Cisco Systems CCNA 2 6.3 Routing Protocols Overview

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.

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.