Cisco Systems CCNA 2 manual Purpose of a routing protocol and autonomous systems

At this point, students do not have to understand the details of an autonomous system. They just need to understand the basic concepts of an autonomous system. Students do not have enough experience to understand policy-based routing.

6.2.3 Purpose of a routing protocol and autonomous systems

The goal of a routing protocol is to fill the routing table with known networks or destinations and the best route to reach these destinations. Although routers can forward packets without a routing protocol configured, using a protocol allows for dynamic updates. The router can be configured with static routes. When static routes are used, the administrator must configure a route for each network. Instruct the students to think of all the networks on the Internet and the different paths to each network. Then instruct the students to think about how fast the Internet changes. A routing protocol will dynamically learn routes to all networks even when the paths change.

The router knowledge needs to reflect an accurate, consistent view of the topology. This view is called convergence. When all routers in an internetwork use the same knowledge, the internetwork is said to have converged. This means all the routers have agreed on the reachable networks.

The purpose of autonomous systems is to segregate the entire network into administrations. If all the routers needed to communicate with all other routers on the Internet, each router would have a tremendous number of routes and would use large amounts of bandwidth to share the routes with the other routers. This is referred to as overhead for the routers. More overhead will increase hardware requirements. When a network is divided into autonomous systems, only the routers inside the local AS receive details about the routing information. Routers in other autonomous systems only need a summary of the routing information. This reduces the number of routes and the amount of routing information that has to be shared, which reduces router overhead. It also improves network stability since routing updates that are caused by topology changes do not have to be shared outside of the local AS. Some routing protocols can be used divide an AS into smaller units to provide the same benefits.

6.2.4 Identifying the classes of routing protocols

Most routing algorithms can be classified as one of three basic algorithms:

•Distance vector

•Link state

•Balanced hybrid

Routers will determine which route to take to a given network based on the type of algorithm that is used. Each of the three types has advantages and disadvantages.

6.2.5 Distance vector routing protocol features

Distance vector routing algorithms are used to send periodic copies of a routing table. Each router receives a routing table from its directly-connected neighboring routers. RIP sends its entire table every 30 seconds and IGRP sends its entire table every 90 seconds. The algorithm eventually accumulates network distances so that it can maintain a database of

70 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 6 Copyright ♥ 2004, Cisco Systems, Inc.