Functions of a Router
Characteristics of a Network
Networks have had a significant impact on our lives. They have changed the way we live, work, and play.
Networks allow us to communicate, collaborate, and interact in ways we never did before. We use the network in a variety of ways, including web applications, IP telephony, video conferencing, interactive gaming, electronic commerce, education, and more.
There are many key structures and performance-related characteristics referred to when discussing networks:
These characteristics and attributes provide a means to compare different networking solutions.
Note: While 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.
Why Routing?
How does clicking a link in a web browser return the desired information in mere seconds? 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.
When a packet arrives on a router interface, the router uses its routing table to determine how to reach the destination network. The destination of the IP packet might be a web server in another country or an email server on the local area network. It is the responsibility of routers to deliver those packets efficiently. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.
Routers Are Computers
Most network capable devices (i.e., computers, tablets, and smartphones) require the following components to operate:
A router is essentially a specialized computer. It requires a CPU and memory to temporarily and permanently store data to execute operating system instructions, such as system initialization, routing functions, and switching functions.
Note: Cisco devices use the Cisco Internetwork Operating System (IOS) as the system software.
The next table summarizes the types of router memory, the volatility, and examples of what is stored in each.
| Memory | Volatile / Non-Volatile | Stores |
| RAM | Volatile | ? Running IOS ? Running configuration file ? IP ROuting an ARP tables ? Packet buffer |
| ROM | Non-Volatile | ? Bootup instructions ? Basic diagnostic software ? Limited IOS |
| NVRAM | Non-Volatile | Startup configuration file |
| Flash | Non-Volatile | ? IOS ? Other system files |
Routers store data using:
Unlike a computer, a router does not have video adapters or sound card adapters. Instead, routers have specialized ports and network interface cards to interconnect devices to other networks. Figure 3 identifies some of these ports and interfaces.
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Routers Interconnect Networks
Most users are unaware of the presence of numerous routers on their own network or on the Internet. Users expect to be able to access web pages, send emails, and download music, regardless of whether the server accessed is on their own network or on another network. Networking professionals know that it is the router that is responsible for forwarding packets from network to network, from the original source to the final destination.
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.
Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both local-area networks (LANs) and wide-area networks (WANs). LANs are commonly Ethernet networks that contain devices, such as PCs, printers, and servers. 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.
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Notice that each site requires the use of a router to interconnect to other sites. Even the Home Office requires a router. In this topology, the router located at the Home Office is a specialized device that performs multiple services for the home network.
Routers Choose Best Paths
The primary functions of a router are to:
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.
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 must forward the packet out of an interface configured with the Point-to-Point 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).
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The animation in the figure follows a packet from the source PC to the destination PC. Notice that it is the responsibility of the router to find the destination network in its routing table and forward the packet on 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 network address is found in the routing table, R1 encapsulates the packet inside a PPP frame and forwards the packet to R2. A similar process is performed by R2.
Note: Routers use static routes and dynamic routing protocols to learn about remote networks and build their routing tables.
Packet Forwarding Mechanisms
Routers support three packet-forwarding mechanisms:
Figures 1 to 3 illustrate the differences between the three packet-forwarding mechanisms. Assume that a traffic flow consisting of five packets are all going to the same destination. As shown in Figure 1, with process switching, each packet must be processed by the CPU individually. Contrast this with fast switching, as shown in Figure 2. With fast switching, notice how only the first packet of a flow is process-switched and added to the fast-switching cache. The next four packets are quickly processed based on the information in the fast-switching cache. Finally, in Figure 3, CEF builds the FIB and adjacency tables, after the network has converged. All five packets are quickly processed in the data plane.
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| Figure 1: Process Switching | Figure 2: Fast Switching | Figure 3: Cisco Express Forwarding |
A common analogy used to describe the three packet-forwarding mechanisms is as follows:
Connect Devices
Connect to a Network
Network devices and end users typically connect to a network using a wired Ethernet or wireless connection. Refer to the figure as a sample reference topology. The LANs in the figure serve as an example of how users and network devices could connect to networks.
Home office devices can connect as follows:
The Branch site devices connect as follows:
The Central site devices connect as follows:
In the Branch and Central LANs, hosts are connected either directly or indirectly (via WAPs) to the network infrastructure using a Layer 2 switch.
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Default Gateways
To enable network access, devices must be configured with IP address information to identify the appropriate:
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, then 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.
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 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 and it, therefore, must send the packet to the Media Access Control (MAC) address of its default gateway. The Packet protocol data unit (PDU) in the figure identifies the source and destination IP and MAC addresses.
Note: A router is also usually configured with its own default gateway. This is sometimes known as the Gateway of Last Resort.
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Document Network Addressing
When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should identify:
As the figure shows, this information is captured by creating two useful network documents:
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| Device | Interface | IP Address | Subnet Mask | Default Gateway | ||||||||
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| R2 |
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| PC1 | N/A | 192.168.1.10 | 255.255.255.0 | 192.168.1.1 | ||||||||
| PC2 | N/A | 192.168.3.10 | 255.255.255.0 | 192.168.3.1 |
Enable IP on a Host
A host can be assigned IP address information either:
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 DHCP server. In large enterprises, dedicated DHCP servers providing services to many LANs are implemented. In a smaller branch or small office setting, DHCP services can be provided by a Cisco Catalyst switch or a Cisco ISR.
Device LEDs
Host computers connect to a wired network using a network interface and RJ-45 Ethernet cable. Most network interfaces have one or two LED link indicators next to the interface. Typically, a green LED means a good connection while a blinking green LED indicates network activity.
