CCNA 2: Chap 1 Routing Concepts
Accidentally using the clock rate command on a DTE interface generates the following informational message:
%Error: This command applies only to DCE interface
Network infrastructure devices require IP addresses to enable remote management. Using the device IP address, the network administrator can remotely connect to the device using Telnet, SSH, HTTP, or HTTPS.
A switch does not have a dedicated interface to which an IP address can be assigned. Instead, the IP address information is configured on a virtual interface called a switched virtual interface (SVI).
Configuring an IPv6 interface is similar to configuring an interface for IPv4. Most IPv6 configuration and verification commands in the Cisco IOS are similar to their IPv4 counterparts. In many cases, the only difference is the use of ipv6 in place of ip in commands.
An IPv6 interface must be ■ Configured with IPv6 address and subnet mask—Use the ipv6 address ipv6address/prefix-length [link-local | eui-64] interface configuration command. ■ Activated—The interface must be activated using the no shutdown command.
Most network-capable devices (such as computers, tablets, and smartphones) require the following components to operate, as shown in Figure 1-3:
CPU ■ Operating system (OS) ■ Memory and storage (RAM, ROM, NVRAM, Flash, hard drive)
A router is essentially a specialized computer. It requires a CPU and memory to t emporarily and permanently store data to execute operating system instructions, such as system initialization, routing functions, and switching functions.
Cisco devices also require an OS; Cisco devices commonly use the Cisco IOS as its system software
Although there are many devices and technologies collaboratively working together to enable this, the primary device is the router. Stated simply, a router connects one network to another network
Communication between networks would not be possible without a router determining the best path to the destination and forwarding traffic to the next router along that path. The router is responsible for the routing of traffic between networks.
In a working network environment, infrastructure devices are commonly accessed remotely using Secure Shell (SSH) or Hypertext Transfer Protocol Secure (HTTPS).
Console access is really only required when initially configuring a device, or if remote access fails.
Cost
Cost indicates the general expense for purchasing of network components, and installation and maintenance of the network.
It is possible for a router to receive a packet that is encapsulated in one type of data link frame and to forward the packet out of an interface that uses a different type of data link frame.
For example, a router may receive a packet on an Ethernet interface, but it must forward the packet out of an interface configured with the Point-toPoint Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium to which it connects. The different data link technologies that a router can connect to include Ethernet, PPP, Frame Relay, DSL, cable, and wireless (802.11, Bluetooth, and so on).
The default gateway is usually the address of the interface on the router connected to the local network. The router maintains routing table entries of all connected networks as well as entries of remote networks, and it determines the best path to reach those destinations.
For example, if PC1 sends a packet to the Web Server located at 176.16.1.99, it would discover that the Web Server is not on the local network. It would therefore send the packet to the MAC address of its default gateway. The packet protocol data unit (PDU) at the top in Figure 1-12 identifies the source and destination IP and MAC addresses.
Depending on the type of interface, additional parameters may be required.
For example, in our lab environment, the serial interface connecting to the serial cable end labeled DCE must be configured with the clock rate command.
A router is also usually configured with its own default gateway. This is known as the
Gateway of Last Resort
The service provider router would typically provide the clock rate to the customer router.
However, in a lab environment, the clock rate command is required on the DCE end when interconnecting two serial interfaces.
Ethernet switches function at the data link layer, Layer 2, and are used to forward Ethernet frames between devices within the same network.
However, when the source IP and destination IP addresses are on different networks, the Ethernet frame must be sent to a router.
,notice that it is the responsibility of the router to find the destination network in its routing table and forward the packet toward its destination.
In this example, router R1 receives the packet encapsulated in an Ethernet frame. After de-encapsulating the packet, R1 uses the destination IP address of the packet to search its routing table for a matching network address. After a destination n etwork address is found in the routing table, R1 encapsulates the packet inside a PPP frame and forwards the packet to R2. R2 performs a similar process.
Optionally, the interface could also be configured with a short description of up to 240 characters using the description command.
It is good practice to configure a description on each interface. On production networks, the benefits of interface descriptions are quickly realized because they are helpful in troubleshooting and identifying a third-party connection and contact information.
Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both LANs and WANs.
LANs are commonly Ethernet networks that contain devices, such as PCs, printers, and s ervers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet service provider (ISP) network.
The cable is connected between the serial port of the host and the console port on the device.
Most computers and notebooks no longer include built-in serial ports; therefore, a USB port can establish a console connection. However, a special USB-to-RS-232 compatible serial port adapter is required when using the USB port.
Host computers connect to a wired network using a network interface and RJ-45 E thernet cable.
Most network interfaces have one or two LED link indicators next to the interface. The significance and meaning of the LED colors vary between manufacturers. However, a green LED typically means a good connection, whereas a blinking green LED indicates network activity.
LAN hosts typically connect to a router using Layer 3 IP addresses. The focus of this topic is to examine how devices connect to a small, routed network.
Network devices and end users typically connect to a network using a wired Ethernet or wireless connection. Refer to Figure 1-11 as a sample reference topology. The LANs in the figure serve as an example of how users and network devices can connect to networks.
