Answers to Chapter 3 Troubleshooting Exercises

Figure 3-37 shows the internetwork diagram for Troubleshooting Exercises 1 through 6.

Figure 3-37. The Internetwork for Troubleshooting Exercises 1 through 6

1:	Example 3-165 shows the BGP configuration of router R2 in Figure 3-37. Example 3-165 BGP Configuration of Router R2 router bgp 10 no synchronization network 0.0.0.0 neighbor 172.16.254.2 remote-as 10 neighbor 172.16.254.2 next-hop-self neighbor 172.16.254.6 remote-as 10 neighbor 172.16.254.6 next-hop-self no auto-summary ! ip classless ip route 0.0.0.0 0.0.0.0 Ethernet10 Example 3-166 shows the BGP table and routing table for R2. Although there are routes to the destinations in the autonomous systems shown in Figure 3-37, pings to those destinations fail. Why? Example 3-166 The BGP and Routing Tables of R2 in Figure 3-37 R2# show ip bgp BGP table version is 7, local router ID is 10.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 0.0.0.0 0.0.0.0 0 32768 i *>i172.17.0.0 172.16.255.21 0 100 0 60 i *>i172.18.0.0 172.16.255.9 0 100 0 30 i *>i172.19.0.0 172.16.255.5 0 100 0 20 i *>i172.20.0.0 172.16.255.13 0 100 0 40 i *>i172.21.0.0 172.16.255.17 0 100 0 50 i R2# 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 Gateway of last resort is 0.0.0.0 to network 0.0.0.0 10.0.0.0 255.255.255.0 is subnetted, 1 subnets C 10.1.1.0 is directly connected, Ethernet11 B 172.20.0.0 [200/0] via 172.16.255.13, 00:01:15 B 172.21.0.0 [200/0] via 172.16.255.17, 00:01:16 172.16.0.0 255.255.255.252 is subnetted, 2 subnets C 172.16.254.0 is directly connected, Ethernet12 C 172.16.254.4 is directly connected, Ethernet13 B 172.17.0.0 [200/0] via 172.16.255.21, 00:01:16 B 172.18.0.0 [200/0] via 172.16.255.9, 00:00:59 B 172.19.0.0 [200/0] via 172.16.255.5, 00:00:59 S* 0.0.0.0 0.0.0.0 is directly connected, Ethernet10 R2#ping 172.17.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.17.1.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R2#
A:	R2 has no routes to the next-hop addresses referenced in the BGP table. R1 and R3 must advertise routes to these addresses or use the neighbor next-hop-self command.
2:	Example 3-167 shows debug output from routers R1 and R5 in Figure 3-37. What problem do the messages indicate ? Example 3-167 debug Output from R1 and R5 in Figure 3-37 R1# debug ip bgp BGP debugging is on R1# BGP: 172.16.255.5 open active, local address 172.16.255.6 BGP: 172.16.255.5 sending OPEN, version 4 BGP: 172.16.255.5 received NOTIFICATION 2/2 (peer in wrong AS) 2 bytes 000A BGP: 172.16.255.5 closing __________________________________________________________________________________ R5# 6d08h: BGP: 172.16.255.6 open active, delay 28272ms 6d08h: BGP: 172.16.255.6 open active, local address 172.16.255.5 6d08h: BGP: 172.16.255.6 sending OPEN, version 4 6d08h: BGP: 172.16.255.6 OPEN rcvd, version 4 6d08h: BGP: 172.16.255.6 bad OPEN, remote AS is 10, expected 30 6d08h: BGP: 172.16.255.6 sending NOTIFICATION 2/2 (peer in wrong AS) 2 bytes 000A 6d08h: BGP: 172.16.255.6 remote close, state CLOSEWAIT 6d08h: BGP: 172.16.255.6 closing
A:	R5's BGP configuration contains the statement neighbor 172.16.255.6 remote-as 30, when the statement should be neighbor 172.16.255.6 remote-as 10.
