RIP Features 729 This command outputs shows the last update was received 20 seconds ago. Observing Multiple Paths to Destination Some routing protocols support multiple paths to the same destination. Unlike single path algorithms, these multipath algorithms permit traffic over multiple lines, provide better throughput, and are more reliable. RIP Features Routing Information Protocol (RIP) was originally specified in RFC 1058 in 1988. Its key characteristics include the following: ■ It is a distance vector routing protocol. ■ Hop count is used as the metric for path selection (see Figure 16-11). ■ If the hop count is greater than 15, the packet is discarded. ■ By default, routing updates are broadcast every 30 seconds. Note in Figure 16-11 that the 19.2-kbps path between the two hosts using the top routers is 2 hops. The lower alternate path using the three T-1 links is 4 hops. Because RIP path selection is based solely on the number of hops, in this case, RIP path selection chooses the 19.2-kbps link instead of the much faster T1 links. Figure 16-11 RIP Uses Hop Count as Its Metric 200.200.200.0/24 auto-summary 200.200.200.0/24 [1] via 192.168.10.2, 00:00:20, Serial0/0 Example 16-4 show ip rip database Command Output (Continued) , 00:00:20 T1 T1 T1 19.2 kpbs chpt_16.fm Page 729 Tuesday, May 27, 2003 2:16 PM 730 Chapter 16: Distance Vector Routing Protocols RIP has evolved over the years from a classful routing protocol, RIP Version 1 (RIP-1), to a classless routing Protocol, RIP Version 2 (RIP-2). RIP-2 enhancements include the following: ■ Capability to carry additional packet routing information ■ Authentication mechanism to secure table updates ■ Capability to support subnet masks RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed in a path is 15. When a router receives a routing update that contains a new or changed entry, the metric value is increased by one. If this causes the metric to be incremented beyond 15, it is considered to be infinity, and the network destination is considered unreachable. RIP includes a number of features that are com- mon in other routing protocols. For example, RIP implements split horizon and hold- down mechanisms to prevent incorrect routing information from being propagated. Enabling RIP on an IP Network The router rip command enables RIP as the routing protocol. The network command is then used to tell RIP which networks are directly connected for the router to adver- tise. The routing process then associates these interfaces with the network addresses and begins using RIP on interfaces. RIP sends routing-update messages at regular intervals and triggered updates when the network topology changes. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The metric value for the path is increased by one, and the source interface of the update is indi- cated as the next hop in the route. RIP routers maintain only the best route to a desti- nation; however, RIP routers can maintain multiple routes to the same destination if those routes have the same metric. After updating its routing table due to a configuration change, the router immediately begins transmitting routing updates to inform other network routers of the change. These updates are sent independently of the regularly scheduled updates that RIP routers forward. RIP advertises classful networks or major class networks only. To enable RIP, use the commands in Table 16-3, beginning in global configuration mode. chpt_16.fm Page 730 Tuesday, May 27, 2003 2:16 PM RIP Features 731 The following commands show the process of enabling RIP and specifying directly connected networks. BHM(config)#router rip ! selects RIP as the routing protocol BHM(config-router)#network 1.0.0.0 ! specifies a directly connected network) BHM(config-router)#network 2.0.0.0 ! specifies a directly connected network The Cisco router interfaces that are connected to networks 1.0.0.0 and 2.0.0.0 send and receive RIP updates. These routing updates enable the router to learn the network topology from a neighboring router that is running RIP. Using the ip classless Command Sometimes, a router receives packets destined for an unknown subnet of a network that has directly connected subnets. To forward these packets to the best supernet route possible, use the ip classless global configuration command. The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this fea- ture, use the no form of this command. When this feature is disabled and a packet is being sent to a subnet of a network that has no network default route, the router discards the packet. Figure 16-12 illustrates this principle. If the host sends a packet to 128.20.4.1 and no network default route exists, the router discards the packet. IP classless affects only the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built. This degree of impact is the essence of classful routing. If part of a major network is known and the destination subnet toward which the packet is destined is unknown, the packet is dropped. Table 16-3 Commands to Enable RIP Command Purpose Router(config)#router rip Enables a RIP routing process, which then switches to the router configuration mode Router(config-router)#network network-number Associates a network with a RIP routing process chpt_16.fm Page 731 Tuesday, May 27, 2003 2:16 PM 732 Chapter 16: Distance Vector Routing Protocols Figure 16-12 No IP Classless Routing The most confusing aspect of this rule is that the router uses the default route only if the major network destination does