IGRP 749 The autonomous system number is one that identifies the IGRP process. It is also used to tag the routing information. To specify a list of networks for IGRP routing processes, use the network router con- figuration command: RouterA(config)# router igrp 101 RouterA(config-router)# network 192.168.1.0 To remove an entry, use the no form of the command: RouterA(config)# no router igrp 101 RouterA(config-router)# no network 192.168.1.0 Example 16-13 demonstrates how to configure IGRP using AS 101 on RouterA and RouterB. Verifying the IGRP Configuration To verify that IGRP has been configured properly, enter the show ip route command and look for IGRP routes signified by an I. Additional commands for checking IGRP configuration are as follows: ■ show interface interface—Verifies that the Ethernet interface is properly configured. ■ show running-config—Verifies that IGRP is enabled on the router. ■ show running-config interface interface—Verifies that the proper IP address has been used. ■ show running-config | begin interface interface—Verifies that IGRP is running on the router’s interfaces starting at a specific interface. Example 16-13 Configuring IGRP RouterA(config)# router igrp 101 RouterA(config-router)# network 192.168.1.0 RouterA(config-router)# network 192.168.2.0 RouterB(config)# router igrp 101 RouterB(config-router)# network 192.168.2.0 RouterB(config-router)# network 192.168.3.0 Lab Activity Configuring IGRP In this lab, you set up an IP addressing scheme using Class C networks and configure IGRP on all routers. chpt_16.fm Page 749 Tuesday, May 27, 2003 2:16 PM 750 Chapter 16: Distance Vector Routing Protocols ■ show running-config | begin igrp—Verifies that IGRP is enabled on the router. ■ show ip protocols—Verifies that IGRP is enabled on the router. To verify that the Ethernet interface is properly configured, enter the show interface fa0/0 command. Example 16-14 illustrates the output. To see if IGRP is enabled on the router, use the commands demonstrated in Example 16-15. To check for proper IP addresses use the command demonstrated in Example 16-16. Example 16-14 show interface Command Output RouterA# show interface fa0/0 FastEthernet0/0 is up, line protocol is up Hardware is AmdFE, address is 0009.7c89.5620 (bia 0009.7c89.5620) Internet address is 192.168.1.1/24 Output Omitted Example 16-15 show ip protocols and show running-config Command Output RouterA# show ip protocols Routing Protocol is "igrp 101" Output Omitted RouterA# show running-config | begin igrp router igrp 101 network 192.168.1.0 network 192.168.2.0 ! Output Omitted Example 16-16 show running-config interface Command Output RouterA# show running-config interface fa0/0 Building configuration Current configuration: ! interface FastEthernet0/0 chpt_16.fm Page 750 Tuesday, May 27, 2003 2:16 PM IGRP 751 Example 16-17 illustrates the output from the show ip route command and displays the routes available via this router’s interfaces. Troubleshooting IGRP Most IGRP configuration errors involve a mistyped network statement, discontiguous subnets, or an incorrect autonomous system number. The following commands are useful when troubleshooting IGRP: ■ show ip protocols—Used to display IP routing protocol information in summary form. ■ show ip route—Used to show the IP routing table on the router. ■ debug ip igrp events—Diplays information about summary IGRP routing that is running on the network. ■ debug ip igrp transactions—Shows message requests from neighbor routers ask- ing for an update and the broadcasts sent from the originating router towards that neighbor router. ip address 192.168.1.1 255.255.255.0 no ip directed-broadcast end Example 16-17 show ip route Command Output RouterA# 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, Loopback0 C 192.168.2.0/24 is directly connected, Serial0/0 I 192.168.3.0/24 [100/80135] via 192.168.2.2, 00:01:00, Serial0/0 Example 16-16 show running-config interface Command Output (Continued) chpt_16.fm Page 751 Tuesday, May 27, 2003 2:16 PM 752 Chapter 16: Distance Vector Routing Protocols ■ ping—A utility used to determine whether a specific IP address is accessible. ■ traceroute—A utility that traces a packet from your computer to an Internet host, showing how many hops the packet requires to reach the host and how long each hop takes. Example 16-18 illustrates output from debug ip igrp events command. Example 16-19 illustrates output from debug ip igrp transactions command. Example 16-18 debug ip igrp events Command Output RouterA# debug ip igrp events IGRP event debugging is on 00:21:38: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1 .1) 00:21:38: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes. 00:21:38: IGRP: Total routes in update: 2 00:21:38: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:21:38: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes. 00:21:38: IGRP: Total routes in update: 1 Example 16-19 debug ip igrp transactions Command Output RouterA# debug ip igrp transactions IGRP protocol debugging is on 00:22:17: IGRP: received update from 192.168.2.2 on Serial0/0 00:22:17: network 192.168.3.0, metric 80135 (neighbor 110) 