more specific than 10.0.0.0/8 Example: Routing Table Dstn Ntwk Next Rtr Port Route Cost 125.0.0.0 137.3.14 1 12 161.5.0.0 197.3.6.6 1 4 134.7.0.0 164.17.3.12 2 10 Which Dstn address is most specific? 2 and 3

· Internet Protocol version 4 (IPv4)

The Internet Protocol version 4 is the dominant protocol for routing traffic on the Internet, specifying "to" and "from" addresses using a dotted decimal such as "122.45.255.0".

The Network Layer

The OSI layer that addresses data packets, routes the packets from a source to a destination through the network, and ensures the delivery of those packets. The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across the network. To accomplish this end-to-end transport, the network layer uses four basic processes: · Addressing end devices - End devices must be configured with a unique IP address for identification on the network. · Encapsulation - The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. The encapsulation process adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. · Routing - The network layer provides services to direct packets to a destination host on another network. To travel to other networks, the packet must be processed by a router. The role of the router is to select the best path and direct packets toward the destination host in a process known as routing. A packet may cross many intermediary devices before reaching the destination host. Each router a packet crosses to reach the destination host is called a hop. · De-encapsulation - When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer. Unlike the transport layer (OSI Layer 4), which manages the data transport between the processes running on each host, network layer protocols specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the data carried in each packet allows the network layer to carry packets for multiple types of communications between multiple hosts.

Connect to a Router

The connections on a Cisco router can be grouped into two categories: In-band router interfaces and management ports. Similar to a Cisco switch, there are several ways to access user EXEC mode in the CLI environment on a Cisco router. These are the most common: Console - This is a physical management port that provides out-of-band access to a Cisco device. Out-of-band access refers to access via a dedicated management channel that is used for device maintenance purposes only. Secure Shell (SSH) - SSH is a method for remotely establishing a secure CLI connection through a virtual interface, over a network. Unlike a console connection, SSH connections require active networking services on the device including an active interface configured with an address. Telnet - Telnet is an insecure method of remotely establishing a CLI session through a virtual interface, over a network. Unlike SSH, Telnet does not provide a securely encrypted connection. User authentication, passwords, and commands are sent over the network in plaintext. Note: Some devices, such as routers, may also support a legacy auxiliary port that was used to establish a CLI session remotely using a modem. Similar to a console connection, the AUX port is out-of-band and does not require networking services to be configured or available. Telnet and SSH require an inband network connection which means that an administrator must access the router through one of the WAN or LAN interfaces. Inband interfaces receive and forward IP packets. Every configured and active interface on the router is a member or host on a different IP network. Each interface must be configured with an IPv4 address and subnet mask of a different network. The Cisco IOS does not allow two active interfaces on the same router to belong to the same network.

IPv6 Packet Header

The fields in the IPv6 packet header include: Version - This field contains a 4-bit binary value set to 0110 that identifies this as an IP version 6 packet. Traffic Class - This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. Flow Label - This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers. Payload Length - This 16-bit field indicates the length of the data portion or payload of the IPv6 packet. Next Header - This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol. Hop Limit - This 8-bit field replaces the IPv4 TTL field. This value is decremented by a value of 1 by each router that forwards the packet. When the counter reaches 0, the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host, indicating that the packet did not reach its destination because the hop limit was exceeded. Source IPv6 Address - This 128-bit field identifies the IPv6 address of the sending host. Destination IPv6 Address - This 128-bit field identifies the IPv6 address of the receiving host. An IPv6 packet may also contain extension headers (EH), which provide optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. EHs are used for fragmentation, security, to support mobility and more. Unlike IPv4, routers do not fragment routed IPv6 packets.

A Router is a Computer

There are many types of infrastructure routers available. In fact, Cisco routers are designed to address the needs of many different types of businesses and networks: Branch - Teleworkers, small businesses, and medium-size branch sites. Includes Cisco Integrated Services Routers (ISR) G2 (2nd generation). WAN - Large businesses, organizations, and enterprises. Includes the Cisco Catalyst Series Switches and the Cisco Aggregation Services Routers (ASR). Service Provider - Large service providers. Includes Cisco ASR, Cisco CRS-3 Carrier Routing System, and 7600 Series routers.