3:	Example 3-168 shows the BGP tables of R1 and R3 in Figure 3-37. The first table indicates that 172.17.0.0/24 can be reached either via R6 (172.16.255.25) or R3 (172.16.254.9). Which path is R1 using, and why? Example 3-168 BGP Tables from R1 and R3 in Figure 3-37 R1# show ip bgp BGP table version is 8, local router ID is 172.20.7.1 Status codes: s suppressed, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *>i0.0.0.0 172.16.254.1 0 100 0 i * i172.17.0.0 172.16.254.9 0 100 0 60 i *> 172.16.255.25 0 0 60 i *> 172.18.0.0 172.16.255.9 0 0 30 i *> 172.19.0.0 172.16.255.5 0 0 20 i *>i172.20.0.0 172.16.254.9 0 100 0 40 i *>i172.21.0.0 172.16.254.9 0 100 0 50 i R1# ___________________________________________________________________________________ R3# show ip bgp BGP table version is 5, local router ID is 172.16.255.22 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i0.0.0.0 172.16.254.5 0 100 0 i * i172.17.0.0 172.16.254.10 0 100 0 60 i *> 172.16.255.21 0 0 60 i * i172.18.0.0 172.16.254.10 0 100 0 30 i * i172.19.0.0 172.16.254.10 0 100 0 20 i *> 172.20.0.0 172.16.255.13 0 0 40 i *> 172.21.0.0 172.16.255.17 0 0 50 i R3#
A:	R1 is using the path through R6, because EBGP paths are preferred over IBGP paths.
4:	Example 3-169 shows the BGP and IGP configurations for R1, R3, R6, and R7 in Figure 3-37. Example 3-169 BGP and IGP Configurations for Routers R1, R3, R6, and R7 R1 router bgp 10 neighbor 172.16.254.1 remote-as 10 neighbor 172.16.254.1 next-hop-self neighbor 172.16.254.9 remote-as 10 neighbor 172.16.254.9 next-hop-self neighbor 172.16.255.5 remote-as 20 neighbor 172.16.255.9 remote-as 30 neighbor 172.16.255.25 remote-as 60 _______________________________________________________________________ R3 router bgp 10 neighbor 172.16.254.5 remote-as 10 neighbor 172.16.254.5 next-hop-self neighbor 172.16.254.10 remote-as 10 neighbor 172.16.254.10 next-hop-self neighbor 172.16.255.13 remote-as 40 neighbor 172.16.255.17 remote-as 50 neighbor 172.16.255.21 remote-as 60 neighbor 172.16.255.21 next-hop-self _______________________________________________________________________ R6 router eigrp 60 redistribute bgp 60 metric 1000 100 255 1 1500 network 172.17.0.0 ! router bgp 60 network 172.17.0.0 neighbor 172.16.255.26 remote-as 10 _______________________________________________________________________ R7 router eigrp 60 redistribute bgp 60 metric 1000 100 255 1 1500 network 172.17.0.0 ! router bgp 60 network 172.17.0.0 neighbor 172.16.255.22 remote-as 10 Example 3-168 shows the BGP tables for R1 and R3. For each of the following destinations, what next-hop address does R6 use? Explain why R6 uses the addresses you name . Destinations: 172.20.7.102 172.18.58.35 10.53.12.6
A:	172.20.7.102: Next-hop 172.17.1.1 172.18.58.35: Next-hop 172.16.255.26 10.53.12.6: Packet is dropped Neither R1 nor R3 has synchronization turned off. As a result, each advertises only the addresses it has learned from EBGP neighbors. R6 has learned 172.18.0.0/24 from R1, but R1 does not advertise 172.20.0.0/24, which it learned from an IBGP neighbor. R3 advertises that route to R7, which advertises it to R6 via EIGRP. Both R1 and R3 have learned the default route from IBGP neighbor R2, so neither router advertises the default.