If the link light is not on, then 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.
Note: 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.
Cisco ISRs use various LED indicators to provide status information. A Cisco 1941 router is shown in the figure. The LEDs on the router help the network administrator conduct some basic troubleshooting. Each device has a unique set of LEDs. Consult the device-specific documentation for an accurate description of the LEDs.
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| # | Port | LED | Color | Description | |||||||||||||||||||||
| 1 | GE0/0 and GE0/1 |
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| 2 | Console | EN |
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| 3 | USB | EN |
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Console Access
In a production 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.
Console access requires:
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. If the host does not have a serial port, the USB port can be used to establish a console connection. A special USB-to-RS-232 compatible serial port adapter is required when using the USB port.
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.
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Enable IP on a Switch
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).
For example, in the figure below, the SVI on the Layer 2 switch S1 is assigned the IP address 192.168.10.2/24 and a default gateway of the router located at 192.168.10.1.
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Basic Settings on a Router
Configure Basic Router Settings
Cisco routers and Cisco switches have many similarities. 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.
When configuring a Cisco switch or router, the following basic tasks should be performed first:
Note: Always save the changes on a router and verify the basic configuration and router operations.
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| Router>enable Router#configure terminalRouter(config)#hostname R1R1(config)# R1(config)#enable secret class R1(config)# R1(config)#line console 0R1(config-line)#password ciscoR1(config-line)#loginR1(config-line)#exitR1(config)# R1(config)#line vty 0 4R1(config-line)#password ciscoR1(config-line)#loginR1(config-line)#exitR1(config)# R1(config)#service password-encryptionR1(config)# R1(config)#banner motd $ Authorized Access Only! $R1(config)# R1(config)#exitR1#copy running-config startup-configDestination filename [startup-config]? Building configuration [OK] R1# |
Configure an IPv4 Router Interface
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.
To be available, an interface must be:
Optionally, the interface could also be configured with a short description. It is good practice to configure a description on each interface. The description text is limited to 240 characters. On production networks, a description can be helpful in troubleshooting by providing information about the type of network to which the interface is connected. If the interface connects to an ISP or service carrier, it is helpful to enter the third party connection and contact information.
Depending on the type of interface, additional parameters may be required. For example, in the lab environment, the serial interface connecting to the serial cable end labeled DCE must be configured with the clock rate command.
Note: Accidentally using the clock rate command on a DTE interface generates a %Error: This command applies only to DCE interface message.
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| R1>enable R1#configure terminal R1(config)#interface gigabitethernet 0/0 R1(config-if)#description Link to LAN 1 R1(config-if)#ip address 192.168.10.1 255.255.255.0R1(config-if)#no shutdownR1(config-if)#exit* Jan 30 22:04:47.551: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Jan 30 22:04:50.899: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Jan 30 22:04:51.899: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface gigabitethernet 0/1 R1(config-if)#description Link to LAN 2 R1(config-if)#ip address 192.168.11.1 255.255.255.0R1(config-if)#no shutdownR1(config-if)#exit* Jan 30 22:06:02.543: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to down * Jan 30 22:06:05.899: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up * Jan 30 22:06:06.899: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up R1(config)# R1(config)#interface serial 0/0/0 R1(config-if)#description Link to R2 R1(config-if)#ip address 209.165.200.225 255.255.255.252 R1(config-if)#clock rate 128000 R1(config-if)#no shutdown R1(config-if)#exit * Jan 30 23:01:17.323: %LINK-3-UPDOWN: Interface Serial0/0/0, changed state to down R1(config)# |
Configure an IPv6 Router Interface
Configuring an IPv6 interface is similar to configuring an interface for IPv4. Most IPv6 configuration and verification commands in the Cisco IOS are very similar to their IPv4 counterparts. In many cases, the only difference uses ipv6 in place of ip in commands.
An IPv6 interface must be:
Note: 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 minimum, 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 interface to have multiple IPv6 global unicast addresses from the same subnet. The following commands can be used to statically create a global unicast or link-local IPv6 address:
In the example topology shown in Figure 1, R1 must be configured to support the following IPv6 network addresses:
When the router is configured using the ipv6 unicast-routing global configuration command, the router begins sending ICMPv6 Router Advertisement messages out the interface. This enables a PC connected to the interface to automatically configure an IPv6 address and to set a default gateway without needing the services of a DHCPv6 server. Alternatively, a PC connected to the IPv6 network can get its IPv6 address statically assigned, as shown in Figure 2. Notice that the default gateway address configured for PC1 is the IPv6 global unicast address of the R1 GigabitEthernet 0/0 interface.
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| R1>enable R1#configure terminal R1(config)#interface gigabitethernet 0/0 R1(config-if)#description Link to LAN 1 R1(config-if)#ipv6 address 2001:db8:acad:1::1/64R1(config-if)#no shutdownR1(config-if)#exit* Feb 3 21:38:37.279: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Feb 3 21:38:40.967: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Feb 3 21:38:41.967: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface gigabitethernet 0/1 R1(config-if)#description Link to LAN 2 R1(config-if)#ipv6 address 2001:db8:acad:2::1/64R1(config-if)#no shutdownR1(config-if)#exit* Feb 3 21:39:21.867: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Feb 3 21:39:24.967: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Feb 3 21:39:25.967: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface serial 0/0/0 R1(config-if)#description Link to R2 R1(config-if)#ipv6 address 2001:db8:acad:3::1/64 R1(config-if)#clock rate 128000 R1(config-if)#no shutdown R1(config-if)#exit * Feb 3 21:39:43.307: %LINK-3-UPDOWN: Interface Serial0/0/0, changed state to down R1(config)# |
Configure an IPv4 Loopback Interface
Another common configuration of Cisco IOS routers is enabling a loopback interface.