Network Layer Protocols

There are several network layer protocols in existence. However, there are only two network layer protocols that are commonly implemented: · Internet Protocol version 4 (IPv4) · Internet Protocol version 6 (IPv6)

Router Bootup Process

There are three major phases to the bootup process. As shown in Figure 1, they are: 1. Perform the POST and load the bootstrap program. 2. Locate and load the Cisco IOS software. 3. Locate and load the startup configuration file or enter setup mode. 1. Performing POST and Load Bootstrap Program During the Power-On Self-Test (POST), the router executes diagnostics from ROM on several hardware components, including the CPU, RAM, and NVRAM. After the POST, the bootstrap program is copied from ROM into RAM. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM. Note: At this point, if you have a console connection to the router, you begin to see the output on the screen. 2. Locating and Loading Cisco IOS The IOS is typically stored in flash memory and is copied into RAM for execution by the CPU. If the IOS image is not located in flash, then the router may look for it using a Trivial File Transfer Protocol (TFTP) server. If a full IOS image cannot be located, a limited IOS is copied into RAM, which can be used to diagnose problems and transfer a full IOS into Flash memory. 3. Locating and Loading the Configuration File The bootstrap program then copies the startup configuration file from NVRAM into RAM. This becomes the running configuration. If the startup configuration file does not exist in NVRAM, the router may be configured to search for a TFTP server. If a TFTP server is not found, then the router displays the setup mode prompt. Note: Setup mode is not used in this course to configure the router. When prompted to enter setup mode, always answer no. If you answer yes and enter setup mode, press Ctrl+C at any time to terminate the setup process.

Default Gateway for a Switch

Typically, a workgroup switch that interconnects client computers is a Layer 2 device. As such, a Layer 2 switch does not require an IP address to function properly. However, if you wish to connect to the switch and administratively manage it over multiple networks, you will need to configure the SVI with an IPv4 address, subnet mask, and default gateway address. The default gateway address is typically configured on all devices that wish to communicate beyond just their local network. In other words, to remotely access the switch from another network using SSH or Telnet, the switch must have an SVI with an IPv4 address, subnet mask, and default gateway address configured. If the switch is accessed from a host within the local network, then the default gateway IPv4 address is not required. To configure a default gateway on a switch, use the ip default-gateway global configuration command. The IP address configured is that of the router interface of the connected switch. Enter global configuration and configure '192.168.10.1' as the default gateway for S1. S1# configure terminal Enter configuration commands, one per line. End with CNTL/Z. S1(config)# ip default-gateway 192.168.10.1 S1(config)# A common misconception is that the switch uses its configured default gateway address to determine where to forward packets originating from hosts connected to the switch and destined for hosts on remote networks. Actually, the IP address and default gateway information is only used for packets that originate from the switch. Packets originating from host computers connected to the switch must already have the default gateway address configured on their host computer operating systems.

IPv4 header functions

Version - Always set to 0100 for IPv4 Differentiatied Services - Identifies the priority of each packet Time-to-Live - Commonly referred to as hop count Protocol - Identifies the upper-layer protocol to be used next Source IP Address - The IP address of the sending host Destination IP Address - The IP address of the destination host

Pv6 Header fields

Version - It is always set to 0110 Payload Length - identifies the size of the data portion of the packet Traffic class - classifies packets for congestion control Next Header - Identifies the application type to the upper-layer protocol Flow Label - To suggest that all packets receive the same type of handling by IPv6 routers Hop Limit - When this value reaches 0, the sender is notified that the packet was not delivered source IP Address - This is the IPv6 address of the sendng host Destination IP Address - This is the IPv6 address of the receiving host

Router Packet Forwarding Decision

When a host sends a packet to another host, it will use its routing table to determine where to send the packet. If the destination host is on a remote network, the packet is forwarded to the default gateway. What happens when a packet arrives at the default gateway, which is usually a router? The router looks at its routing table to determine where to forward packets. The routing table of a router can store information about: Directly-connected routes - These routes come from the active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is connected to a different network segment. Remote routes - These routes come from remote networks connected to other routers. Routes to these networks can be manually configured on the local router by the network administrator or dynamically configured by enabling the local router to exchange routing information with other routers using a dynamic routing protocol. Default route - Like a host, routers also use a default route as a last resort if there is no other route to the desired network in the routing table.

