RIP
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39. RIP
39.1
Introduction to RIP
RIP is first introduced in ARPANET, this is a protocol dedicated to small, simple
networks. RIP is a distance vector routing protocol based on the Bellman-Ford algorithm.
Network devices running vector routing protocol send two kind of information to the
neighboring devices regularly:
• Number of hops to reach the destination network, or metrics to use or number of
networks to pass.
• What is the next hop, or the director (vector) to use to reach the destination network.
The distance vector Layer 3 switch send all their route selecting tables to the
neighbor layer3 switches at regular interval. A layer3 switch will build their own route
selecting information table based on the information received from the neighbor layer3
switches. Then, it will send this information to its own neighbor layer3 switches. As a result,
the route selection table is built on second hand information, route beyond 15 hops will be
deemed as unreachable.
RIP protocol is an optional routing protocol based on UDP. Hosts using RIP send and
receive packets on UDP port 520. All layer3 switches running RIP send their route table to
all neighbor layer3 switches every 30 seconds for update. If no information from the
partner is received in 180 seconds, then the device is deemed to have failed and the
network connected to that device is considered to be unreachable. However, the route of
that layer3 switch will be kept in the route table for another 120 seconds before deletion.
As layer3 switches running RIP built route table with second hand information, infinite
count may occur. For a network running RIP routing protocol, when an RIP route becomes
unreachable, the neighboring RIP layer3 switch will not send route update packets at once,
instead, it waits until the update interval timeout (every 30 seconds) and sends the update
packets containing that route. If before it receives the updated packet, its neighbors send
packets containing the information about the failed neighbor, ―infinite count‖ will be
resulted. In other words, the route of unreachable layer3 switch will be selected with the
metrics increasing progressively. This greatly affects the route selection and route
aggregation time.
To prevent ―infinite count‖, RIP provides mechanism such as ―split horizon‖ and
―triggered update‖ to solve route loop. ―Split horizon‖ is done by avoiding sending to a
gateway routes leaned from that gateway. There are two split horizon methods: ―simple
split horizon‖ and ―poison reverse split horizon‖. Simple split horizon deletes from the