A P P E N D I X D

Comparisons of 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.

Chapters 5, “RIP, IGRP, and Static Route Concepts and Configuration”, and 6, “OSPF and EIGRP Concepts and Configuration”, in this book cover the details of four different routing protocols – RIP, IGRP, EIGRP, and OSPF. However, the ICND exam lists one exam topic that implies that you should be able to pick the right routing protocol, given a set of requirements. So, this appendix contains an excerpt from the INTRO Exam Certification Guide that lists some definitions, lists comparisons of the routing protocols, as well as giving a few insights into two other IP routing protocols.

For those of you who have a copy of the INTRO exam certification guide: all the information in this chapter is taken from that book, specifically from chapter 14.

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.

568Appendix D: Comparisons of Dynamic Routing Protocols

■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 protocol—it travels through the network from the source to the destination. The routing protocol is like the fellow whose job it is to decide what to paint on the various road signs. As long as all the road signs have correct information, the guy in the car should make it to the right town just by reading the road signs. Likewise, as long as the routing protocol puts the right routes in the various routing tables, the routers should deliver packets successfully.

All routing protocols have several general goals, as summarized in the following list:

■To dynamically learn and fill the routing table with a route to all subnets in the network.

■If more than one route to a subnet is available, to place the best route in the routing table.

■To notice when routes in the table are no longer valid, and to remove those routes from the routing table.

■If a route is removed from the routing table and another route through another neighboring router is available, to add the route to the routing table. (Many people view this goal and the preceding one as a single goal.)

■To add new routes, or to replace lost routes with the best currently available route, as quickly as possible. The time between losing the route and finding a working replacement route is called convergence time.

■To prevent routing loops.

So, all routing protocols have the same general goals. Cisco IOS Software supports a large variety of IP routing protocols. IP’s long history and continued popularity have resulted in the specification and creation of several different competing routing protocol options. So, classifying IP routing protocols based on their differences is useful.

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Table D-1 Major Comparison Points Between Interior Routing Protocols (Continued)

You will see lots of coverage for most of the IP routing protocols in chapters 5 and 6. For the few that are not covered in depth on the ICND exam, the next few sections outline the basics..

Routing Through the Internet with the Border Gateway Protocol

ISPs use BGP today to exchange routing information between themselves and other ISPs and customers. Whereas interior routing protocols might be concerned about advertising all subnets inside a single organization, with a large network having a few thousand routes in the IP routing table, exterior routing protocols try to make sure that advertising routes reach every organization’s network. Exterior routing protocols also deal with routing tables that, with a lot of work done to keep the size down, still exceed

100,000 routes.

BGP advertises only routing information to specifically defined peers using TCP. By using TCP, a router knows that any routing updates will be re-sent if they happen to get lost in transit.

BGP uses a concept called autonomous systems when describing each route. An autonomous system (AS) is a group of devices under the control of a single organization—in other words, that organization has autonomy from the other

Routing Protocol Overview 571

interconnected parts of the Internet. An AS number (ASN) is assigned to each AS, uniquely identifying each AS in the Internet. BGP includes the ASNs in the routing updates to prevent loops. Figure D-1 shows the general idea.

Figure D-1 BGP Uses ASNs to Prevent Routing Loops

B

Enterprise B –

ASN 22

Notice that in the figure, the BGP updates sent to each successive AS show the ASNs in the route. When R1 receives the BGP update from R4, it notices that its own ASN in found inside the AS path and ignores that particular route.

BGP does not use a metric like internal routing protocols. Because BGP expects to be used between different ISPs and between ISPs and customers, BGP allows for a very robust set of alternatives for deciding what route to use; these alternatives are called policies. Routing policy can be based on the fact that an ISP might have a better business relationship with a particular ISP. For instance, in Figure D-1, packets from Enterprise B toward Enterprise A can take the “high” route (from ASN 3, to ASN 2, and then to ASN 1) if ISP3 has a better business relationship with ISP2, as compared with ISP4.