00:23:07: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1 .1) 00:23:07: network 192.168.2.0, metric=80125 00:23:07: network 192.168.3.0, metric=80135 00:23:07: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:23:07: network 192.168.1.0, metric=110 chpt_16.fm Page 752 Tuesday, May 27, 2003 2:16 PM IGRP 753 If it is discovered that the wrong AS number is being used, correcting it as in Example 16-20 results in the corrected output. Example 16-20 debug ip igrp transactions RouterA(config)# no router igrp 102 RouterA(config)# router igrp 101 RouterA(config-router)# network 192.168.1.0 RouterA(config-router)# network 192.168.2.0 00:27:50: IGRP: broadcasting request on FastEthernet0/0 00:27:50: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1.1) 00:27:51: IGRP: Update contains 0 interior, 0 system, and 0 exterior routes. 00:27:51: IGRP: Total routes in update: 0 - suppressing null 00:28:01: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1.1) 00:28:01: network 192.168.2.0, metric=80125 00:28:01: network 192.168.3.0, metric=80135 00:28:01: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes. 00:28:01: IGRP: Total routes in update: 2 00:28:01: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:28:01: network 192.168.1.0, metric=110 00:28:01: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes. 00:28:01: IGRP: Total routes in update: 1 Lab Activity Default Routing with RIP and IGRP In this lab, you configure a default route and use RIP to propagate this default information to other routers. When you have this configuration working, you must migrate the network from RIP to IGRP and configure default routing to work with that protocol as well. Lab Activity Unequal Cost Load Balancing with IGRP In this lab, you configure and tune IGRP for unequal-cost load balancing and observe load balancing using debug commands. chpt_16.fm Page 753 Tuesday, May 27, 2003 2:16 PM 754 Chapter 16: Distance Vector Routing Protocols Summary In this chapter, you learned the following: ■ Routing information is maintained through updates as the topology changes in a network. ■ Routing loops occur in a network from alternate routes, slow convergence, and inconsistent routing updates. ■ Defining a maximum can be achieved to prevent count to infinity. ■ Three methods of preventing routing loops: split horizon, triggered updated, and hold-down timers. ■ Route poisoning is used by various distance vector protocols to overcome large routing loops and offer information about when a subnet or network is not accessible. ■ To configure routing protocols: RIP and IGRP. ■ To use the ip classless command. ■ To troubleshoot routing protocols: RIP and IGRP. ■ To verify routing protocols: RIP and IGRP. ■ To configure default routes. To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. chpt_16.fm Page 754 Tuesday, May 27, 2003 2:16 PM Key Terms 755 Key Terms adjacent neighbor Two directly connected routers that participate in the exchange of routing information are said to be adjacent. convergence The speed and capability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology. count to infinity A problem that can occur in routing algorithms that are slow to converge in which routers continuously increment the hop count to particular net- works. Typically, some arbitrary hop-count limit is imposed to prevent this problem. exterior routes Routes to networks outside the autonomous system that are consid- ered when identifying a gateway of last resort. flush timer Indicates how much time passes before a route is flushed from the routing table. The IGRP default is seven times the routing update timer. holddowns Prevent regular update messages from inappropriately reinstating a route that might not be up. hold-time timer Specifies the amount of time for which information about better routes is ignored. The IGRP default for this variable is 3 times the update timer period plus 10 seconds. IGRP (Interior Gateway Routing Protocol) An IGP developed by Cisco to address the problems associated with routing in large, heterogeneous networks. Compare with EIGRP. See also IGP, OSPF, and RIP. interior routes Routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes. invalid timer Specifies how long a router waits in the absence of routing-update mes- sages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. poison reverse updates Updates that are necessary to defeat larger routing loops. Generally speaking, increases in routing metrics indicate routing loops. Poison reverse updates are then sent to remove the route and place it in holddown. RIP (Routing Information Protocol) An IGP supplied with UNIX BSD systems. The most common IGP in the Internet. RIP uses hop count as a routing metric. 