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C H A P T E R14
Introduction to Dynamic Routing Protocols
The United States Postal Service routes a huge number of letters and packages each day. To do so, the postal sorting machines run fast, sorting lots of letters. Then the letters are placed in the correct container and onto the correct truck or plane to reach the final destination. However, if no one programs the letter-sorting machines to know where letters to each ZIP code should be sent, the sorter can’t do its job. Similarly, Cisco routers can route many packets, but if the router doesn’t know any routes, it can’t do its job.
This chapter introduces the basic concepts behind IP routing protocols and lists some of the key features of each of the IP routing protocols covered on the INTRO exam. Cisco expects CCNAs to demonstrate a comfortable understanding of the logic behind the routing of packets and the different but related logic behind routing protocols—the protocols used to discover routes. To fully appreciate the nuances of routing protocols, you need a thorough understanding of routing—the process of forwarding packets. You might even want to review the section “IP Routing and Routing Protocols,“ in Chapter 5, “Fundamentals of IP,“ for a review of routing, before proceeding with this chapter.
For those of you studying for the CCNA exam, if you are following the reading plan outlined in the introduction, you will move to the CCNA ICND Exam Certification Guide after this chapter. For those of you studying just for the INTRO exam, this chapter completes the coverage of topics related to IP and IP routing.
“Do I Know This Already?“ Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The eight-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 14-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?“ quiz questions that correspond to those topics.
404 Chapter 14: Introduction to Dynamic Routing Protocols
Table 14-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundations Topics Section
Routing Protocol Overview
Questions Covered in This Section
1–8
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 this question wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security.
1. Which of the following routing protocols are considered to use distance vector logic?
a. RIP b. IGRP
c. EIGRP d. OSPF e. BGP
2. Which of the following routing protocols are considered to use link-state logic?
a. RIP V1 b. RIP V2 c. IGRP d. EIGRP e. OSPF
f. BGP
g. Integrated IS-IS
3. Which of the following routing protocols use a metric that is, by default, at least partially affected by link bandwidth?
a. RIP V1 b. RIP V2 c. IGRP
“Do I Know This Already?“ Quiz 405
d. EIGRP e. OSPF
f. BGP
g. Integrated IS-IS
4. Which of the following interior routing protocols support VLSM?
a. RIP V1 b. RIP V2 c. IGRP d. EIGRP e. OSPF
f. Integrated IS-IS
5. Which of the following situations would cause RIP to remove all the routes learned from a particular neighboring router?
a. Keepalive failure
b. No longer receiving updates from that neighbor
c. Updates received 5 or more seconds after the last update was sent to that neighbor d. Updates from that neighbor have the global “route bad“ flag
6. Which of the following interior routing protocols are considered to be capable of converging quickly?
a. RIP V1 b. RIP V2 c. IGRP d. EIGRP e. OSPF
f. Integrated IS-IS
406 Chapter 14: Introduction to Dynamic Routing Protocols
7. Which of the following interior routing protocols use hop count as their metric?
a. RIP V1 b. RIP V2 c. IGRP d. EIGRP e. OSPF
f. Integrated IS-IS
8. What update timer is used by IGRP?
a. 5 seconds b. 10 seconds c. 30 seconds d. 60 seconds e. 90 seconds
f. None of the above
The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:
6 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections and the Q&A section.
7 or 8 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the Q&A section. Otherwise, move to the next chapter.
Routing Protocol Overview 407
Foundation Topics
To pass the INTRO exam, you need to know some basic information about several IP routing protocols. For the ICND exam, you will need to know distance vector concepts, as well as how to configure two distance vector IP routing protocols—the Routing Information Protocol (RIP) and the Interior Gateway Routing Protocol (IGRP). You will also need to know the concepts behind Enhanced IGRP (EIGRP), as well as Open Shortest Path First (OSPF)—two other IP routing protocols.
This chapter provides overview of routing protocols and the underlying logic used by these protocols.
Routing Protocol Overview
IP routing protocols have one primary goal—to fill the IP routing table with the current best routes it can find. The goal is simple, but the process and options can be complicated.
Terminology can get in the way when you’re learning about routing protocols. This book’s terminology relating to routing and routing protocols is consistent with the authorized Cisco courses, as well as with most Cisco documentation. So, just to make sure you have the terminology straight before diving into the details, a quick review of a few related terms might be helpful:
A routing protocol fills the routing table with routing information. Examples include RIP and IGRP.
A routed protocol is a protocol with OSI Layer 3 characteristics that define logical addressing and routing. The packets defined by the network layer (Layer 3) portion of these protocols can be routed. Examples of routed protocols include IP and IPX.
The term routing type has been used in other Cisco courses, so you should also know this term. It refers to the type of routing protocol, such as link-state or distance vector.
IP routing protocols fill the IP routing table with valid, (hopefully) loop-free routes. Although the primary goal is to build a routing table, each routing protocol has a very important secondary goal of preventing loops. The routes added to the routing table include a subnet number, the interface out which to forward packets so that they are delivered to that subnet, and the IP address of the next router that should receive packets destined for that subnet (if needed).
An analogy about routing protocols can help. Imagine that a stubborn man is taking a trip to somewhere he has never been. He might look for a road sign referring to the destination town and pointing him to the next turn. By repeating the process at each intersection, he eventually should make it to the correct town. Of course, if a routing loop occurs (in other words, he’s lost!) and he stubbornly never asks for directions, he could drive around forever—or at least until he runs out of gas. In this analogy, the guy in the car is like a routed
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