ROM
Nonvolatile memory that provides permanent storage for the following: ■ Bootup instructions ■ Basic diagnostic software ■ Limited IOS in case the router cannot load the full-featured IOS
Flash
Nonvolatile memory that provides permanent storage for the following: ■ IOS ■ Other system-related files
NVRAM
Nonvolatile memory that provides permanent storage for the following: ■ Startup configuration file (startup-config)
Routers support three packet-forwarding mechanisms:
Process switching Fast switching Cisco Express forwarding (CEF)
Reliability
Reliability indicates the dependability of the components that make up the network, such as the routers, switches, PCs, and servers. R eliability is often measured as a probability of failure or as the mean time between failures (MTBF).
One distinguishing feature between switches and routers is the type of interfaces supported by each. For example, Layer 2 switches support LANs and, therefore, have multiple FastEthernet or Gigabit Ethernet ports.
Routers support LANs and WANs and can interconnect different types of networks; therefore, they support many types of interfaces. For example, G2 ISRs have one or two integrated Gigabit Ethernet interfaces and High-Speed WAN Interface Card (HWIC) slots to accommodate other types of network interfaces, including serial, DSL, and cable interfaces.
Scalability
Scalability indicates how easily the network can accommodate more users and data transmission requirements. If a network design is optimized to only meet current requirements, it can be very difficult and expensive to meet new needs when the network grows.
Security
Security indicates how protected the network is, including the information that is transmitted over the network. The subject of security is important, and techniques and practices are constantly evolving. Consider security whenever actions are taken that affect the network.
Cisco Express Forwarding (CEF)
Shown in Figure 1-10, CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered, such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains precomputed reverse lookups, next-hop information for routes including the interface, and Layer 2 information. CEF is the fastest forwarding mechanism and the preferred choice on Cisco routers.
Process switching
Shown in Figure 1-8, this is an older packet-forwarding mechanism still available for Cisco routers. When a packet arrives on an interface, it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, and then it determines the exit interface and forwards the packet. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets. This process-switching mechanism is slow and rarely implemented in modern networks.
Speed
Speed is a measure of the data rate in bits per second (b/s) of a given link in the network
If the link light is not on, there may be a problem with either the network cable or the network itself. The switch port where the connection terminates would also have an LED indicator lit. If one or both ends are not lit, try a different network cable.
The actual function of the LEDs varies between computer manufacturers. Similarly, network infrastructure devices commonly use multiple LED indicators to provide a quick status view. For example, a Cisco Catalyst 2960 switch has several status LEDs to help monitor system activity and performance. These LEDs are generally lit green when the switch is functioning normally and lit amber when there is a malfunction.
Although the term "speed" is commonly used when referring to the network bandwidth, it is not technically accurate
The actual speed that the bits are transmitted does not vary over the same medium. The difference in bandwidth is due to the number of bits transmitted per second, not how fast they travel over wire or wireless medium.
When a host sends a packet to a device on a different IP network, the packet is forwarded to the default gateway because a host device cannot communicate directly with devices outside of the local network
The default gateway is the intermediary device that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.
When a packet arrives on a router interface, the router uses its routing table to determine how to reach the destination network
The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.
Statically assigned addresses are commonly used to identify specific network resources, such as network servers and printers.
They can also be used in smaller networks with few hosts. However, most host devices acquire their IPv4 address information by accessing a DHCPv4 server. In large enterprises, dedicated DHCPv4 servers providing services to many LANs are implemented. In a smaller branch or small office setting, DHCPv4 services can be provided by a Cisco Catalyst switch or a Cisco ISR.
The Cisco ISR G2 supports a USB serial console connection. To establish connectivity, a USB Type-A to USB Type-B (mini-B USB) is required, as well as an operating system device driver.
This device driver is available from www.cisco.com. Although these routers have two console ports, only one console port can be active at a time. When a cable is plugged into the USB console port, the RJ-45 port becomes inactive. When the USB cable is removed from the USB port, the RJ-45 port becomes active.
An interface can generate its own IPv6 link-local address without having a global unicast address by using the ipv6 enable interface configuration command.
Unlike IPv4, IPv6 interfaces will typically have more than one IPv6 address. At a m inimum, an IPv6 device must have an IPv6 link-local address but will most likely also have an IPv6 global unicast address. IPv6 also supports the ability for an i nterface to have multiple IPv6 global unicast addresses from the same subnet. From
Router memory is classified as volatile or nonvolatile
Volatile memory loses its content when the power is turned off, whereas nonvolatile memory does not lose its content when the power is turned off.
RAM
Volatile memory that provides temporary storage for various applications and processes including the following: ■ Running IOS ■ Running configuration file ■ IP routing and ARP tables ■ Packet buffer
When a host sends a packet to a device that is on the same IP network, the packet is simply forwarded out of the host interface to the destination device.
When a host sends a packet to a device on a different IP network, the packet is forwarded to the default gateway because a host device cannot communicate directly with devices outside of the local network. The default gateway is the destination that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.
