Foundation Summary 245

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

■Route summarization—Route summarization reduces the size of the network’s routing tables by causing a number of more specific routes to be replaced with a single route that includes all the IP addresses covered by the subnets in the original routes.

■Variable-length subnet masking—VLSM occurs when more than one mask is used in a single Class A, B, or C network. Although route summarization causes more than one mask to be used, requiring support for VLSM, you can also simply design a network to use multiple subnet masks.

Table 7-4 lists the routing protocols and their support (or nonsupport) of VLSM.

Table 7-4 Interior IP Routing Protocol VLSM Support

The following list describes a generalized process by which you can summarize a group of subnets into one summary route. This process attempts to find the “best” summary that includes all subnets, as opposed to finding all summary routes that include all subnets:

Step 1 Find the longest part of the subnet numbers that are identical, moving left to right. (For our purposes, consider this first part the “in common” part.)

Step 2 The summary route’s subnet number has the same value in the “in common” part of the summarized subnets and binary 0s in the second part.

Step 3 The subnet mask for the summary route has binary 1s in the “in common” part and binary 0s in the rest of the mask.

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Step 4 Check your work by calculating the range of valid IP addresses implied by the new summary route, comparing the range to the summarized subnets. The new summary should encompass all IP addresses in the summarized subnets.

Table 7-5 lists the routing protocols, whether they transmit mask information, support VLSM, and are classless or classful.

Table 7-5 Interior IP Routing Protocol: Classless or Classful?

■Classless and classful routing protocols—With classful routing protocols, the routing protocol must consider class rules; classless routing protocols do not. Specifically, classful routing protocols must automatically summarize routing information at network boundaries, meaning that they cannot support discontiguous networks.

Classful routing protocols also cannot support VLSM. Classless routing protocols can support discontiguous networks, and support VLSM.

■Classless and classful routing—With classful routing, the only time the default route is used is when a packet’s destination Class A, B, or C network number is not in the routing table. With classless routing, the default is used whenever the packet does not match a more specific route in the routing table.

Q&A 247

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

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

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?

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?

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

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?

6.Which interior IP routing protocols support route summarization?

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?

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.

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?

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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?

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?

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

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

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

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?

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?

17.Compare and contrast route summarization and autosummarization.

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

19.What command switches a router from classless routing to classful routing?

This chapter covers the following subjects:

■Scaling the IP Address Space for the Internet

■Miscellaneous TCP/IP Topics