not exist in the routing table. By default, a router assumes that all subnets of a directly connected network are present in the routing table. If a packet is received with an unknown destination address within an unknown subnet of an attached network, the router assumes that the subnet does not exist. Therefore, the router drops the packet, even if a default route exists. Configuring ip classless on the router resolves this problem by instructing the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route, as Figure 16-13 demonstrates. Common RIP Configuration Issues RIP routers must rely on neighboring routers for network information. RIP uses a dis- tance vector routing algorithm. All distance vector routing protocols have problems that result in slow convergence. Some of these issues include routing loops and counting to infinity. These problems both result in inconsistencies due to routing update messages with outdated routes being propagated around the internetwork. To reduce routing loops and counting to infinity, RIP uses the following methods: ■ Split horizon ■ Poison reverse ■ Hold-down timer ■ Triggered updates chpt_16.fm Page 732 Tuesday, May 27, 2003 2:16 PM RIP Features 733 Figure 16-13 IP Classless Some of these methods might require some configuration, while others do not require or rarely require configuration. RIP permits a maximum hop count of 15. Any destination greater than 15 hops away is tagged as unreachable. The maximum hop count for RIP greatly restricts its use in large internetworks, but prevents a problem called count to infinity from causing end- less network routing loops. The split-horizon rule is based on the fact that it is usually not useful to send informa- tion about a route back in the direction from which the route came. In some network configurations, it might be necessary to disable split horizon. It is disabled on a per- interface basis. To disable split horizon, use the following command: Router(config-if)# no ip split-horizon The hold-down timer is another mechanism that might need some changes. Hold-down timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. This holddown prevents any inferior route from being updated, but might also prevent a valid alternative route from being installed. The hold-down timer can be decreased to speed up convergence, but take such action with caution. The ideal solution is to set the timer just longer than the longest possible update time for the internetwork. In Figure 16-14, the loop consists of four routers. With each router having an update time of 30 seconds, the longest loop possible is 120 seconds. Therefore, the hold-down timer should be set to slightly more than 120 seconds. ip classless chpt_16.fm Page 733 Tuesday, May 27, 2003 2:16 PM 734 Chapter 16: Distance Vector Routing Protocols Figure 16-14 Hold-Down Timers To change the hold-down timer, use the following command: GAD(config-router)# holdown-timer seconds One additional item that affects convergence and can be configured is the update inter- val. By default, Cisco IOS Software runs IP RIP updates every 30 seconds. This time can be reconfigured for longer intervals to conserve bandwidth or for a shorter interval to decrease convergence time. To change the update interval, use the following command: GAD(config-router)#update-timer seconds As discussed earlier in this chapter, another issue with routing protocols is the unwanted advertisement of routing updates out a particular interface. When a network command is issued, RIP sends advertisements out all interfaces within that network address range. To control the set of interfaces that exchanges routing updates, the network administrator can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. Because RIP is a broadcast protocol, the network administrator might have to config- ure RIP to exchange routing information in a nonbroadcast network such as Frame Relay. In this type of network, RIP needs to be informed of other neighboring RIP routers. To define a neighboring router with which to exchange routing information, use the following command: GAD(config-router)neighbor ip address By default, the software receives RIP-1 and RIP-2 packets, but sends only RIP-1 packets. The network administrator can configure the router to receive and send only RIP-1 packets or to send only RIP-2 packets. To configure the router to send and receive packets from only one version, use the commands in the router configuration mode as specified in Table 16-4. chpt_16.fm Page 734 Tuesday, May 27, 2003 2:16 PM RIP Features 735 To control how packets received from an interface are processed, use the following commands described in Table 16-5. Verifying RIP Configuration Several commands can be used to verify that RIP is properly configured. Two of the most commonly used commands are show ip route and show ip protocols. The show ip protocols command outputs information about all the IP routing proto- cols configured on the router, as demonstrated in Example 16-5. This output can be Table 16-4 Specifying the RIP Version Command Purpose (config-router)#version {1 | 2 } Configures the software to receive and send only RIP-1 or only RIP-2 packets. (config-if)#ip rip send version 1 Configures an interface to send only RIP-1 packets. (config-if)#ip rip send version 2 Configures an interface to send only RIP-2 packets. (config-if)#ip rip send version 1 2 Configures an interface to send only RIP-1 or RIP-2 packets. Table 16-5 Controlling Packets Command Purpose (config-if)#ip