# Overview

RIP is an interior gateway protocol (IGP) typically used in small, homogeneous networks. RIP uses distance-vector routing to route information through IP networks.

Distance-vector routing requires that each router simply inform its neighbors of its routing table. For each network path, the receiving router picks the neighbor advertising the lowest metric, then adds this entry into its routing table for readvertisement.

Any host that uses RIP is assumed to have interfaces to one or more networks. These networks are considered to be directly connected networks. RIP relies on access to certain information about each of these networks. The most important information is the network’s metric.

## RIP Metric

RIP uses the hop count as the metric (also known as cost) to compare the value of different routes. The hop count is the number of routers that data packets must traverse between RIP networks. Metrics range from 0 for a directly connected network to 16 for an unreachable network. This small range prevents RIP from being useful for large networks.

## RIP Messages

RIP exchanges routing information via User Datagram Protocol (UDP) data packets. Each RIP router sends and receives datagrams on UDP port number 520, the RIP version 1/RIP version 2 port. All communications intended for another router’s RIP process area are sent from the RIP port.

Every RIP message contains a RIP header that consists of a command and a version number. The router supports RIP version 1 (RIPv1) and RIP version 2 (RIPv2) extensions.

RIP employs the following message types:

- Request—A request for the responding router to send all or part of its routing table.
- Response—A message containing all or part of the sender’s routing table. This message is sent in response to a request or is an unsolicited routing update generated by the sender.

The RIP request and response messages also contain a list of route entries. Each route entry contains the following:

- Address Entry Identifier—The type of address
- Destination IP address—The destination address of the message
- Cost to reach the destination—A value between 1 and 15, which specifies the current metric for reaching the destination