5:	Example 3-170 shows the BGP configurations for R1 and R3 in Figure 3-37. Example 3-170 BGP Configurations for Routers R1 and R3 R1 router bgp 10 no synchronization aggregate-address 172.16.0.0 255.255.248.0 summary-only neighbor 172.16.254.1 remote-as 10 neighbor 172.16.254.1 next-hop-self neighbor 172.16.254.9 remote-as 10 neighbor 172.16.254.9 next-hop-self neighbor 172.16.255.5 remote-as 20 neighbor 172.16.255.9 remote-as 30 neighbor 172.16.255.25 remote-as 60 _______________________________________________________________________ R3 router bgp 10 no synchronization aggregate-address 172.16.0.0 255.255.248.0 summary-only neighbor 172.16.254.5 remote-as 10 neighbor 172.16.254.5 next-hop-self neighbor 172.16.254.10 remote-as 10 neighbor 172.16.254.10 next-hop-self neighbor 172.16.255.13 remote-as 40 neighbor 172.16.255.17 remote-as 50 neighbor 172.16.255.21 remote-as 60 neighbor 172.16.255.21 next-hop-self The objective is to suppress all the more-specific routes and advertise only an aggregate. R8's BGP table, in Example 3-171, still shows the more-specific routes. What is wrong? Example 3-171 The BGP Table of R8 in Figure 3-37 R8# show ip bgp BGP table version is 163, local router ID is 172.21.1.1 Status codes: s suppressed, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 0.0.0.0 172.16.255.18 0 10 i *> 172.17.0.0 172.16.255.18 0 10 60 i *> 172.18.0.0 172.16.255.18 0 10 30 i *> 172.19.0.0 172.16.255.18 0 10 20 i *> 172.20.0.0 172.16.255.18 0 10 40 i *> 172.21.0.0 0.0.0.0 0 32768 i R8#
A:	The mask specified in the aggregate-address command should be 255.248.0.0. The aggregate specified does not match anything in the routing tables of R1 or R3 and so is not advertised.
6:	Packets from AS 60 destined for any of the other autonomous systems shown in Figure 3-37 should be forwarded across the link between R6 and R1. The link between R7 and R3 should be used only as a backup for this traffic, although packets destined for the Internet can still use this link. To implement this policy, R3 should advertise only the default route and the aggregate 172.16.0.0/13. R1 should advertise the more-specific routes. Example 3-172 shows the configurations for R1, R3, R6, and R7. Example 3-172 Configurations for Routers R1, R3, R6, and R7 R1 router bgp 10 no synchronization neighbor 172.16.254.1 remote-as 10 neighbor 172.16.254.1 next-hop-self neighbor 172.16.254.9 remote-as 10 neighbor 172.16.254.9 next-hop-self neighbor 172.16.255.5 remote-as 20 neighbor 172.16.255.9 remote-as 30 neighbor 172.16.255.25 remote-as 60 _______________________________________________________________________ R3 router bgp 10 no synchronization aggregate-address 172.16.0.0 255.248.0.0 summary-only neighbor 172.16.254.5 remote-as 10 neighbor 172.16.254.5 next-hop-self neighbor 172.16.254.10 remote-as 10 neighbor 172.16.254.10 next-hop-self neighbor 172.16.255.13 remote-as 40 neighbor 172.16.255.17 remote-as 50 neighbor 172.16.255.21 remote-as 60 neighbor 172.16.255.21 next-hop-self _______________________________________________________________________ R6 redistribute bgp 60 metric 1000 100 255 1 1500 network 172.17.0.0 ! router bgp 60 network 172.17.0.0 neighbor 172.16.255.26 remote-as 10 _______________________________________________________________________ R7 router eigrp 60 redistribute bgp 60 metric 1000 100 255 1 1500 network 172.17.0.0 ! router bgp 60 network 172.17.0.0 neighbor 172.16.255.22 remote-as 10 Example 3-173 shows R7's routing table. Has the objective been accomplished? If not, why not? Example 3-173 R7's Routing Table for Troubleshooting Exercise 6 R7# 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 T - traffic engineered route Gateway of last resort is 172.16.255.22 to network 0.0.0.0 172.17.0.0/24 is subnetted, 3 subnets C 172.17.1.0 is directly connected, Ethernet0 D 172.17.3.0 [90/409600] via 172.17.1.2, 09:18:50, Ethernet0 C 172.17.2.0 is directly connected, Ethernet1 172.16.0.0/30 is subnetted, 1 subnets C 172.16.255.20 is directly connected, Serial0 D EX 172.19.0.0/16 [170/2611200] via 172.17.1.2, 00:19:08, Ethernet0 D EX 172.18.0.0/16 [170/2611200] via 172.17.1.2, 00:19:08, Ethernet0 B* 0.0.0.0/0 [20/0] via 172.16.255.22, 00:18:37 B 172.16.0.0/13 [20/0] via 172.16.255.22, 00:18:09 R7#
A:	The objective has not been fully accomplished. The routes to 172.18.0.0/24 and 172.19.0.0/24 are correct, but the routes to 172.20.0.0 and 172.21.0.0/24 are not in the routing table. The aggregate-address command at R3 has suppressed the advertisement of more-specific routes not only to R3's EBGP peers, but also to its IBGP peers. As a result, R1 does not know about 172.20.0.0/24 and 172.21.0.0/24.