A router connects multiple networks, which means that it has multiple interfaces that each belong to a different IP network
When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. The interface that the router uses to forward the packet may be the final destination, or it may be a network connected to another router that is used to reach the destination network.
The router uses its routing table to determine the best path to use to forward a packet.
When the router receives a packet, it examines the destination address of the packet and uses the routing table to search for the best path to that network. The routing table also includes the interface to be used to forward packets for each known network. When a match is found, the router encapsulates the packet into the data link frame of the outgoing or exit interface, and the packet is forwarded toward its destination.
physical topology
is the arrangement of the cables, network devices, and end systems. It describes how the network devices are actually interconnected with wires and cables
logical topology
is the path over which the data is transferred in a network. It describes how the network devices appear connected to network users.
Even the Home Office requires a router.
the router located at the Home Office is a specialized device that performs multiple services for the home network.
Availability
—Availability is the likelihood that the network is available for use when it is required.
Fast switching
—Shown in Figure 1-9, this is a common packet-forwarding mechanism that uses a fast-switching cache to store next-hop information. When a packet arrives on an interface, it is forwarded to the control plane, where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is also stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is reused without CPU intervention.
To be available, an interface must be both of the following:
■ Configured with an IP address and a subnet mask—Use the ip address ip-address subnet-mask interface configuration command. ■ Activated—By default, LAN and WAN interfaces are not activated ( shutdown). To enable an interface, it must be activated using the no shutdown command. (This is similar to powering on the interface.) The interface must also be connected to another device such as a switch or another router for the physical layer to be active.
Console access requires the following:
■ Console cable—RJ-45-to-DB-9 serial cable or a USB serial cable ■ Terminal emulation software—Tera Term, PuTTY
The Branch site devices connect as follows:
■ Corporate resources (that is, file servers and printers) connect to Layer 2 switches using Ethernet cables. ■ Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables. ■ Laptops and smartphones connect wirelessly to wireless access points (WAP). ■ The WAPs connect to switches using Ethernet cables. ■ Layer 2 switches connect to an Ethernet interface on the edge router using Ethernet cables. An edge router is a device that sits at the edge or boundary of a network and routes between that network and another, such as between a LAN and a WAN. ■ The edge router connects to a WAN service provider (SP). ■ The edge router also connects to an ISP for backup purposes.
The Central site devices connect as follows:
■ Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables. ■ Layer 2 switches connect redundantly to multilayer Layer 3 switches using Ethernet fiber-optic cables (orange connections). ■ Layer 3 multilayer switches connect to an Ethernet interface on the edge router using Ethernet cables. ■ The corporate website server is connected using an Ethernet cable to the edge router interface. ■ The edge router connects to a WAN SP. ■ The edge router also connects to an ISP for backup purposes.
Following are the primary functions of a router:
■ Determine the best path to send packets ■ Forward packets toward their destination
What are the primary functions and features of a router?
■ Determine the best path to send packets ■ Forward packets toward their destination -connects one network to another network -responsible for the delivery of packets across different networks - uses its routing table to determine the best path to use to forward a packet. - responsibility of the routers to deliver those packets in a timely manner
When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should identify the following:
■ Device names ■ Interfaces used in the design ■ IP addresses and subnet masks ■ Default gateway addresses
To enable network access, devices must be configured with IP address information to identify the appropriate
■ IP address—Identifies a unique host on a local network. ■ Subnet mask—Identifies with which network subnet the host can communicate. ■ Default gateway—Identifies the IP address of the router to send a packet to when the destination is not on the same local network subnet.
Home Office devices can connect as follows:
■ Laptops and tablets connect wirelessly to a home router. ■ A network printer connects using an Ethernet cable to the switch port on the home router. ■ The home router connects to the service provider cable modem using an Ethernet cable. ■ The cable modem connects to the ISP network.
Cisco routers and Cisco switches are a lot alike. They support a similar modal operating system, similar command structures, and many of the same commands. In addition, both devices have similar initial configuration steps. For instance, the following configuration tasks should always be performed:
■ Name the device—Distinguishes it from other routers. ■ Secure management access—Secures privileged EXEC, user EXEC, and remote access. ■ Configure a banner—Provides legal notification of unauthorized access.
A common analogy used to describe the three packet-forwarding mechanisms is as follows
■ Process switching solves a problem by doing math long hand, even if it is the identical problem. ■ Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems. ■ CEF solves every possible problem ahead of time in a spreadsheet.
A host can be assigned IP address information in one of two ways:
■ Statically—The host is manually assigned a unique IP address, subnet mask, and default gateway. The DNS server IP address can also be configured. ■ Dynamically—The host receives its IP address information automatically from a DHCP server. The DHCP server offers the host a valid IP address, subnet mask, and default gateway information. The DHCP server may provide other information.
This information is captured by creating two useful network documents:
■ Topology diagram—As shown in Figure 1-13, the topology diagram provides a visual reference that indicates the physical connectivity and logical Layer 3 addressing. Often created using diagramming software, such as Microsoft Visio. Figure 1-13 Topology Diagram ■ An addressing table—A table, such as Table 1-2, is used to capture device names, interfaces, IPv4 addresses, subnet masks, and default gateway addresses.
