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514 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

Chapter 5

“Do I Know This Already?” Quiz

1.Which of the following must be true before IOS lists a route as “S” in the output of a show ip route command?

Answer: C. The “S” designation implies that the route is a static route. Static routes are created with the ip route command.

2.Which of the following commands correctly configures a static route?

Answer: A

3.Which of the following distance vector features prevents routing loops by causing the routing protocol to advertise only a subset of known routes, as opposed to the

full routing table?

Answer: D. Split horizon causes a router to not advertise routes out an interface if the route would cause packets to be sent out that same interface.

4.Which of the following features prevents routing loops by advertising an infinite metric route when a route fails?

Answer: E. Route poisoning means advertising the failed route with an “infinite” metric, as opposed to simply ceasing to advertise the route. Poison reverse is route poisoning by advertising a route that previously was not advertised because of split horizon.

5.Router1 has interfaces with addresses 9.1.1.1 and 10.1.1.1. Router 2, connected to Router1 over a serial link, has addresses 10.1.1.2 and 11.1.1.2. Which of the

following commands would be part of a complete RIP configuration on Router2, with which Router2 advertises out all interfaces, and about all routes?

Answer: A, E, H. A router only needs network commands matching its own interfaces.

6.Which of the following situations would cause RIP or IGRP to remove all the routes learned from a particular neighboring router?

Answer: B. Distance vector protocols rely on the regular receipt of routing updates from their neighbors to continue believing that the routes through that neighbor are still valid.

7.Which of the following network commands, following a router rip command, would cause RIP to send updates out two interfaces whose IP addresses are 10.1.2.1 and 10.1.1.1, mask 255.255.255.0?

Answer: A. The network command has a Class A, B, or C network number as the parameter. The router enables RIP on all interfaces in that network.

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8.What command(s) list(s) information identifying the neighboring routers that are sending routing information to a particular router?

Answer: B, D

9.What part of the output of the show ip route command identifies the metric associated with the route?

Answer: E. The first number in brackets is the administrative distance, and the second number is the metric.

Q&A

1.What type of routing protocol algorithm uses a hold-down timer? What is its purpose?

Answer: Distance vector. Holddown helps prevent counting-to-infinity problems. After learning that a route has failed, a router waits for a hold-down timer before believing any new information about the route.

2.Define what split horizon means to the contents of a routing update. Does this apply to both the distance vector algorithm and the link-state algorithm?

Answer: Routing updates sent out an interface do not contain routing information about subnets learned from updates entering the same interface. Split horizon is used only by distance vector routing protocols.

3.What steps would you take to migrate from RIP to IGRP in a router whose current RIP configuration includes only router rip followed by a network 10.0.0.0 command?

Answer: Issue the following commands in configuration mode:

router igrp 5 network 10.0.0.0 no router rip

If RIP still were configured, IGRP’s routes would be chosen over RIP. The Cisco IOS software considers IGRP a better source of routing information by default, as defined in the administrative distance setting (the defaults are 120 for RIP and 100 for IGRP).

4.How does the Cisco IOS software designate a subnet in the routing table as a directly connected network? What about a route learned with IGRP or RIP?

Answer: The show ip route command lists routes with a designator on the left side of the command output. C represents connected routes, I is used for IGRP, and R represents routes derived from RIP.

516Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

5.Create a configuration for IGRP on a router with these interfaces and addresses: e0 using 10.1.1.1, e1 using 224.1.2.3, s0 using 10.1.2.1, and s1 using 199.1.1.1. Use process ID 5.

Answer:

router igrp 5 network 10.0.0.0 network 199.1.1.0

If you noticed that 224.1.2.3 is not a valid Class A, B, or C address, you get full credit. A new address is needed for Ethernet1, with a matching network command.

6.Create a configuration for IGRP on a router with these interfaces and addresses: to0 using 200.1.1.1, e0 using 128.1.3.2, s0 using 192.0.1.1, and s1 using 223.254.254.1.

Answer:

router igrp 1 network 200.1.1.0 network 128.1.0.0 network 192.0.1.0 network 223.254.254.0

Because four different networks are used, four network commands are required. If you noticed that this question does not specify the process ID (1 in this example) but configured one, you get full credit. A few of these network numbers are used in examples; memorize the range of valid Class A, B, and C network numbers.

7.From a router’s user mode, without using debugs or privileged mode, how can you determine what routers are sending you routing updates?

Answer: The show ip protocol command output lists the routing sources—the IP addresses of routers sending updates to this router. Knowing how to determine a fact without looking at the configuration will better prepare you for the exam. Also, the show ip route command lists next-hop router IP addresses. The next-hop routers listed identify the routers that are sending routing updates.

8.Imagine that a router has an interface E0 with IP address 168.10.1.1 and E1 with IP address 10.1.1.1. If the commands router rip and network 10.0.0.0, with no other network commands, are configured in the router, does RIP send updates out Ethernet0?

Answer: No. There must be a network statement for network 168.10.0.0 before RIP advertises out that interface. The network command simply selects the connected interfaces on which to send and receive updates.

9.Imagine that a router has an interface E0 with IP address 168.10.1.1 and E1 with IP address 10.1.1.1. If the commands router igrp 1 and network 10.0.0.0 are configured in the router, does IGRP advertise 168.10.0.0?

Answer: No. There must be a network statement for network 168.10.0.0 before IGRP advertises that directly connected subnet.

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10.If the commands router igrp 1 and network 10.0.0.0 are configured in a router that has an Ethernet0 interface with IP address 168.10.1.1, mask 255.255.255.0, does this router have a route to 168.10.1.0?

Answer: Yes. The route is in the routing table because it is a directly connected subnet, not because of any action by IGRP.

11.Must IGRP metrics for multiple routes to the same subnet be equal for the multiple routes to be added to the routing table? If not, how close in value do the metrics have to be?

Answer: IGRP (and EIGRP) use a concept called variance, which represents how close the metrics to the same subnet must be before they are considered equal. The variance router subcommand is used to set the value.

12.When you’re using RIP, what configuration command controls the number of equal-cost routes that can be added to the routing table at the same time? What is the maximum number of equal-cost routes to the same destination that can be included in the IP routing table at once?

Answer: The maximum-paths x router subcommand is used in RIP configuration mode to set the number. The maximum is 6, and the default is 4.

13.When you’re using IGRP, what configuration command controls the number of equalcost routes that can be added to the routing table at the same time? What is the maximum number of equal-cost routes to the same destination that can be included in the IP routing table at once?

Answer: The maximum-paths x router subcommand is used in IGRP configuration mode to set the number. The maximum is 6, and the default is 4.

14.Which command lists all the IP routes learned via RIP?

Answer: The show ip route rip command lists only RIP-learned routes.

15.Which command or commands list all IP routes in network 172.16.0.0?

Answer: show ip route 172.16.0.0 lists all the routes in 172.16.0.0. Also, the show ip route list 1 command lists routes in network 172.16.0.0 assuming that the access-list 1 permit 172.16.0.0 0.0.255.255 configuration command also exists.

16.True or false: Distance vector routing protocols learn routes by transmitting routing updates.

Answer: False. Routes are learned by receiving routing updates from neighboring routers.