7:	Reexamine Figure 3-19 and Example 3-98 and the associated discussion. Meribel advertises its local route 172.17.0.0 to its EBGP peers with an ORIGIN of Incomplete, whereas Lillehammer advertises the route back to Meribel with an ORIGIN of IGP. Will this cause Meribel to prefer the route from Lillehammer, thereby causing a routing loop?
A:	No. Although an ORIGIN of IGP takes precedence over an ORIGIN of Incomplete in the BGP decision process, administrative weight takes precedence over ORIGIN. By default, Meribel assigns a weight of 32768 to locally originated routes and a weight of 0 to learned routes, so the local route is preferred.
8:	Example 3-174 shows the configuration for the router named Colorado in Figure 3-24. Example 3-174 Configuration for Router Colorado in Figure 3-24 router bgp 100 network 10.1.11.0 mask 255.255.255.0 network 10.1.12.0 mask 255.255.255.0 neighbor CLIENTS peer-group neighbor CLIENTS ebgp-multihop 2 neighbor CLIENTS update-source Loopback2 neighbor CLIENTS filter-list 2 in neighbor CLIENTS filter-list 1 out neighbor 10.1.255.2 remote-as 200 neighbor 10.1.255.2 peer-group CLIENTS neighbor 10.1.255.3 remote-as 300 neighbor 10.1.255.3 peer-group CLIENTS neighbor 10.1.255.4 remote-as 400 neighbor 10.1.255.4 peer-group CLIENTS neighbor 10.1.255.5 remote-as 500 neighbor 10.1.255.5 peer-group CLIENTS neighbor 10.1.255.6 remote-as 600 neighbor 10.1.255.6 peer-group CLIENTS no auto-summary ! ip classless ip route 10.1.255.2 255.255.255.255 Serial0/1.305 ip route 10.1.255.3 255.255.255.255 Serial0/1.306 ip route 10.1.255.4 255.255.255.255 Serial0/1.307 ip route 10.1.255.5 255.255.255.255 Serial0/1.308 ! ip as-path access-list 1 permit ^$ ip as-path access-list 2 permit ^[2-6]00$ All router IDs shown in Figure 3-24 are configured on loopback interfaces, and no routing protocol other than BGP is running on any of the routers. Assuming that all the links shown in the figure are functioning properly, are all the other five routers EBGP peers of Colorado? If not, why not?
A:	No. Router NewHampshire is not a peer, because there is no static route entry at Colorado for 10.1.255.6/32.
9:	Refer to the configuration shown in Troubleshooting Exercise 8 for router Colorado in Figure 3-24. What will be the result of removing the no auto-summary statement from the configuration?
A:	Removing the statement has no effect on the topology shown in Figure 3-82, because all the router IDs and all the addresses of all the autonomous systems are subnets of 10.0.0.0.
10:	Refer again to the configuration shown in Troubleshooting Exercise 8. What routes does the incoming route filter permit?
A:	The incoming route filter refers to AS_PATH list 2. The one line of that list permits any route whose AS_PATH meets the following criteria: The route's AS_PATH consists of a single AS number. The decimal representation of the AS number must be three digits. The first digit must be a number between 2 and 6, inclusive. The second and third digits must be 0s.
11:	Refer to Figure 3-24 and the configuration for router Colorado in Troubleshooting Exercise 8. What subnets, other than those local to its own AS or the inter-AS links, can a host on subnet 10.1.3.0/24 ping?
A:	Only subnets 10.1.11.0/24, 10.1.12.0/24, and 10.1.255.1/32. The outgoing route filter at Colorado prevents its EBGP peers from learning of any routes other than local routes.