Cisco Systems CCNA 2 manual Routing Protocols Overview, Link-state routing protocol features

network topology information. This is measured in hop counts, or the number of routers in the path to a destination network.

Distance vector algorithms do not allow a router to know the exact topology of an internetwork. The router only uses hop count to determine the best path. Distance vector algorithms require each router to send its entire routing table to each of its neighbors. This creates network traffic and there is a limit to the number of hops a distance vector routing protocol will use. The RIP maximum hop count is 15 and IGRP is 255. Explain that distance vector routing protocols use the view of neighboring routers to develop their view of the internetwork. The router will use copies of neighboring routing tables to build its routing table.

6.2.6 Link-state routing protocol features

The second basic algorithm used for routing is the link-state algorithm. Link-state algorithms are also known as Dijkstras algorithms.

Link-state routing uses the following:

•A topological database

•The SPF algorithm and the resulting SPF tree

•A routing table of paths and ports to each network

•A link-state advertisement (LSA), which is a small packet sent between routers that contains link information

Link-state routing requires more memory. Routers send updates when there is a change in the table. There is less network traffic because the routers are not sending updates every 30 or 90 seconds. The routers in an area elect a Designated Router (DR) and a Backup Designated Router (BDR). When a change is made in the network, the router that notices the change sends an update to the DR. When an update occurs, only the change is sent instead of the entire routing table. The DR then sends the network change to all routers in the area with a multicast.

An important concept to mention is that routers that use a link-state routing protocol develop a common view of the internetwork. A link-state protocol collects links from neighboring routers to create a routing table. Students also need to understand that the updates from the routers contain information about the links. These links can be locally connected or received from other routers. Students also need to know that the updates are partial updates.

6.3 Routing Protocols Overview

Essential Labs:	None
Optional Labs:	None
Core TIs:	All
Optional TIs:	none

Course-Level Claim: Students can evaluate the characteristics of routing protocols.

71 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 6 Copyright ♥ 2004, Cisco Systems, Inc.