CCNA FAQ: Distance Vector Routing Protocols
Q1. What are the mitigation methods distance vector routing protocols use to avoid routing loops?
Answer: Because distance vector routing protocols are susceptible to routing loops, they have incorporated several countermeasures to help mitigate any routing loop anomalies. For instance, all distanc vector routing protocols have a maximum hop count to avoid counting to infinity. In addition, split horizon prevents routers from advertising networks out the same interface in which they were learned. Upon learning about a failing network, routers poison the route by setting it to an infinite metric and send a triggered update to the router’s neighbors. The router will not process new inferior information about the failed network until the hold-down timer expires to ensure that the failed network does not get accidentally reinstated.
Q2. What are the characteristics of RIP?
Answer: RIP is a classful distance vector routing protocol that uses hop count as its metric (maximum of 15). RIP broadcasts the contents of the routing table to its directly connected neighbors every 30 seconds.
Q3. What additional features are present in RIPv2 that are not present in RIP?
Answer: RIPv2 supports classless routing updates if the no auto-summary command is used. In addition, RIPv2 updates are sent as multicasts and can be authenticated using an MD5 password.
Q4. What is the purpose of the passive-interface command?
Answer: The passive-interface command indicates that the specified interface will not send routing updates; however, the interface can still receive and process updates. This is useful to control which routers are sent updates and to save wasted bandwidth and processing of broadcasts or multicasts on LAN segments.
Q5. What are the fundamental configuration steps for RIPv2?
Answer: To enable RIP, you must enter the routing process by using the keyword router followed by the routing protocol. When you are in the routing configuration process, you must advertise the directly connected classful networks attached to the router by using the network command followed by the classful network. To enable RIPv2, you have to add the command version 2 in the routing configuration.
Q6. Given the following output, which of the following is a true statement?
A. The 172.16.0.0 has an administrative distance of 1.
B. The 220.127.116.11 network is in a hold-down state.
C. The configuration for this router to advertise RIP should have a network 172.16.0.0 and a network 18.104.22.168 statement.
D. None of the above
Answer: B. Because the routing table update shows the 22.214.171.124/8 network as possibly down, it is currently in a hold-down state and waiting for the hold-down timer to expire before accepting a route with a higher metric. Answer A is incorrect because the AD is 120 and the hip count is 1. C is false because those network entries were learned via RIP, not advertised. D is incorrect, because B is correct.
Q7. Given the following output, which of the following statements is false regarding the 0.0.0.0/0 network?
A. The 0.0.0.0 network was statically configured in this router.
B. The gateway of last resort is the 192.168.1.9 router.
C. The default route was advertised to the local router via RIP.
D. The default route was automatically redistributed in Router 192.168.1.9.
Answer: A. Because the default route has an R statement next to it, it was redistributed automatically by the neighbor at 192.168.1.9. If it was statically configured, it would have an S indication next to the route. Answers B, C, and D are true because the 0.0.0.0 route was redistributed and advertised via RIP by the router with the IP address of 192.168.1.9.
Q8. Based on the following output, which network may not show up in the routing table?
RouterA#debug ip rip
RIP protocol debugging is on
00:26:27: RIP: received v1 update from 192.168.1.6 on Serial0/0
00:26:27: 192.168.1.8 in 12 hops
00:26:27: 192.168.1.12 in 15 hops
00:26:27: 172.17.0.0 in 14 hops
00:26:27: 172.18.0.0 in 16 hops
Answer: D. Because the 172.18.0.0 has an infinite metric for RIP being advertised, it is most likely a poisoned route or a router that is not showing up in the routing table. Answers A, B, and C will show up in the routing table because their hop count does not exceed the maximum hop count for RIP (15).
Q9. Which two commands enable a distance vector routing protocol to be classless?
A. version 2
C. no auto-summary
D. router classless
Answer: A, C. The only distance vector routing protocol that can be classless is RIPv2. The command to enable RIPv2 is version 2. To make it classless, you use the no auto-summary command. Answer B is incorrect because the passive-interface command is used to keep updates from being sent out that specific interface. Answer D is not a valid command.
