Chapter 3. EIGRP Principles

This chapter covers the following topics:

EIGRP Features and Advantages—Describes the key features and advantages of EIGRP, including neighborship and reliable incremental updates, neighbor discovery and recovery, the sophisticated metric used by EIGRP, DUAL, and queries.
Understanding EIGRP Tables—Describes neighbors, topology, and routing tables.
EIGRP Network Design—Describes the factors that affect the scaling of EIGRP.

Enhanced Interior Gateway Routing Protocol (EIGRP) is a distance vector protocol that uses the same sophisticated metric as IGRP and uses the Diffusing Update Algorithm (DUAL) convergence algorithm. EIGRP is able to converge quickly and use little bandwidth because it—like OSPF—has separate keepalives and reliable updates. EIGRP is sometimes referred to as a hybrid routing protocol, although advanced distance vector routing protocol is probably a more accurate description.

EIGRP is an efficient but proprietary solution for large networks. Its ability to scale is limited only by the design of the network.

This chapter covers EIGRP used with IPv4. You can find a short discussion of EIGRP support for IPv6 in Chapter 21, "IPv6 Routing Protocols, Configuration, and Transitioning from IPv4," but the concepts of EIGRP are essentially the same.

This chapter provides a theoretical description of how EIGRP works. The operation of EIGRP, available options, and design considerations are explained in this chapter, particularly in reference to scaling EIGRP and its use over a nonbroadcast multiaccess (NBMA) WAN environment.

The topics in this chapter directly reflect questions on the BSCI exam. EIGRP is designed for use in large networks. As a proprietary routing protocol for Cisco, it is therefore an obligatory subject in a Cisco exam on IP routing protocols.

The purpose of the "Do I Know This Already?" quiz is to help you decide which parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions.

The 13-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 3-1 outlines the major topics discussed in this chapter and the corresponding quiz questions.

Table 3-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics Section	Questions Covered in This Section	Score
EIGRP Features and Advantages	1–7
Understanding EIGRP Tables	8–12
EIGRP Network Design	13
Total Score

Caution

The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark the question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guessed skews your results and might provide you with a false sense of security.

1.	Which of the following describes how a reply to an EIGRP query is sent? Multicast Best effort unicast A reliable unicast A reliable multicast
2.	Which of the following are advantages to using EIGRP as a routing protocol instead of RIP? Rapid convergence Reduced bandwidth consumption Link state routing Increased vendor support
3.	Cisco identifies four main components of EIGRP. Select the two correct components from the following list. Neighbor discovery SPF algorithm Areas RTP
4.	Given the default constant values, which variables are included in the EIGRP metric? Bandwidth Load Reliability Delay
5.	For EIGRP to form a neighbor relationship, which of the following must be true? Authentication must be enabled. The K-values of the metric must be the same on both routers. The autonomous system number must be the same on both routers. The holddown timer must be the same.
6.	What do the letters SIA stand for? Stuck in Active Shortest IP Address Stuck in Area Simple IP Address
7.	What is the EIGRP reserved Multicast address? 224.0.0.1 224.0.0.5 224.0.0.9 224.0.0.10
8.	The neighbor table contains which of the following information? The administrative distance The metric of neighbors The feasible distance between neighbors The status of the links between neighbors
9.	What do the letters SRTT stand for? Shortest Remote-Trip Time Smooth Round-Trip Time Shortest Reliable-Trip Time Single Remote Timer Test

10.	Which of the following would trigger the topology table to recalculate? LSP received SRT packet received A new router coming online Link loss detected
11.	Feasible successor information is stored in which table? Topology Routing Neighbor Autonomous System
12.	What is EIGRP Active Mode? When the router is actively forwarding When the router is actively recalculating paths When the router is searching for replacement paths When the router is discovering neighbors
13.	The factors that can affect the scaling of EIGRP include which of the following? The amount of information sent between neighbors The number of routers that receive updates The distance between neighboring routers The number of alternative paths to remote networks

You can find the answers to the "Do I Know This Already?" quiz in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

9 or less correct—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.
10 or 11—Begin with the "Foundation Summary" section, and then go to the "Q&A" section at the end of the chapter. If you have trouble answering the Q&A questions, read the appropriate sections in "Foundation Topics."
12 or 13 overall score—If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. Otherwise, move to the next chapter.