User's Manual

37-1
Chapter 37 RIPng
37.1 Introduction to RIPng
RIPng is first introduced in ARPANET; this is a protocol dedicated to small, simple networks. RIPng is a
distance vector routing protocol based on the Bellman-Ford algorithm. Network devices running vector routing
protocol send 2 kinds 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.
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 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.
RIPng is an optional routing protocol based on UDP. Hosts using RIPng 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 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 RIPng build route table with second hand information, infinite count may occur.
For a network running RIPng routing protocol, when a RIPng route becomes unreachable, the neighboring
RIPng 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 avoid “infinite count”, RIPng 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 other than wait for the 30 sec timer.
So far the RIPng protocol has got only one version -- Version1: RIPng protocol is introduced in RFC 2080.
RIPng transmits updating data packet by multicast data packet (multicast address FF02::9)