The loopback interface is a logical interface internal to the router. It is not assigned to a physical port and can therefore never be connected to any other device. It is considered a software interface that is automatically placed in an UP state, as long as the router is functioning.
The loopback interface is useful in testing and managing a Cisco IOS device because it ensures that at least one interface will always be available. For example, it can be used for testing purposes, such as testing internal routing processes, by emulating networks behind the router.
Additionally, the IPv4 address assigned to the loopback interface can be significant to processes on the router that use an interface IPv4 address for identification purposes, such as the Open Shortest Path First (OSPF) routing process. By enabling a loopback interface, the router will use the always available loopback interface address for identification, rather than an IP address assigned to a physical port that may go down.
Enabling and assigning a loopback address is simple:
| Router(config)#interface loopback number Router(config-if)#ip address ip-address subnet-mask Router(config-if)#exit |
Multiple loopback interfaces can be enabled on a router. The IPv4 address for each loopback interface must be unique and unused by any other interface.
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| R1(config)#interface loopback 0 R1(config-if)#ip address 10.0.0.1 255.255.0 R1(config-if)#exit R1(config)# * Jan 30 22:04:50.899: %LINK-3-UPDOWN: Interface loopback0, changed state to up * Jan 30 22:04:51.899: %LINEPROTO-5-UPDOWN: Line protocol on Interface loopback0, changed state to up |
Verify Connectivity of Directly Connected Networks
Verify Interface Settings
There are several show commands that can be used to verify the operation and configuration of an interface. The following three commands are especially useful to quickly identify an interface status:
The show ip interface brief in the example below reveals that the LAN interfaces and the WAN link are all activated and operational as indicated by the Status of “up” and Protocol of “up”. A different output would indicate a problem with either the configuration or the cabling.
Note: The Embedded-Service-Engine0/0 interface is displayed because Cisco ISRs G2 have dual core CPUs on the motherboard. The Embedded-Service-Engine0/0 interface is outside the scope of this course.
Look at the output of the show ip route command. Notice the three directly connected network entries and the three local host route interface entries. A local host route has an administrative distance of 0. It also has a /32 mask for IPv4, and a /128 mask for IPv6. The local host route is for routes on the router owning the IP address. It is used to allow the router to process packets destined to that IP.
The output of the show running-config interface command displays the current commands configured on the specified interface.
The following two commands are used to gather more detailed interface information:
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| R1#show ip interface brief Interface IP-Address OK? Method Status Protocol Embedded-Service-Engine0/0 unassigned YES unset administratively down down GigabitEthernet0/0 192.168.10.1 YES manual up up GigabitEthernet0/1 192.168.11.1 YES manual up up Serial0/0/0 209.165.200.225 YES manual up up Serial0/0/1 unassigned YES unset administratively down down R1# R1#show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP <output omitted> Gateway of last resort is not set 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.11.0/24 is directly connected, GigabitEthernet0/1 L 192.168.11.1/32 is directly connected, GigabitEthernet0/1 209.165.200.0/24 is variably subneted, 2 subnets, 2 masks C 209.165.200.224/24 is directly connected, Serial0/0/0 L 209.165.200.225/32 is directly connected, Serial0/0/0 R1# R1#show running-config interface gigabitEthernet 0/0Building configuration... Current configuration : 128 bytes !interface GigabitEthernet0/0 description Link to LAN 1 ip address 192.168.10.1 255.255.255.0 duplex auto speed auto end R1# |
Verify IPv6 Interface Settings
The commands to verify the IPv6 interface configuration are similar to the commands used for IPv4.
The show ipv6 interface brief command displays a summary for each of the interfaces. The [up/up] output on the same line as the interface name indicates the Layer 1/Layer 2 interface state. This is the same as the Status and Protocol columns in the equivalent IPv4 command.
The output displays two configured IPv6 addresses per interface. One address is the IPv6 global unicast address that was manually entered. The other address, which begins with FE80, is the link-local unicast address for the interface. A link-local address is automatically added to an interface whenever a global unicast address is assigned. An IPv6 network interface is required to have a link-local address, but not necessarily a global unicast address.
The show ipv6 interface gigabitethernet 0/0 command output shown displays the interface status and all of the IPv6 addresses belonging to the interface. Along with the link local address and global unicast address, the output includes the multicast addresses assigned to the interface, beginning with prefix FF02.
The show ipv6 route command can be used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table. The show ipv6 route command will only display IPv6 networks, not IPv4 networks.
Within the routing table, a ‘C’ next to a route indicates that this is a directly connected network. When the router interface is configured with a global unicast address and is in the “up/up” state, the IPv6 prefix and prefix length is added to the IPv6 routing table as a connected route.
The IPv6 global unicast address configured on the interface is also installed in the routing table as a local route. The local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with the interface address of the router as the destination.
The ping command for IPv6 is identical to the command used with IPv4 except that an IPv6 address is used. As shown in Figure 4, the ping command is used to verify Layer 3 connectivity between R1 and PC1.
