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42C4911, January 2007 155
CHAPTER 9
Routing Information Protocol
In a routed environment, routers communicate with one another to keep track of available
routes. Routers can learn about available routes dynamically using the Routing Information
Protocol (RIP). Alteon OS software supports RIP version 1 (RIPv1) and RIP version 2 (RIPv2)
for exchanging TCP/IP route information with other routers.
Distance Vector Protocol
RIP is known as a distance vector protocol. The vector is the network number and next hop,
and the distance is the cost associated with the network number. RIP identifies network reach-
ability based on cost, and cost is defined as hop count. One hop is considered to be the distance
from one switch to the next which is typically 1. This cost or hop count is known as the metric.
When a switch receives a routing update that contains a new or changed destination network
entry, the switch 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.
Stability
RIP includes a number of other stability features that are common to many routing protocols.
For example, RIP implements the split horizon and hold-down mechanisms to prevent incor-
rect routing information from being propagated.
RIP prevents routing loops from continuing indefinitely by implementing a limit on the num-
ber of hops allowed in a path from the source to a destination. The maximum number of hops
in a path is 15. The network destination network is considered unreachable if increasing the
metric value by 1 causes the metric to be 16 (that is infinity). This limits the maximum diame-
ter of a RIP network to less than 16 hops.
RIP is often used in stub networks and in small autonomous systems that do not have many
redundant paths.