R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide

20
Configuring RIP
Routing Information Protocol (RIP) is a distance-vector IGP suited to small-sized networks. It employs UDP
to exchange route information through port 520.
Overview
RIP uses a hop count to measure the distance to a destination. The hop count from a router to a directly
connected network is 0. The hop count from a router to a directly connected router is 1. To limit
convergence time, RIP restricts the metric range from 0 to 15. A destination with a metric value of 16 (or
greater) is considered unreachable. For this reason, RIP is not suitable for large-sized networks.
RIP route entries
RIP stores routing entries in a database. Each routing entry contains the following elements:
• Destination address—IP address of a destination host or a network.
• Next hop—IP address of the next hop.
• Egress interface—Egress interface of the route.
• Metric—Cost from the local router to the destination.
• Route time—Time elapsed since the last update. The time is reset to 0 when the routing entry is
updated.
• Route tag—Used for route control. For more information, see "Configuring routing policies."
Routing loop prevention
RIP uses the following mechanisms to prevent routing loops:
• Counting to infinity—A destination with a metric value of 16 is considered unreachable. When a
routing loop occurs, the metric value of a route will increment to 16 to avoid endless looping.
• Triggered updates—RIP immediately advertises triggered updates for topology changes to reduce
the possibility of routing loops and to speed up convergence.
• Split horizon—Disables RIP from sending routing information on the interface from which the
information was learned to prevent routing loops and save bandwidth.
• Poison reverse—Enables RIP to set the metric of routes received from a neighbor to 16 and sends
these routes back to the neighbor so the neighbor can delete such information from its routing table
to prevent routing loops.