Other useful IPv6 verification commands include:
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| R1#show ipv6 interface brief GigabitEthernet0/0 [up/up] FE80::FE99:47FF:FE75:C3E0 2001:DB8:ACAD:1::1 GigabitEthernet0/1 [up/up] FE80::FE99:47FF:FE75:C3E1 2001:DB8:ACAD:2::1 Serial0/0/0 [up/up] FE80::FE99:47FF:FE75:C3E0 2001:DB8:ACAD:3::1 Serial0/0/0 [administratively down/down] unassigned R1# |
Filter Show Command Output
Commands that generate multiple screens of output are, by default, paused after 24 lines. At the end of the paused output, the --More-- text displays. Pressing Enter displays the next line and pressing the spacebar displays the next set of lines. Use the terminal length number command to specify the number of lines to be displayed. A value of 0 (zero) prevents the router from pausing between screens of output.
Another very useful feature that improves the user experience in the command-line interface (CLI) is the filtering of show output. Filtering commands can be used to display specific sections of output. To enable the filtering command, enter a pipe (|) character after the show command and then enter a filtering parameter and a filtering expression.
The filtering parameters that can be configured after the pipe include:
Note: Output filters can be used in combination with any show command.
| R1#show running-config | section line vty line vty 0 4 password 7 030752180500 login transport input all R1# R1#show ip interface brief Interface IP-Address OK? Method Status Protocol Embedded-Service-Engine0/0 unassigned YES unset administratively down down GigabitEthernet0/0 192.168.10.1 YES manual up up GigabitEthernet0/1 192.168.11.1 YES manual up up Serial0/0/0 209.165.200.225 YES manual up up Serial0/0/1 unassigned YES unset administratively down down R1# R1#show ip interface brief | include upInterface IP-Address OK? Method Status Protocol GigabitEthernet0/0 192.168.10.1 YES manual up up GigabitEthernet0/1 192.168.11.1 YES manual up up Serial0/0/0 209.165.200.225 YES manual up up R1# R1#show ip interface brief | exclude unassigned Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 192.168.10.1 YES manual up up GigabitEthernet0/1 192.168.11.1 YES manual up up Serial0/0/0 209.165.200.225 YES manual up up R1# R1#show ip route | begin Gateway Gateway of last resort is not set 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.11.0/24 is directly connected, GigabitEthernet0/1 L 192.168.11.1/32 is directly connected, GigabitEthernet0/1 209.165.200.0/24 is variably subneted, 2 subnets, 2 masks C 209.165.200.224/24 is directly connected, Serial0/0/0 L 209.165.200.225/32 is directly connected, Serial0/0/0 R1# |
Command History Feature
The command history feature is useful, because it temporarily stores the list of executed commands to be recalled.
To recall commands in the history buffer, press Ctrl+P or the Up Arrow key. The command output begins with the most recent command. Repeat the key sequence to recall successively older commands. To return to more recent commands in the history buffer, press Ctrl+N or the Down Arrow key. Repeat the key sequence to recall successively more recent commands.
By default, command history is enabled and the system captures the last 10 command lines in its history buffer. Use the show history privileged EXEC command to display the contents of the buffer.
It is also practical to increase the number of command lines that the history buffer records during the current terminal session only. Use the terminal history size user EXEC command to increase or decrease the size of the buffer.
| R1#terminal history size 200R1# R1#show history show ip interface brief show interface g0/0 show ip route show ip route 209.165.200.224 show running-config interface s0/0/0 terminal history size 200 show history R1# |
Routing Decisions
Switching Packets Between Networks
Router Switching Function
A primary function of a router is to forward packets toward their destination. This is accomplished by using a switching function, which is the process used by a router to accept a packet on one interface and forward it out of another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.
Note: In this context, the term “switching” literally means moving packets from source to destination and should not be confused with the function of a Layer 2 switch.
After the router has determined the exit interface using the path determination function, the router must encapsulate the packet into the data link frame of the outgoing interface.
What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:
As shown in the figure, devices have Layer 3 IPv4 addresses and Ethernet interfaces have Layer 2 data link addresses. For example, PC1 is configured with IPv4 address 192.168.1.10 and an example MAC address of 0A-10. As a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. This is because the Layer 3 PDU does not change. However, the Layer 2 data link addresses change at every hop as the packet is de-encapsulated and re-encapsulated in a new Layer 2 frame by each router.
It is common for packets to require encapsulation into a different type of Layer 2 frame than the one which was received. For example, a router might receive an Ethernet encapsulated frame on a FastEthernet interface, and then process that frame to be forwarded out of a serial interface.
Notice in the figure that the ports between R2 and R3 do not have associated MAC addresses. This is because this is a serial link. MAC addresses are only required on multi-access networks, such as Ethernet. A serial link is a point-to-point connection and uses a different Layer 2 frame that does not require the use of a MAC address. In this example, when Ethernet frames are received on R2 from the Fa0/0 interface, destined for PC2, it is de-encapsulated and then re-encapsulated for the serial interface, such as a Point-to-Point Protocol (PPP) encapsulated frame. When R3 receives the PPP frame, it is de-encapsulated again and then re-encapsulated into an Ethernet frame with a destination MAC address of 0B-20, prior to being forwarded out the Fa0/0 interface.
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Send a Packet
In the animation in the figure, PC1 is sending a packet to PC2. PC1 must determine if the destination IPv4 address is on the same network. PC1 determines its own subnet by doing an AND operation on its own IPv4 address and subnet mask. This produces the network address that PC1 belongs to. Next, PC1 does this same AND operation using the packet destination IPv4 address and the PC1 subnet mask.
