Samsung Ubigate iBG3026, Configuration Manual, Page: 134 / 718

CHAPTER 13. RIP
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When a router receives a routing update that contains a new or changed
destination network entry, the router adds 1 to the metric value indicated in the
update and enters the network in the routing table. The IP address of the
sender is used as the next hop.
RIP prevents routing loops from continuing indefinitely by implementing a
limit on the number of hops allowed in a path from the source to a destination.
The maximum number of hops in a path is 15. If a router receives a routing
update that contains a new or changed entry, and if increasing the metric value
by 1 causes the metric to be infinity(that is, 16), the network destination is
considered unreachable. The downside of this stability feature is that it limits
the maximum diameter of a RIP network to less than 16 hops.
RIP includes a number of other stability features that are common to many
routing protocols. These features are designed to provide stability despite
potentially rapid changes in a network's topology. For example, RIP
implements the split horizon and hold down mechanisms to prevent incorrect
routing information from being propagated.
Split horizon helps reduce convergence time by not allowing routers to
advertise networks in the direction from which those networks were learned.
The only information sent in RIP announcements are for those networks that
are beyond the neighboring router in the opposite direction. Networks learned
from the neighboring router are not included. Split horizon eliminates count-
to-infinity and routing loops during convergence in single-path internetworks
and reduces the chances of count-to-infinity in multi-path internetworks.
Split horizon with poison reverse differs from simple split horizon because it
announces all networks. However, those networks learned in a given direction
are announced with a hop count of 16, indicating that the network is
unreachable. In a single-path internetwork, split horizon with poison reverse
has no benefit beyond split horizon. However, in a multipath internetwork,
split horizon with poison reverse greatly reduces count-to-infinity and routing
loops. Split horizon with poison reverse does have the disadvantage of
additional RIP message overhead because all networks are advertised.
RIP uses numerous timers to regulate its performance. These include a
routing-update timer, a route-timeout timer, and a route-flush timer.
The routing-update timer clocks the interval between periodic routing updates.
Generally, it is set to 30 seconds, with a small random amount of time added
whenever the timer is reset.