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Chapter 7 523

Answer: OSPF converges fast, like EIGRP, but it is an open standard, unlike EIGRP. Therefore, OSPF would more easily allow routers from multiple vendors to be used. EIGRP converges quickly, like OSPF, but requires little or no engineering design, whereas OSPF requires significant engineering design effort in larger networks.

Chapter 7

“Do I Know This Already?” Quiz

1.Which of the following summarized subnets is the smallest summary route that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?

Answer: D

2.Which of the following summarized subnets is not a valid summary that includes subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?

Answer: C. You can check your answer by deriving the range of valid IP addresses for each subnet shown, as compared with the range of valid IP addresses in the three subnets stated in the question.

3.What does the acronym VLSM stand for?

Answer: A

4.Imagine a router that lists three routes to subnets in network 10.0.0.0. Assume that VLSM is in use. How many times does the show ip route command list mask/prefix information about routes in network 10.0.0.0?

Answer: C. When VLSM is used, the mask information is listed beside each route, because each route can have a different mask associated with it.

5.Which routing protocol(s) perform(s) autosummarization by default?

Answer: A, B, C. Of these routing protocols, only EIGRP allows autosummarization to be disabled.

6.Which of the following routing protocols are classless?

Answer: C, D

524Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

7.Which of the following is affected by whether a router is performing classful or classless routing?

Answer: A. Classless routing always matches a default route if one exists. Classful routing matches a default route only if the destination Class A, B, or C network is not in the routing table.

8.Which of the following routing protocols support route summarization?

Answer: C, D. RIP-1 and IGRP perform autosummarization. If you chose A and B as well, but you know that they support only autosummarization, give yourself credit for a right answer!

Q&A

1.Name the two commands typically used to create a default route for a router.

Answer: The ip default-network command and the ip route 0.0.0.0 0.0.0.0 commands accomplish the goal of having the router use a known route as the default for packets that are not matched in the routing table. The ip route 0.0.0.0 0.0.0.0 command uses the fact that network 0.0.0.0 is used by Cisco IOS software to represent the default network.

2.Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and subnets are known, except that there is also a default route (0.0.0.0) in the routing table. A packet destined for 192.1.1.1 arrives at the router. What configuration command determines whether the default route is used in this case?

Answer: The packet is routed using the default route, regardless of other configuration commands. In this scenario, in which the Class A, B, or C network is known, there is no match for the destination in the known subnets, and a default exists, so the default must be used.

3.Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and its subnets are known, except that there is also a default route

(0.0.0.0) in the routing table. A packet destined for 10.1.1.1 arrives at the router, but no known subnet of network 10 matches this destination address. What configuration command determines whether the default route is used in this case?

Answer: If the command ip classless is configured, the packet is routed using the default route. If no ip classless is configured, the packet is discarded.

4.What feature supported by EIGRP allows it to support VLSM?

Answer: The association and transmission of mask information with each route allows VLSM support with any routing protocol.

Chapter 7 525

5.List the interior IP routing protocols that have autosummarization enabled by default. Which of these protocols allow autosummarization to be disabled using a configuration command?

Answer: RIP-1, IGRP, EIGRP, and RIP-2 all have autosummarization enabled by default. EIGRP and RIP-2 can disable this feature.

6.Which interior IP routing protocols support route summarization?

Answer: EIGRP, OSPF, IS-IS, and RIP-2 support route summarization. (Give yourself credit on this question if you included OSPF and EIGRP.)

7.Assume that several subnets of network 172.16.0.0 exist in a router’s routing table. What must be true about these routes for the output of the show ip route command to list mask information only on the line that lists network 172.16.0.0 but that doesn’t show mask information on each route for each subnet?

Answer: If all the subnets of 172.16.0.0 use the same mask, the output of the show ip route command lists only the mask in the heading line for the network. If VLSM were in use, each route for each subnet would reflect the mask used in that case.

8.Router A and Router B are connected via a point-to-point serial link. Router A’s interfaces use IP address 172.16.1.1, mask 255.255.255.0 and address 172.16.2.1, mask 255.255.255.0. Router B’s interfaces use address 172.16.2.2, mask 255.255.255.0 and address 10.1.1.1, mask 255.255.254.0. Is VLSM in use? Explain your answer.

Answer: Although two different masks are used, VLSM is not used. VLSM implies that two different masks are in use in the same Class A, B, or C network. In this example, only one mask is used for each classful network.

9.What is the smallest summarized route that summarizes the subnets 10.1.63.0, 10.1.64.0, 10.1.70.0, and 10.1.71.0, all with mask 255.255.255.0?

Answer: Only the first 17 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.1.0.0, mask 255.255.128.0.

10.What is the smallest summarized route that summarizes the subnets 10.5.111.0, 10.5.112.0, 10.5.113.0, and 10.5.114.0, all with mask 255.255.255.0?

Answer: The first 19 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.5.96.0, mask 255.255.224.0.

526Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

11.What is the smallest summarized route that summarizes the subnets 10.5.110.32, 10.5.110.48, and 10.5.110.64, all with mask 255.255.255.248?

Answer: The first 25 bits of these subnet numbers are in common. Therefore, the smallest summary is 10.5.110.0, mask 255.255.255.128.

12.Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which are classless?

Answer: EIGRP and OSPF

13.Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which support VLSM?

Answer: EIGRP and OSPF

14.Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which advertise mask information along with subnet numbers?

Answer: EIGRP and OSPF

15.Of the terms classful routing, classful routing protocol, classless routing, and classless routing protocol, which describe a feature that affects when a router uses the default route?

Answer: Classful routing and classless routing define rules by which a router chooses whether to use the default route.

16.What allows for the successful use of a discontiguous Class A, B, or C IP network— classful routing, classful routing protocol, classless routing, or classless routing protocol?

Answer: Whether a routing protocol is or is not classless defines whether it can support discontiguous networks. Only a classless routing protocol supports discontiguous networks.

17.Compare and contrast route summarization and autosummarization.

Answer: Route summarization allows an engineer to choose a summary route to configure, advertising the summary as opposed to the more specific routes. Autosummarization summarizes only a Class A, B, or C network number, and only at the boundary between that network and another network.

18.Of the routing protocols RIP-1, IGRP, EIGRP, and OSPF, which use autosummarization by default and also cannot have autosummarization disabled?

Answer: RIP-1 and IGRP