If the destination network address is the same network as PC1, then PC1 does not use the default gateway. Instead, PC1 refers to its ARP cache for the MAC address of the device with that destination IPv4 address. If the MAC address is not in the cache, then PC1 generates an ARP request to acquire the address to complete the packet and send it to the destination. If the destination network address is on a different network, then PC1 forwards the packet to its default gateway.
To determine the MAC address of the default gateway, PC1 checks its ARP table for the IPv4 address of the default gateway and its associated MAC address.
If an ARP entry does not exist in the ARP table for the default gateway, PC1 sends an ARP request. Router R1 sends back an ARP reply. PC1 can then forward the packet to the MAC address of the default gateway, the Fa0/0 interface of router R1.
A similar process is used for IPv6 packets. Instead of the ARP process, IPv6 address resolution uses ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages. IPv6-to-MAC address mapping are kept in a table similar to the ARP cache, called the neighbor cache.
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Forward to the Next Hop
The following processes take place when R1 receives the Ethernet frame from PC1:
- R1 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R1, therefore, copies the frame into its buffer.
- R1 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
- R1 de-encapsulates the Ethernet frame.
- Because the destination IPv4 address of the packet does not match any of the directly connected networks of R1, R1 consults its routing table to route this packet. R1 searches the routing table for a network address that would include the destination IPv4 address of the packet as a host address within that network. In this example, the routing table has a route for the 192.168.4.0/24 network. The destination IPv4 address of the packet is 192.168.4.10, which is a host IPv4 address on that network.
The route that R1 finds to the 192.168.4.0/24 network has a next-hop IPv4 address of 192.168.2.2 and an exit interface of FastEthernet 0/1. This means that the IPv4 packet is encapsulated in a new Ethernet frame with the destination MAC address of the IPv4 address of the next-hop router.
Because the exit interface is on an Ethernet network, R1 must resolve the next-hop IPv4 address with a destination MAC address using ARP:
- R1 looks up the next-hop IPv4 address of 192.168.2.2 in its ARP cache. If the entry is not in the ARP cache, R1 would send an ARP request out of its FastEthernet 0/1 interface and R2 would send back an ARP reply. R1 would then update its ARP cache with an entry for 192.168.2.2 and the associated MAC address.
- The IPv4 packet is now encapsulated into a new Ethernet frame and forwarded out the FastEthernet 0/1 interface of R1.
The animation in the figure illustrates how R1 forwards the packet to R2.
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Packet Routing
The following processes take place when R2 receives the frame on its Fa0/0 interface:
- R2 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R2, therefore, copies the frame into its buffer.
- R2 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
- R2 de-encapsulates the Ethernet frame.
- Because the destination IPv4 address of the packet does not match any of the interface addresses of R2, R2 consults its routing table to route this packet. R2 searches the routing table for the destination IPv4 address of the packet using the same process R1 used.
The routing table of R2 has a route to the 192.168.4.0/24 network, with a next-hop IPv4 address of 192.168.3.2 and an exit interface of Serial 0/0/0. Because the exit interface is not an Ethernet network, R2 does not have to resolve the next-hop IPv4 address with a destination MAC address.
- The IPv4 packet is now encapsulated into a new data link frame and sent out the Serial 0/0/0 exit interface.
When the interface is a point-to-point (P2P) serial connection, the router encapsulates the IPv4 packet into the proper data link frame format used by the exit interface (HDLC, PPP, etc.). Because there are no MAC addresses on serial interfaces, R2 sets the data link destination address to an equivalent of a broadcast.
The animation in the figure illustrates how R2 forwards the packet to R3.
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Reach the Destination
The following processes take place when the frame arrives at R3:
- R3 copies the data link PPP frame into its buffer.
- R3 de-encapsulates the data link PPP frame.
- R3 searches the routing table for the destination IPv4 address of the packet. The routing table has a route to a directly connected network on R3. This means that the packet can be sent directly to the destination device and does not need to be sent to another router.
Because the exit interface is a directly connected Ethernet network, R3 must resolve the destination IPv4 address of the packet with a destination MAC address:
- R3 searches for the destination IPv4 address of the packet in its Address Resolution Protocol (ARP) cache. If the entry is not in the ARP cache, R3 sends an ARP request out of its FastEthernet 0/0 interface. PC2 sends back an ARP reply with its MAC address. R3 then updates its ARP cache with an entry for 192.168.4.10 and the MAC address that is returned in the ARP reply.
- The IPv4 packet is encapsulated into a new Ethernet data link frame and sent out the FastEthernet 0/0 interface of R3.
- When PC2 receives the frame, it examines the destination MAC address, which matches the MAC address of the receiving interface, its Ethernet network interface card (NIC). PC2, therefore, copies the rest of the frame into its buffer.
- PC2 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
- PC2 de-encapsulates the Ethernet frame and passes the IPv4 packet to the IPv4 process of its operating system.
The animation in the figure illustrates how R3 forwards the packet to PC2.
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Path Determination
Routing Decisions
A primary function of a router is to determine the best path to use to send packets. To determine the best path, the router searches its routing table for a network address that matches the destination IP address of the packet.
The routing table search results in one of three path determinations:
The logic flowchart in the figure illustrates the router packet forwarding decision process.
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Best Path
Determining the best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or shortest path to reach that network. Whenever multiple paths to the same network exist, each path uses a different exit interface on the router to reach that network.
The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric.
Dynamic routing protocols typically use their own rules and metrics to build and update routing tables. The routing algorithm generates a value, or a metric, for each path through the network. Metrics can be based on either a single characteristic or several characteristics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric.
The following lists some dynamic protocols and the metrics they use:
The animation in the figure highlights how the path may be different depending on the metric being used.
