Chapter 3: Medium-Sized Routed Network Construction

Cisco Press

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Summary of Implementing Variable-Length Subnet Masks

The following list summarizes the key points discussed in this section:

  • Subnetting lets you efficiently allocate addresses by taking one large broadcast domain and breaking it up into smaller, more manageable broadcast domains.

  • VLSMs let you more efficiently allocate IP addresses by adding multiple layers of the addressing hierarchy.

  • The benefits of route summarization include smaller routing tables and the ability to isolate topology changes.

Chapter Summary

The list that follows summarizes the key points that were discussed in this chapter.

  • Routers gather and maintain routing information to enable the transmission and receipt of packets. Various classes of routing protocols contribute to the different features in each network. The Routing Information Protocol (RIP), the Enhanced Interior Gateway Routing Protocol (EIGRP), and the Open Shortest Path First (OSPF) routing protocol each provide different features and capabilities.

  • A distance vector routing algorithm sends its entire routing table to its neighbors. Link-state routing algorithms maintain a complex database of topology information, which routers use to maintain full awareness of distant routers.

  • OSPF is a classless, link-state routing protocol that is widely deployed in many networks. EIGRP is a classless routing protocol that behaves like a classful routing protocol by default.

  • You can further tune these routing protocols by implementing variable-length subnet masks (VLSMs) and route summarization. It is up to network administrators to be knowledgeable about each protocol so that they can implement the most appropriate routing protocol based on the individual needs of their networks.

Review Questions

Use the questions here to review what you learned in this chapter. The correct answers and solutions are found in the appendix, "Answers to Chapter Review Questions."

  1. Which statement most accurately describes static and dynamic routes?

    1. Dynamic routes are manually configured by a network administrator, whereas static routes are automatically learned and adjusted by a routing protocol.

    2. Static routes are manually configured by a network administrator, whereas dynamic routes are automatically learned and adjusted by a routing protocol.

    3. Static routes tell the router how to forward packets to networks that are not directly connected, whereas dynamic routes tell the router how to forward packets to networks that are directly connected.

    4. Dynamic routes tell the router how to forward packets to networks that are not directly connected, whereas static routes tell the router how to forward packets to networks that are directly connected.

  2. Which of the following protocols is an example of an Exterior Gateway Protocol?

    1. RIP

    2. BGP

    3. OSPF

    4. EIGRP

  3. In which situation is an administrative distance required?

    1. Whenever static routes are defined

    2. Whenever dynamic routing is enabled

    3. When the same route is learned via multiple routing sources

    4. When multiple paths are available to the same destination, and they are all learned via the same routing protocol

  4. How does a distance vector router learn about paths for networks that are not directly connected?

    1. From the source router.

    2. From neighboring routers.

    3. From the destination router.

    4. A distance vector router can only learn directly connected networks.

  5. What does a distance vector router send to neighboring routers as part of a periodic routing-table update?

    1. The entire routing table

    2. Information about new routes

    3. Information about routes that have changed

    4. Information about routes that no longer exist

  6. With distance vector routing, count to infinity can be prevented by setting a maximum for what value?

    1. Metric

    2. Update time

    3. Hold-down time

    4. Administrative distance

  7. What does split horizon specify?

    1. That information about a route should not be sent in any direction

    2. That information about a route should not be sent back in the direction from which the original information came

    3. That information about a route should always be sent back in the direction from which the original information came

    4. That information about a route should be sent back only in the direction from which the original information came

  8. When a router sets the metric for a down network to the maximum value, what is it doing?

    1. Triggering the route

    2. Poisoning the route

    3. Applying split horizon

    4. Putting the route in holddown

  9. If a route for a network is in holddown and an update arrives from a neighboring router with the same metric as was originally recorded for the network, what does the router do?

    1. Ignores the update

    2. Increments the hold-down timer

    3. Marks the network as "accessible" and removes the hold-down timer

    4. Marks the network as "accessible" but keeps the hold-down timer on

  10. If a router has a network path in holddown and an update arrives from a neighboring router with a better metric than originally recorded for the network, what two things does it do? (Choose two.)

    1. Removes the holddown

    2. Continues the holddown

    3. Marks the route as "accessible"

    4. Marks the route as "inaccessible"

    5. Marks the route as "possibly down"

  11. How can link-state protocols limit the scope of route changes?

    1. By supporting classless addressing

    2. By sending the mask along with the address

    3. By sending only updates of a topology change

    4. By segmenting the network into area hierarchies

  12. What is the purpose of link-state advertisements?

    1. To construct a topological database

    2. To specify the cost to reach a destination

    3. To determine the best path to a destination

    4. To verify that a neighbor is still functioning

  13. What are two characteristics of OSPF? (Choose two.)

    1. Hierarchical

    2. Proprietary

    3. Open standard

    4. Similar to RIP

    5. Distance vector protocol

  14. OSPF routes packets within a single _____.

    1. Area

    2. Network

    3. Segment

    4. Autonomous system

  15. How many subnets are gained by subnetting 172.17.32.0/20 into multiple /28 subnets?

    1. 16

    2. 32

    3. 256

    4. 1024

  16. How many hosts can be addressed on a subnet that has seven host bits?

    1. 7

    2. 62

    3. 126

    4. 252

  17. How many hosts can be addressed with a prefix of /30?

    1. 1

    2. 2

    3. 4

    4. 30

  18. Which subnet mask would be appropriate for a Class C address used for 9 LANs, each with 12 hosts?

    1. 255.255.255.0

    2. 255.255.255.224

    3. 255.255.255.240

    4. 255.255.255.252

  19. How can you most effectively summarize the IP range of addresses from 10.1.32.0 to 10.1.35.255?

    1. 10.1.32.0/23

    2. 10.1.32.0/22

    3. 10.1.32.0/21

    4. 10.1.32.0/20

  20. How can you most effectively summarize the IP range of addresses from 172.168.12.0/24 to 172.168.13.0/24?

    1. 172.168.12.0/23

    2. 172.168.12.0/22

    3. 172.168.12.0/21

    4. 172.168.12.0/20

Copyright © 2007 Pearson Education. All rights reserved.

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