PLANET XGS3-24042 - Manual (Page 283)

36-1
Chapter 36 RIP
36.1 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 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 packets by packets broadcast, subnet
mask and authentication is not supported. Some fields in the RIP-I packets are not used and are required to
be all 0’s; for this reason, such all 0's fields should be checked when using RIP-I, the RIP-I packets should be
discarded if such fields are non-zero. RIP-II is a more improved version than RIP-I. RIP-II sends route update
packets by multicast packets (multicast address is 224.0.0.9). Subnet mask field and RIP authentication filed