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Load Balancing
What happens if a routing table has two or more paths with identical metrics to the same destination network?
When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. This is called equal cost load balancing. The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. The router forwards packets using the multiple exit interfaces listed in the routing table.
If configured correctly, load balancing can increase the effectiveness and performance of the network. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes.
Note: Only EIGRP supports unequal cost load balancing.
The animation in the figure provides an example of equal cost load balancing.
The animation in the figure provides an example of equal cost load balancing.
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Administrative Distance
It is possible for a router to be configured with multiple routing protocols and static routes. If this occurs, the routing table may have more than one route source for the same destination network. For example, if both RIP and EIGRP are configured on a router, both routing protocols may learn of the same destination network. However, each routing protocol may decide on a different path to reach the destination based on that routing protocol’s metrics. RIP chooses a path based on hop count, whereas EIGRP chooses a path based on its composite metric. How does the router know which route to use?
Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the "trustworthiness" of the route; the lower the AD, the more trustworthy the route source. For example, a static route has an AD of 1, whereas an EIGRP-discovered route has an AD of 90. Given two separate routes to the same destination, the router chooses the route with the lowest AD. When a router has the choice of a static route and an EIGRP route, the static route takes precedence. Similarly, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1.
The table lists various routing protocols and their associated ADs.
| Route Source | Administrative Distance |
| Connected | 0 |
| Static | 1 |
| EIGRP summary route | 5 |
| External BGP | 20 |
| Internet BGP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| External EIGRP | 170 |
| Internal BGP | 200 |
Router Operation
Analyze the Routing Table
The Routing Table
The routing table of a router stores information about:
Specifically, a routing table is a data file in RAM that is used to store route information about directly connected and remote networks. The routing table contains network or next hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the next hop on the way to the final destination. The next hop association can also be the outgoing or exit interface to the next destination.
The figure identifies the directly connected networks and remote networks of router R1.
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Routing Table Sources
On a Cisco IOS router, the show ip route command can be used to display the IPv4 routing table of a router. A router provides additional route information, including how the route was learned, how long the route has been in the table, and which specific interface to use to get to a predefined destination.
Entries in the routing table can be added as:
The sources of the routing table entries are identified by a code. The code identifies how the route was learned. For instance, common codes include:
L - Identifies the address assigned to a router’s interface. This allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded.
Note: Other codes are beyond the scope of this chapter.
The figure shows the routing table of R1 in a simple network.
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| R1#show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX -EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF external NSSA type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks D 10.1.1.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0 D 10.1.2.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0 192.168.10.0/24 is variably subnetted, 2 subnets, 3 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 192.168.11.0/24 is variably subnetted, 2 subnets, 3 masks C 192.168.11.0/24 is directly connected, GigabitEthernet0/1 L 192.168.11.1/32 is directly connected, GigabitEthernet0/1 209.165.200.0/24 is variably subneted, 2 subnets, 3 masks C 209.165.200.224/24 is directly connected, Serial0/0/0 L 209.165.200.225/32 is directly connected, Serial0/0/0 |
Remote Network Routing Entries
As a network administrator, it is imperative to know how to interpret the content of an IPv4 and IPv6 routing table. The figure displays an IPv4 routing table entry on R1 for the route to remote network 10.1.1.0.
The entry identifies the following information:
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Directly Connected Routes
Directly Connected Interfaces
A newly deployed router, without any configured interfaces, has an empty routing table, as shown in the figure.
Before the interface state is considered up/up and added to the IPv4 routing table, the interface must:
When the interface is up, the network of that interface is added to the routing table as a directly connected network.
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| R1#show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX -EIGRP external, O - OSPF, - IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF external NSSA type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is not set R1# |
Directly Connected Routing table Entries
An active, properly configured, directly connected interface actually creates two routing table entries. The figure displays the IPv4 routing table entries on R1 for the directly connected network 192.168.10.0.
The routing table entry for directly connected interfaces is simpler than the entries for remote networks. The entries contain the following information:
Note: Prior to IOS 15, local route routing table entries (L) were not displayed in the IPv4 routing table. Local route (L) entries have always been a part of the IPv6 routing table.
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Directly Connected Examples
The example in the figure show the steps to configure and activate the interfaces attached to R1. Notice the Layer 1 and 2 informational messages generated as each interface is activated.
As each interface is added, the routing table automatically adds the connected (‘C’) and local (‘L’) entries. The figure provides an example of the routing table with the directly connected interfaces of R1 configured and activated.
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| R1(config)#interface gigabitethernet 0/0 R1(config-if)#description Link to LAN 1 R1(config-if)#ip address 192.168.10.1 255.255.255.0R1(config-if)#no shutdown R1(config-if)#exit * Feb 1 13:37:35.035: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Feb 1 13:37:38:211: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Feb 1 13:37:38:211: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface gigabitethernet 0/0 R1(config-if)#description Link to LAN 2 R1(config-if)#ip address 192.168.10.1 255.255.255.0R1(config-if)#no shutdown R1(config-if)#exit * Feb 1 13:38:01.471: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to down * Feb 1 13:38:04:211: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up * Feb 1 13:38:04:211 %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up R1(config)# R1(config)#interface serial 0/0/0 R1(config-if)#description Link to R2 R1(config-if)#ip address 209.165.200.225 255.255.255.252R1(config-if)#no shutdown R1(config-if)#exit * Feb 1 13:38:22:723: %LINK-3-UPDOWN: Interface Serial0/0/0 changed state to up * Feb 1 13:38:23.723: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0, changed state to up R1(config)# |
Directly Connected IPv6 Example
The example in the figure below shows the configuration steps for the directly connected interfaces of R1 with the indicated IPv6 addresses. Notice the Layer 1 and Layer 2 informational messages generated as each interface is configured and activated.