rip receive version 1 Configures an interface to receive only RIP-1 packets. (config-if)#ip rip receive version 2 Configures an interface to receive only RIP-2 packets. (config-if)#ip rip receive version 1 2 Configures an interface to receive only RIP-1 or RIP-2 packets. Lab Activity Configuring RIP In this lab, you set up an IP addressing scheme using Class C networks and configure RIP on all routers. chpt_16.fm Page 735 Tuesday, May 27, 2003 2:16 PM 736 Chapter 16: Distance Vector Routing Protocols used to verify most, if not all, of the RIP configuration. Some of the most common configuration items to verify are the following: ■ That RIP is configured ■ That the correct interfaces are sending and receiving RIP updates ■ That the correct RIP version is being sent and received ■ That the router is advertising the correct networks The show ip route command can be used to verify that the routes received by RIP are being installed in the routing table, as Example 16-6 shows. Examine the output of the command and look for RIP routes, signified by “R.” Remember that the network takes some time to converge so the routes might not appear immediately. Example 16-5 show ip protocols Command Output GAD# show ip protocols Routing Protocol is "rip“ ! Line above verifies that RIP is configured Sending updates every 30 seconds, next due in 5 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip Default version control: send version 1, receive any version ! Line above verifies the RIP version Interface Send Recv Triggered RIP Key-chain ! Line above verifies RIP interfaces FastEthernet0/0 1 1 2 Serial0/0 1 1 2 Routing for Networks: 192.168.1.0 192.168.2.0 ! Lines above verify networks being advertised Routing Information Sources: Gateway Distance Last Update 192.168.2.2 120 00:00:11 Distance: (default is 120) chpt_16.fm Page 736 Tuesday, May 27, 2003 2:16 PM RIP Features 737 The last entry, beginning with the letter R, indicates that network 192.168.3.0 was learned from RIP and it can be reached via adjacent router (next hop) interface 192.168.2.2, which is remotely attached to this router’s Serial 0/0 interface. Additional commands you can use to check RIP configuration are as follows: ■ show interface interface ■ show ip interface interface ■ show running-config These commands are useful when it is necessary to find out information about a par- ticular interface. Using the show interface command displays all information about an interface, including whether it is up or down and what type of protocol, IP address, or encapsulation type might be configured on the interface. Basically, these commands provide an administrator with all the configuration information that is available about a particular interface. The show running-config command, however, is used to show the current configuration on the router and all its interfaces. Recall that a Cisco router runs each protocol separately from other protocols. For this reason including the ip in the show ip interface command is necessary to list specifically just IP information regarding the specific interface. Example 16-6 show ip route Command Output GAD# show ip route Codes: C - connected, S - static, I - IGRP, 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 NSSA external 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 C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0 R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:07, Serial0/0 ! line above verifies RIP routes received chpt_16.fm Page 737 Tuesday, May 27, 2003 2:16 PM 738 Chapter 16: Distance Vector Routing Protocols Troubleshooting RIP Most of the RIP configuration errors involve an incorrect network statement, discon- tinuous subnets, or split horizon. The primary tool for finding RIP update issues is the debug ip rip command. The debug ip rip command displays RIP routing updates as they are sent and received. Figure 16-15 and Example 16-7 demonstrate a router using debug ip rip and receiving an update. Figure 16-15 RIP Network for Debugging Example 16-7 debug ip rip Output BHM# debug ip rip RIP event debugging is on BHM# 00:45:36 RIP:received v1 update from 192.168.13.2 on Serial0/0 00:45:36 192.168.14.0 in 1 hop 00:45:36 172.31.0.0 in 2 hops 00:45:36 172.29.0.0 in 15 hops 00:45:36 RIP sending v1 update to 255.255.255.255 via Serial0/0 (192.168.13.1) 00:45:36 network 10.0.0.0, metric 1 00:45:36 RIP sending v1 update to 255.255.255.255 via FastEthernet0/0 (10.0.0.254) 00:45:36 network 192.168.13.0 metric 1 00:45:33 network 192.168.14.0 metric 2 00:45:33 network 172.31.0.0 metric 3 00:45:36 network 172.29.0.0 metric 16 BHM GAD BOAZ S0/0 S0/1 S0/0 S0/1 Fa0/0 192.168.13.1 10.0.0.254 172.31.31.1 192.168.13.2 192.168.14.1 192.168.14.2 S0/0 Ethernet See debug ip rip results in Example 25-7. chpt_16.fm Page 738 Tuesday, May 27, 2003 2:16 PM . network 17 2. 29.0.0 metric 16 BHM GAD BOAZ S0/0 S0 /1 S0/0 S0 /1 Fa0/0 19 2 .16 8 .13 .1 10.0.0 .25 4 17 2. 31. 31. 1 19 2 .16 8 .13 .2 19 2 .16 8 .14 .1 1 92 .16 8 .14 .2 S0/0 Ethernet See debug ip rip results in Example 25 -7. chpt _16 .fm. 19 2 .16 8 .14 .0 in 1 hop 00:45:36 17 2. 31. 0.0 in 2 hops 00:45:36 17 2. 29.0.0 in 15 hops 00:45:36 RIP sending v1 update to 25 5 .25 5 .25 5 .25 5 via Serial0/0 (19 2 .16 8 .13 .1) 00:45:36 network 10 .0.0.0, metric 1 00:45:36. faster T1 links. Figure 16 -11 RIP Uses Hop Count as Its Metric 20 0 .20 0 .20 0.0 /24 auto-summary 20 0 .20 0 .20 0.0 /24 [1] via 19 2 .16 8 .10 .2, 00:00 :20 , Serial0/0 Example 16 -4 show ip rip database Command Output