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Chapter 36 RIPng
36.1 Introduction to RIPng
RIP ng is first introduc ed in ARPA NET, this is a protocol dedicated to small, simple networks. RIP ng is a
distance vector routing protocol based on the Bellman-Ford algorithm. Net work devices running vector routing
protocol send 2 kind of information to the neighboring devices regularly:
• Number of hops to reach the destination network, or met rics to use or number of networks to pass.
• What is the next hop, or the director (vector) to use to reac h the destination network.
Distance vector layer3 switches 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 lay er3 switches. Then, it will send this information to its own neighbor layer3 switches. As a
result, the route selection table is built on sec ond hand information, route beyond 15 hops will be deemed as
unreachable.
RIP ng is an optional routing protocol bas ed on UDP. Hosts using RIP ng send and receive packets on UDP
port 521. 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 net work connected to that device is considered t o 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 ng build route table wit h second hand information, infinite count may occur.
For a network running RIPng routing protoc ol, when a RIPng route becomes unreachable, the neighboring
RIP ng layer3 switch will not send rout e 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 met rics
increasing progressively. This greatly affects the route selection and rout e aggregation time.
To avoid "infinit e count", RIP ng provides mechanism such as "split horizon" and "triggered updat e" to solve
route loop. "Split horiz on" 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
infinit e. "Triggering update" mechanism defines whenever rout e metric changed by the gateway, the gateway
advertise the update packets immediately other than wait for the 30 sec timer.
So far the RIP ng protocol has got only one version----Version1: RIPng prot ocol is introduced in RFC 2080.
RIP ng transmits updating data packet by multicast data packet (multicast address FF02::9)
Each layer3 switch running RIPng has a rout e database, which contains all route ent ries for reachable
destination, and route table is built based on this databas e. When a RIPng layer3 switch sent route update
packets to its neighbor devices, the complete route table is included in the packets. Therefore, in a large
network, routing data to be trans ferred and processed for each layer3 switch is quite large, causing degraded
network performance.
36-1
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Troubleshooting