The show ipv6 route command shown in Figure 2 is used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table. Like IPv4, a ‘C’ next to a route indicates that this is a directly connected network. An ‘L’ indicates the local route. In an IPv6 network, the local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with a destination address of the interface of the router.
Notice that there is also a route installed to the FF00::/8 network. This route is required for multicast routing.
The show ipv6 route command can be combined with a specific network destination to display the details of how that route was learned by the router.
Watch how connectivity to R2 can be verified using the ping command.
Notice what happens when the G0/0 LAN interface of R2 is the target of the ping command. The pings are unsuccessful. This is because R1 does not have an entry in the routing table to reach the 2001:DB8:ACAD:4::/64 network.
R1 requires additional information to reach a remote network. Remote network route entries can be added to the routing table using either:
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| R1(config)#interface gigabitethernet 0/0 R1(config-if)#description Link to LAN 1 R1(config-if)#ipv6 address 2001:db8:acad:1::1/64R1(config-if)#no shutdownR1(config-if)#exit* Feb 3 21:38:37.279: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Feb 3 21:38:40.967: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Feb 3 21:38:41.967: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface gigabitethernet 0/1 R1(config-if)#description Link to LAN 2 R1(config-if)#ipv6 address 2001:db8:acad:2::1/64R1(config-if)#no shutdownR1(config-if)#exit* Feb 3 21:39:21.867: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down * Feb 3 21:39:24.967: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up * Feb 3 21:39:25.967: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up R1(config)# R1(config)#interface serial 0/0/0 R1(config-if)#description Link to R2 R1(config-if)#ipv6 address 2001:db8:acad:3::1/64 R1(config-if)#clock rate 128000 R1(config-if)#no shutdown R1(config-if)#exit * Feb 3 21:39:43.307: %LINK-3-UPDOWN: Interface Serial0/0/0, changed state to down R1(config)# |
Statically Learned Routes
Static Routes
After directly connected interfaces are configured and added to the routing table, then static or dynamic routing can be implemented.
Static routes are manually configured. They define an explicit path between two networking devices. Unlike a dynamic routing protocol, static routes are not automatically updated and must be manually reconfigured if the network topology changes. The benefits of using static routes include improved security and resource efficiency. Static routes use less bandwidth than dynamic routing protocols, and no CPU cycles are used to calculate and communicate routes. The main disadvantage to using static routes is the lack of automatic reconfiguration if the network topology changes.
There are two common types of static routes in the routing table:
A static route can be configured to reach a specific remote network. IPv4 static routes are configured using the ip route network mask {next-hop-ip | exit-intf} global configuration command. A static route is identified in the routing table with the code ‘S’.
A default static route is similar to a default gateway on a host. The default static route specifies the exit point to use when the routing table does not contain a path for the destination network.
A default static route is useful when a router has only one exit point to another router, such as when the router connects to a central router or service provider.
To configure an IPv4 default static route, use the ip route 0.0.0.0 0.0.0.0 {exit-intf | next-hop-ip} global configuration command.
The figure provides a simple scenario of how default and static routes can be applied.
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Static Route Examples
Figure 1 shows the configuration of an IPv4 default static route on R1 to the Serial 0/0/0 interface. Notice that the configuration of the route generated an ‘S*’ entry in the routing table. The ‘S’ signifies that the route source is a static route while the asterisk (*) identifies this route as a possible candidate to be the default route. In fact, it has been chosen as the default route as evidenced by the line that reads, “Gateway of Last Resort is 0.0.0.0 to network 0.0.0.0.”
Figure 2 shows the configuration of two static routes from R2 to reach the two LANs on R1. The route to 192.168.10.0/24 has been configured using the exit interface while the route to 192.168.11.0/24 has been configured using the next hop IPv4 address. Although both are acceptable, there are some differences in how they operate. For instance, notice how different they look in the routing table. Also notice that because these static routes were to specific networks, the output indicates that the Gateway of Last Resort is not set.
Note: Static and default static routes are discussed in detail in the next chapter.
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Static IPv6 Route Examples
Like IPv4, IPv6 supports static and default static routes. They are used and configured like IPv4 static routes.
To configure a default static IPv6 route, use the ipv6 route ::/0 {ipv6-address | interface-type interface-number} global configuration command.
The figure shows the configuration of a default static route on R1 to the Serial 0/0/0 interface.
Notice in the output shown in Figure 2 that the default static route configuration generated an ‘S’ entry in the routing table. The ‘S’ signifies that the route source is a static route. Unlike the IPv4 static route, there is no asterisk (*) or Gateway of Last Resort explicitly identified.
Like IPv4, static routes are routes explicitly configured to reach a specific remote network. Static IPv6 routes are configured using the ipv6 route ipv6-prefix/prefix-length{ipv6-address|interface-type interface-number} global configuration command.
The example in Figure 2 shows the configuration of two static routes from R2 to reach the two LANs on R1. The route to the 2001:0DB8:ACAD:2::/64 LAN is configured with an exit interface, while the route to the 2001:0DB8:ACAD:1::/64 LAN is configured with the next hop IPv6 address. The next hop IPv6 address can be either an IPv6 global unicast or link-local address.
The right figure shows the routing table with the new static routes installed.
