Chapter Glossary

This glossary provides an official Cisco definition for key words and terms introduced in this chapter. I have supplied my own definition for terms that the Cisco glossary does not contain. The words listed here are identified in the text by italics. A complete glossary, including all the chapter terms and additional terms, can be found in Appendix C, "Glossary."

ACK—A hello packet with no data that is an acknowledgment of packets sent reliably.

active—Route state in which when a network change is seen, but on interrogation of the topology table, there is no FC. The router queries its neighbors for alternative routes.

advertised distance (AD)—The cost of the path to the remote network from the neighbor (the metric from the next-hop router).

Diffusing Update Algorithm (DUAL)—An algorithm performed on the topology table to converge the network. It is based on a router detecting a network change within a finite time, with the change being sent reliably and in sequence. Because the algorithm is calculated simultaneously, in order and within a finite time frame on all effected routers, it ensures a loopfree network.

feasible condition (FC)—When a neighbor reports a path (AD) that is lower than the router's FD to a network, the neighbor's (next-hop router's) path has a lower metric than the router's path.

feasible distance (FD)—The lowest-cost distance (metric) to a remote network.

feasible successor (FS)—The neighbor reporting the AD that is lower than the router's FD becomes the feasible successor. This is the next-hop router that meets the FC.

hello—Used to find and maintain neighbors in the topology table. They are sent periodically and are sent reliably.

holdtime—Sent in the hello packet. It determines how long the router waits for hellos from a neighbor before declaring it unavailable. This information is held in the neighbor table.

neighbor—A router running EIGRP that is directly connected.

neighbor table—A list of every neighbor, including the IP address, the outgoing interface, the holdtime, the SRTT, and the uptime, or how long since the neighbor was added to the table. The table is built from information on hellos received from adjacent routers (neighbors).

passive—An operational route is passive. If the path is lost, the router examines the topology table to find an FS. If there is an FS, it is placed in the routing table, and the router does not query the others, which would send it into active mode.

query—Message sent from the router when it loses a path to a network. If there is no alternate route (feasible successor), the router will send out queries to neighbors inquiring whether they have a feasible successor. This makes the route state change to active. The queries are sent reliably.

query scoping—Another term for SIA.

Reliable Transport Protocol (RTP)—Requires that the packets be delivered in sequence and be guaranteed.

reply—A response to the query. If a router has no information to send in a reply, it will send queries to all its neighbors. A unicast is sent reliably.

Retransmission Timeout (RTO)—Timer that is calculated in reference to the SRTT. RTO determines how long the router waits for an ACK before retransmitting the packet.

route table—The routing table, or list of available networks and the best paths. A path is moved from the topology table to the routing table when a feasible successor is identified.

Smooth Round Trip Time (SRTT)—The time that the router waits after sending a packet reliably to hear an acknowledgment. This is held in the neighbor table and is used to calculate the RTO.

Stuck in Active (SIA)—State in which a router has sent out network packets and is waiting for ACKs from all its neighbors. The route is active until all the ACKs have been received. If they do not appear after a certain time, the router is Stuck in Active for the route.

successor—The next-hop router that passes the FC. It is chosen from the FSs as having the lowest metric to the remote network.

topology table—Table that contains all the paths advertised by neighbors to all the known networks. This is a list of all the successors, feasible successors, the feasible distance, the advertised distance, and the outgoing interface. DUAL acts on the topology table to determine successors and feasible successors by which to build a routing table.

update—An EIGRP packet containing change information about the network. It is sent reliably. It is sent only when there is a change in the network to affected routers:

• When a neighbor first comes up

• When a neighbor transitions from active to passive for a destination

• When there is a metric change for a destination


The following questions test your understanding of the topics covered in this chapter. The final questions in this section repeat of the opening "Do I Know This Already?" questions. These are repeated to enable you to test your progress. After you have answered the questions, find the answers in Appendix A. If you get an answer wrong, review the answer and ensure that you understand the reason for your mistake. If you are confused by the answer, refer to the appropriate text in the chapter to review the concepts.

1 If a router does not have a feasible successor, what action will it take?

2 When does EIGRP need to be manually redistributed into another EIGRP process?

3 Which timers are tracked in the neighbor table?

4 What is the difference between an update and a query?

5 When does EIGRP recalculate the topology table?

6 EIGRP has a default limit set on the amount of bandwidth that it can use for EIGRP packets. What is the default percentage limit?

7 State two rules for designing a scalable EIGRP network.

8 What is the preferred configuration for a hybrid multipoint NBMA network when one VC has a CIR of 56 kbps and the other five VCs each have a CIR of 256 kbps?

9 With four Frame Relay circuits in a multipoint solution and a bandwidth configuration of 224, what is the allocation per circuit, and where would the bandwidth command be configured?

10 Explain the purpose of the command no auto-summary.

11 Explain the meaning of the command ip bandwidth-percent eigrp 63 100.

12 EIGRP may be used to send information about which three routing protocols?

13 Which EIGRP packets are sent reliably?

14 In what instances will EIGRP automatically redistribute?

15 How long is the holdtime, by default?

16 What is an EIGRP topology table, and what does it contain?

17 What is the advertised distance in EIGRP, and how is it distinguished from the feasible distance?

18 What EIGRP algorithm is run to create entries for the routing table?

19 When does EIGRP place a network in active mode?

20 By default, EIGRP summarizes at which boundary?

21 What is Stuck in Active?

22 What is the variance command used for?

23 State two factors that influence EIGRP scalability.

24 What command is used to display which routes are in passive or active mode?

25 What command is used in EIGRP to perform manual summarization?

26 For Frame Relay, when would you configure the physical interface (as opposed to a subinterface) with the bandwidth command?

Which command is used to display all types of EIGRP packets that are both received and sent by a router?

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