1102.book Page 755 Tuesday, May 20, 2003 2:53 PM 756 Chapter 16: Distance Vector Routing Protocols routing metric A method by which a routing algorithm determines that one route is better than another. This information is stored in routing tables and sent in routing updates. Metrics include bandwidth, communication cost, delay, hop count, load, MTU, path cost, and reliability. Sometimes referred to simply as a metric. routing protocol A protocol that accomplishes routing through the implementation of a specific routing algorithm. Examples of routing protocols are IGRP, OSPF, and RIP. routing table A table stored in a router or some other internetworking device that keeps track of routes to particular network destinations and, in some cases, metrics associated with those routes. routing update A message sent from a router to indicate network reachability and associated cost information. Routing updates are typically sent at regular intervals and after a change in network topology. Compare with flash update. split horizon A routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split- horizon updates are useful in preventing routing loops. system routes Routes to networks within an autonomous system. Cisco IOS Software derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information. triggered update A triggered update is an update that is sent without waiting for the update timer to expire. update timer Specifies how frequently routing update messages are sent. The IGRP default for this variable is 90 seconds. Check Your Understanding Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1. What kind of entries does a router initially refer to? A. Entries about networks or subnets that are directly connected B. Entries that it has learned about from the Cisco IOS Software C. Entries whose IP address and mask information are known D. Entries that it has learned about from other routers 1102.book Page 756 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 757 2. Which of the following best describes a static route? A. A routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table B. A route that is explicitly configured and entered into the routing table and that takes precedence over routes chosen by dynamic routing protocols C. A route that adjusts automatically to network topology or traffic changes D. A route that adjusts involuntarily to direct frames within a network topology 3. Which of the following best describes a default route? A. A routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table B. A route that is explicitly configured and entered into the routing table C. A route that adjusts automatically to network topology or traffic changes D. A route that adjusts involuntarily to direct frames within a network topology 4. What are exterior routing protocols used for? A. To transmit between nodes on a network B. To deliver information within a single autonomous system C. To communicate between autonomous systems D. To set up a compatibility infrastructure between networks 5. What are interior routing protocols used for? A. They are used to set up a compatibility infrastructure between networks. B. They are used to communicate between autonomous systems. C. They are used to transmit between nodes on a network. D. They are used within a single autonomous system. 6. Which of the following is a global task? A. Addressing IP network numbers by specifying subnet values B. Enabling a routing protocol such as RIP or IGRP C. Assigning network/subnet addresses and the appropriate subnet mask D. Setting up a routing metric to find the best path to each network 1102.book Page 757 Tuesday, May 20, 2003 2:53 PM 758 Chapter 16: Distance Vector Routing Protocols 7. What metric does RIP use to determine the best path for a message to travel on? A. Bandwidth B. Hop count C. Varies with each message D. Administrative distance 8. You suspect that one of the routers connected to your network is sending bad routing information. What command can you use to check? A. router(config)# show ip route B. router# show ip route C. router> show ip protocol D. router(config-router)# show ip protocol 9. Why would you display the IP routing table? A. To set the router update schedule B. To identify destination network addresses and next-hop pairs C. To trace where datagrams are coming from D. To set the parameters and filters for the router 10. If you want to learn which routing protocol a router was configured with, what command structure should you use? A. router> show router protocol B. router(config)> show ip protocol C. router(config)# show router protocol D. router> show ip protocol 11. In the following command, what does the last number stand for? Router (config)# ip route 2.0.0.0 255.0.0.0 1.0.0.2 5 A. The number of hops B. The number of routes to the destination C. The administrative distance D. The destination’s reference number in the routing table 1102.book Page 758 Tuesday, May 20, 2003 2:53 PM . FastEthernet0/0 (19 2 .16 8 .1 .1) 00 :23 :07: network 19 2 .16 8 .2. 0, metric=8 0 12 5 00 :23 :07: network 19 2 .16 8.3.0, metric=8 013 5 00 :23 :07: IGRP: sending update to 25 5 .25 5 .25 5 .25 5 via Serial0/0 (19 2 .16 8 .2 .1) 00 :23 :07:. on 00 :22 :17 : IGRP: received update from 19 2 .16 8 .2. 2 on Serial0/0 00 :22 :17 : network 19 2 .16 8.3.0, metric 8 013 5 (neighbor 11 0) 00 :23 :07: IGRP: sending update to 25 5 .25 5 .25 5 .25 5 via FastEthernet0/0 (19 2 .16 8 .1 . Serial0/0 I 19 2 .16 8.3.0 /24 [10 0/8 013 5] via 19 2 .16 8 .2. 2, 00: 01: 00, Serial0/0 Example 16 -16 show running-config interface Command Output (Continued) chpt _16 .fm Page 7 51 Tuesday, May 27 , 20 03 2 :16 PM 7 52 Chapter