Basic Switch Configuration Steps

configure terminal Enter global configuration mode hostname name Configure a name for the device ip address x.x.x.x x.x.x.x Configure interface IP address no shutdown Enable the interface exit Return to global configuration mode interface fastethernet 0/1 Enter the interface to assign VLAN switchport mode access Define the VLAN membership mode for the port switchport access vlan X Assign the port to a VLAN duplex auto Configure the interface duplex mode to enable AUTO duplex configuration speed auto Configure the interface speed and enable AUTO speed configuration mdix auto Enable auto-MDIX on the interface ip default-gateway x.x.x.x Configure the default gateway on the switch ip http authentication enable Configure the HTTP server for authentication using the enable parameter ip http server Enable HTTP server line console 0 Switch from global configuration mode to line configuration mode for console 0 password cisco Set cisco as the password for the console login Set the console line to require the password to be entered before access is granted line vty 0 15 Switch from global configuration mode to line configuration mode for vty terminals 0-15 enable password cisco Configure cisco as the enable password to enter privileged EXEC mode enable secret class Configure class as the enable secret password to enter privileged EXEC mode service password-encryption Encrypt all the system passwords that are stored in clear text banner login #text# Configure a login banner. # delimits the beginning and end of the banner banner motd #text# Configure a message of the day login banner, # delimits the beginning and end of the banner copy run start Save the running configuration to the switch startup configuration.

Configure Router Interfaces

or routers to be reachable, the in-band router interfaces must be configured. There are many different types of interfaces available on Cisco routers. In this example, the Cisco 1941 router is equipped with: Two Gigabit Ethernet interfaces - GigabitEthernet 0/0 (G0/0) and GigabitEthernet 0/1 (G0/1) A serial WAN interface card (WIC) consisting of two interfaces - Serial 0/0/0 (S0/0/0) and Serial 0/0/1 (S0/0/1) R1# configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)# interface gigabitethernet 0/0 R1(config-if)# ip address 192.168.10.1 255.255.255.0 R1(config-if)# description LAN-10 R1(config-if)# no shutdown R1(config)# interface gigabitethernet 0/1 R1(config-if)# ip address 192.168.11.1 255.255.255.0 R1(config-if)# description LAN-11 R1(config-if)# no shutdown

Introducing the IPv4 Routing Table

routing table, there are no column headings identifying the information contained in a router's routing table. It is important to learn the meaning of the different items included in each entry of the routing table.

Verify Interface Configuration

show ip interface brief show ip route - Displays the contents of the IPv4 routing table stored in RAM. show interfaces - Displays statistics for all interfaces on the device. show ip interface - Displays the IPv4 statistics for all interfaces on a router. verify connectivity from the interface using the ping command.

IPv4 Router Routing Table

the show ip route command can be used to display the router's IPv4 routing table, as shown in the figure. In addition to providing routing information for directly-connected networks and remote networks, the routing table also has information on how the route was learned, the trustworthiness and rating of the route, when the route was last updated, and which interface to use to reach the requested destination. When a packet arrives at the router interface, the router examines the packet header to determine the destination network. If the destination network matches a route in the routing table, the router forwards the packet using the information specified in the routing table. If there are two or more possible routes to the same destination, the metric is used to decide which route appears in the routing table.

Show Version Output

the show version command displays information about the version of the Cisco IOS software currently running on the router, the version of the bootstrap program, and information about the hardware configuration, including the amount of system memory.