HP HPE FlexNetwork 7500 series, Configuration Manual

Page: 37 / 592

25
Configuring RIP
Overview
Routing Information Protocol (RIP) is a distance-vector IGP suited to small-sized networks. It
employs UDP to exchange route information through port 520.
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 value range of the metric 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 routes through the interface where the routes were
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. The neighbor can delete such information from its
routing table to prevent routing loops.
RIP operation
RIP works as follows:
1. RIP sends request messages to neighboring routers. Neighboring routers return response
messages that contain their routing tables.
2. RIP uses the received responses to update the local routing table and sends triggered update
messages to its neighbors. All RIP routers on the network do this to learn latest routing
information.
3. RIP periodically sends the local routing table to its neighbors. After a RIP neighbor receives the
message, it updates its routing table, selects optimal routes, and sends an update to other
neighbors. RIP ages routes to keep only valid routes.