C.4 RIP
Router implements the Routing Information Protocol (RIP) in order to
propagate routing information throughout the network. The protocol is a
straightforward implementation of vector-distance routing for local
networks that partitions participants into active and passive (silent)
machines. Active routers advertise their routes to others; passive
machines listen and update their routes based on advertisements, but do
not advertise.
A router running RIP will broadcast a message every 30 seconds to its
neighbouring routers containing information taken from the router's
current routing database. Each message consists of pairs, where each pair
contains an IP network address and an integer distance to that network.
RIP uses a hop count metric to measure the distance to a destination. In
the RIP metric, a router is defined to be one hop from the directly-
connected network, two hops from networks that are reachable through
one other router, and so on. Thus, the number of hops along a path from a
given source to a given destination refers to the number of routers that a
datagram encounters along that path. When a router receives a broadcast
from another device across a directly-connected network, the advertised
routes are compared to the existing routes held in the router. The
advertised routes are only installed in the current routing table if they
offer a smaller metric to the same destination, or if no route to the
advertised destination exists. If an advertised route is offered via a router
that is already being used as the next hop to that destination, the
advertised metric is used regardless of size. In order to detect faults in the
network, RIP specifies that routes that are not advertised for 180 seconds
become invalid. Therefore, if a router that is being used as a next hop fails,
it will be detected within a period of 180 seconds, and an alternative route
will be found if possible. If no alternative routes exist, the router will be
forced to use the default route.
X870-300351 Issue 1 C-9 Rev.0
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