Chapter 2. Understanding Routing Information Protocol (RIP)

This chapter covers the following key topics about Routing Information Protocol (RIP):

• Metric

• Timers

• Split horizon

• Split horizon with poison reverse

• RIP-1 packet format

• RIP behavior

• Why RIP doesn't support discontiguous networks

• Why RIP doesn't support variable-length subnet masking (VLSM)

• Default routes and RIP

• Protocol extension to RIP

• Compatibility issues

RIP is a distance vector protocol that uses hop count as its metric. This protocol is very simple and was intended for small networks. RIP is similar to gated, which was distributed by the FreeBSD version of UNIX. Before the RFC for RIP Version 1 (RIP-1) was written, several versions of RIP were floating around.

NOTE

Hop count refers to the number of routers being traversed. For example, a hop count of 2 means that the destination is two routers away.

RIP is a classful protocol, which means that it doesn't carry subnet mask information in its routing update. Because it doesn't carry any subnet mask information, it is incapable of supporting variable-length subnet masking (VLSM) and discontiguous networks. RIP enables devices to exchange information about networks that they are directly connected to, as well as any other networks that they have learned from other RIP devices.

RIP sends its routing information every 30 seconds, which is the default update timer. This timer is configurable. The hold-down timer determines how long a router should wait before flushing the information from the routing table.

RFC 1058 was written to provide a standard for RIP, which uses the Bellman-Ford algo-rithm to compute its metric.