SGS-6341-Series User Manual

26.4 RIP
26.4.1 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 Layer 3
switches at regular interval. A Layer 3 switch will build their own route selecting information
table based on the information received from the neighbor Layer 3 switches. Then, it will send
this information to its own neighbor Layer 3 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 Layer 3 switches running RIP send their route table to all
neighbor Layer 3 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 Layer 3 switch will
be kept in the route table for another 120 seconds before deletion.
As Layer 3 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 Layer 3 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 Layer 3 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 route to be sent to the
neighbor gateways the routes learnt from the neighbor gateways; poison reverse split horizon
not only deletes the abovementioned routes, but set the costs of those routes to infinite.
“Triggering update” mechanism defines whenever route metric changed by the gateway, the
gateway advertise the update packets immediately, regardless of the 30 second update timer
status.
There two versions of RIP, version 1 and version 2. RFC1058 introduces RIP-I protocol,
RFC2453 introduces RIP-II, which is compatible with RFC1723 and RFC1388. RIP-I updates
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User’s Manual of SGS-6341 series