Q10. Which of the following is a possible reason for the following debug ip rip output?
*Aug 10 02:02:24.023: RIP: sending v2 update to 126.96.36.199 via Serial0/0/0 (192.168.1.5)
*Aug 10 02:02:24.023: RIP: build update entries
*Aug 10 02:02:24.023: 172.16.0.0/16 via 0.0.0.0, metric 1, tag 0
*Aug 10 01:57:50.823: RIP: received packet with MD5 authentication
*Aug 10 01:57:50.823: RIP: ignored v2 packet from 192.168.1.6 (invalid authentication)
A. The interface that received the update is running as a passive interface.
B. Both routers are not configured for RIPv2.
C. The key strings do not match.
D. Split horizon will not allow the update to be received because that same network was sent on the same interface.
Answer: C. Because the output indicated that the update that was received was ignored because of authentication failure, the logical choice is that the key strings in the update authentication are not matching. Answer A is incorrect because a passive interface would not ignore incoming updates since it suppressed only outgoing updates. Answer B is incorrect because the output shows the updates as version 2 and only version 2 uses authentication. Answer D is incorrect because split horizon does not ignore incoming updates.
Q11. Which of the following is not a mechanism to avoid routing loops?
A. Split horizon
B. Update authentication
C. Hold-down timers
D. Route poisoning
Answer: B. Update authentication is not a mechanism to avoid routing loops. Split horizon, hold-down timers, and router poisoning are all mechanisms to avoid routing loops so answers A, C, and D are incorrect.
Q12. Given the following output of a show ip protocols, which of the following commands was not configured on this router?
A. version 2
B. ip rip authentication key-chain shorty
C. maximum-paths 6
D. network 192.168.0.0
Answer: D. The network statement is incorrect. The correct network command would be network 192.168.1.0. Answer A is incorrect because the output shows that version 2 is being sent and received. Answer B is incorrect because the output shows that the shortykey chain is applied. Answer C is incorrect because the output shows that the maximum paths have been increased from 4 to 6.
Q13. Which command should you implement before doing any debug commands?
A. show running-config
B. show processes
C. undebug all
D. copy running-config startup-config
Answer: B. Before running any debug commands, you should check your router’s current and past utilization with the show processes command. Answer A does not have any effect on the debug process. Answer C will turn off any debugging after the debugging process has been initiated. Answer D does not have any effect on the debugging process.
Q14. Given the following output of a debug ip rip, which of the following is true?
00:57:27: RIP: received v2 update from 192.168.1.6 on Serial0/0
00:57:27: 192.168.1.8/30 via 0.0.0.0 in 1 hops
00:57:27: 192.168.1.12/30 via 0.0.0.0 in 1 hops
00:57:27: 172.17.0.0/16 via 0.0.0.0 in 1 hops
00:57:27: 172.18.0.0/16 via 0.0.0.0 in 1 hops
A. The routing updates are broadcast to their neighbors.
B. The router automatically summarizes these networks.
C. Networks 172.17.0.0 and 172.18.0.0 have an infinite metric.
D. The no auto-summary command is used in RIPv2.
Answer: D. The update is indicative of a RIPv2 update that has been configured as classless with the no auto-summary command. This is true because the updates contain the subnet masks. Answer A is false because RIPv2 multicasts its updates to 188.8.131.52. B is false because the no auto-summary command disables automatic summarization. C is incorrect because an infinite metric for RIPv2 is 16.
Q15. Which characteristic does not go with its respective routing protocol?
A. Multicasts updates—RIP
B. 180 invalid timer—RIP
Answer: A. RIPv2 sends its updates as multicasts, not version 1. Answer B is true because RIP has an invalid timer of 180 seconds. Answers C and D are also true because RIPv2 is classful by default but can be configured as classless.
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