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The next example confirms remote network connectivity to the 2001:0DB8:ACAD:4::/64 LAN on R2 from R1.
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| R1#ping 2000:0db8:acad:4::1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2001:DB8:ACAD:4::1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/16 ms R1# |
Dynamic Routing
Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including network discovery and maintaining routing tables.
Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of depending on manually configured static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks, and the best path to each, are added to the routing table of the router, and identified as a network learned by a specific dynamic routing protocol.
During network discovery, routers exchange routes and update their routing tables. Routers have converged after they have finished exchanging and updating their routing tables. Routers then maintain the networks in their routing tables.
The figure provides a simple scenario of how two neighboring routers would initially exchange routing information. In this simplified message, exchange R1 introduces itself and the networks it can reach. R2 responds and provides R1 with its networks.
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IPv4 Routing Protocols
A router running a dynamic routing protocol does not only make a best path determination to a network, it also determines a new best path if the initial path becomes unusable (or if the topology changes). For these reasons, dynamic routing protocols have an advantage over static routes. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.
Cisco ISR routers can support a variety of dynamic IPv4 routing protocols including:
To determine which routing protocols are supported by the IOS, use the router ? command in global configuration mode as shown in the figure.
Note: The focus of this course is on EIGRP and OSPF. RIP will be discussed only for legacy reasons; the other routing protocols supported by the IOS are beyond the scope of the CCNA certification.
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| R1(config)#router ? bgp Border Gateway Protocol (BGP) eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) isis ISO IS-IS iso-igrp IGRP for OSI networks mobile Mobile routes odr On demand stub Routes ospf Open Shortest Path First (OSPF) ospfv3 OSPFv3 rip Routing Information Protocol (RIP) R1(config)# |
IPv4 Dynamic Routing Examples
In this dynamic routing example, assume that R1 and R2 have been configured to support the dynamic routing protocol EIGRP. The routers also advertise directly connected networks. R2 advertises that it is the default gateway to other networks.
The output in the figure displays the routing table of R1 after the routers have exchanged updates and converged. Along with the connected and link local interfaces, there are three ‘D’ entries in the routing table.
The entry beginning with ‘D*EX’ identifies that the source of this entry was EIGRP (‘D’). The route is a candidate to be a default route (‘*’), and the route is an external route (‘*EX’) forwarded by EIGRP.
The other two ‘D’ entries are routes installed in the routing table based on the update from R2 advertising its LANs.
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| R1#show ip route | begin Gateway Gateway of last resort is 209.165.200.226 to network 0.0.0.0 D*EX 0.0.0.0/0 [170/2297856] via 209.165.200.226, 00:07:27, Serial0/0/0/0 10.0.0.0/24 is subnetted, 2 subnets D 10.1.1.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0 D 10.1.2.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0 192.168.10.0/24 is variably subnetted, 2 subnets, 3 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 192.168.11.0/24 is variably subnetted, 2 subnets, 3 masks C 192.168.11.0/24 is directly connected, GigabitEthernet0/1 L 192.168.11.1/32 is directly connected, GigabitEthernet0/1 209.165.200.0/24 is variably subneted, 2 subnets, 3 masks C 209.165.200.224/24 is directly connected, Serial0/0/0 L 209.165.200.225/32 is directly connected, Serial0/0/0 |
IPv6 Routing Protocols
As shown in the figure, ISR routers can support dynamic IPv6 routing protocols including:
Support for dynamic IPv6 routing protocols is dependent on hardware and IOS version. Most of the modifications in the routing protocols are to support the longer IPv6 addresses and different header structures.
To enable IPv6 routers to forward traffic, you must configure the ipv6 unicast-routing global configuration command.
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| R1(config)#ipv6 router ? eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) ospf Open Shortest Path First (OSPF) rip Routing Information Protocol (RIP) R1(config)# |
IPv6 Dynamic Routing Examples
Routers R1 and R2 have been configured with the dynamic routing protocol EIGRP for IPv6. (This is the IPv6 equivalent of EIGRP for IPv4.)
To view the routing table on R1, enter the show ipv6 route command, as shown in the figure. The output in the figure displays the routing table of R1 after the routers have exchanged updates and converged. Along with the connected and local routes, there are two ‘D’ entries (EIGRP routes) in the routing table.
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| R1#show ipv6 route IPv6 Routing Table - default - 9 entries Codes: C - Connected, L - Local, S - Static, U - Per-user Static route B - BGP, R - RIP, H - NHRP, I1 - ISIS L1 I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP EX - EIGRP external, ND - ND Default, NDp - ND Prefix, DCE - Destination NDr - Redirect, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1 OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 C 2001:0DB8:ACAD:1::/64 [0/0] via GigabitEthernet0/0, directly connected L 2001:0DB8:ACAD:1::1/128 [0/0] via GigabitEthernet0/0, receive C 2001:0DB8:ACAD:2::/64 [0/0] via GigabitEthernet0/1, directly connected L 2001:0DB8:ACAD:2::1/128 [0/0] via GigabitEthernet0/1, receive C 2001:0DB8:ACAD:3::/64 [0/0] via Serial0/0/0, directly connected L 2001:0DB8:ACAD:3::1/128 [0/0] via Serial0/0/0, receive D 2001:DB8:ACAD:4::/64 [90/2172416] via FE80::D68C:B5FF:FECE:A120, Serial0/0/0 D 2001:DB8:ACAD:5::/64 [90/2172416] via FE80::D68C:B5FF:FECE:A120, Serial0/0/0 L FF00::/8 [0/0] via Null0, receive |
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