Troubleshooting Exercises

1	In the first offset list example, the access list in Barney is changed from access-list 5 permit 10.33.32.0 0.0.0.0 to access-list 5 deny 10.33.32.0 0.0.0.0 access-list 5 permit any What is the result?
2	Figure 5-13 shows a network in which the IP address masks on one router have been misconfigured. Example 5-27 through Example 5-29 show the route tables of RTA, RTB, and RTC, respectively. Based on what you know about the way RIP advertises and receives updates, explain each entry in RTB's route table. Explain why RTB's entry for subnet 172.16.26.0 indicates a 32-bit mask. If any entries are missing in any of the route tables, explain why. Figure 5-13. Network for Troubleshooting Exercises 2 and 3. Example 5-27. Route table of RTA in Figure 5-13. RTA#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 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, * - candidate default, U - per-user static route Gateway of last resort is not set 172.16.0.0/16 is subnetted, 4 subnets R 172.16.24.0 [120/1] via 172.16.18.3, 00:00:01, Ethernet0 R 172.16.26.0 [120/2] via 172.16.18.3, 00:00:01, Ethernet0 C 172.16.20.0 is directly connected, Ethernet1 C 172.16.18.0 is directly connected, Ethernet0 RTA# Example 5-28. Route table of RTB in Figure 5-13. RTB#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, * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks R 172.16.24.0/22 [120/1] via 172.16.18.3, 00:00:20, Ethernet0 R 172.16.26.0/32 [120/2] via 172.16.18.3, 00:00:20, Ethernet0 C 172.16.20.0/22 is directly connected, Ethernet1 C 172.16.16.0/22 is directly connected, Ethernet0 RTB# Example 5-29. Route table of RTC in Figure 5-13. RTC#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, * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 172.16.0.0/23 is subnetted, 4 subnets C 172.16.24.0 is directly connected, Serial0 R 172.16.26.0 [120/1] via 172.16.24.2, 00:00:09, Serial0 R 172.16.20.0 [120/1] via 172.16.18.5, 00:00:25, Ethernet0 C 172.16.18.0 is directly connected, Ethernet0 RTC#
3	Users on subnet 172.16.18.0/23 in Figure 5-13 are complaining that connectivity to subnet 172.16.26.0/23 is intermittentsometimes it can be reached, sometimes it can't. (The bad subnet masks of RTB have been corrected.) A first examination of the route tables of RTC and RTD (Example 5-30) seems to show no problems. All subnets are in both tables. Yet a minute or so later, RTC shows subnet 172.16.26.0/23 to be unreachable (Example 5-31), whereas RTD still shows all subnets. A few minutes after that, the subnet is back in RTC's route table (Example 5-32). In each of the three figures, the route table of RTD shows no change. A careful examination of the route tables in Example 5-30 through Example 5-32 will reveal the problem. What is it?

Example 5-30. Route tables of RTC and RTD in Figure 5-13.

RTC#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, * - candidate default,        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/23 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 R       172.16.26.0 [120/1] via 172.16.24.2, 00:02:42, Serial0 R       172.16.20.0 [120/1] via 172.16.18.5, 00:00:22, Ethernet0 R       172.16.22.0 [120/1] via 172.16.18.4, 00:00:05, Ethernet0 C       172.16.18.0 is directly connected, Ethernet0 ________________________________________________________________________________ RTD#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        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, * - candidate default,        U - per-user static route Gateway of last resort is not set      172.16.0.0/16 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 C       172.16.26.0 is directly connected, TokenRing0 R       172.16.20.0 [120/2] via 172.16.24.1, 00:00:00, Serial0 R       172.16.22.0 [120/2] via 172.16.24.1, 00:00:00, Serial0 R       172.16.18.0 [120/1] via 172.16.24.1, 00:00:00, Serial0

Example 5-31. Route tables of RTC and RTD, examined approximately 60 seconds after Example 5-30.

RTC#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, * - candidate default,        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/23 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 R       172.16.26.0/23 is possibly down,            routing via 172.16.24.2, Serial0 R       172.16.20.0 [120/1] via 172.16.18.5, 00:00:19, Ethernet0 R       172.16.22.0 [120/1] via 172.16.18.4, 00:00:24, Ethernet0 C       172.16.18.0 is directly connected, Ethernet0 ________________________________________________________________________________ RTD#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        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, * - candidate default,        U - per-user static route Gateway of last resort is not set      172.16.0.0/16 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 C       172.16.26.0 is directly connected, TokenRing0 R       172.16.20.0 [120/2] via 172.16.24.1, 00:00:15, Serial0 R       172.16.22.0 [120/2] via 172.16.24.1, 00:00:15, Serial0 R       172.16.18.0 [120/1] via 172.16.24.1, 00:00:15, Serial0

Example 5-32. Route tables of RTC and RTD, examined approximately 120 seconds after Example 5-31.

RTC#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, * - candidate default,        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/23 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 R       172.16.26.0 [120/1] via 172.16.24.2, 00:00:09, Serial0 R       172.16.20.0 [120/1] via 172.16.18.5, 00:00:11, Ethernet0 R       172.16.22.0 [120/1] via 172.16.18.4, 00:00:18, Ethernet0 C       172.16.18.0 is directly connected, Ethernet0 ________________________________________________________________________________ RTD#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        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, * - candidate default,        U - per-user static route Gateway of last resort is not set      172.16.0.0/16 is subnetted, 5 subnets C       172.16.24.0 is directly connected, Serial0 C       172.16.26.0 is directly connected, TokenRing0 R       172.16.20.0 [120/2] via 172.16.24.1, 00:00:19, Serial0 R       172.16.22.0 [120/2] via 172.16.24.1, 00:00:19, Serial0 R       172.16.18.0 [120/1] via 172.16.24.1, 00:00:19, Serial0