Document ID: RDWR-ALOS-V2900_AG1302
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Chapter 9 – Routing Information Protocol
This chapter discusses the Alteon implementation of the Routing Information Protocol (RIP). It
includes the following topics:
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Distance Vector Protocol, page 121
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RIP Configuration Example, page 124
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 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 reachability based
on cost, and cost is defined as the hop count. One hop is considered to be the distance from one
Alteon to the next, which is typically 1. This cost or hop count is known as the metric.
When Alteon receives a routing update that contains a new or changed destination network entry, it
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 stability features that are common to many routing protocols. For
example, RIP implements the split horizon and hold-down mechanisms to prevent incorrect routing
information.
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. 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 diameter 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.
Routing Updates
RIP sends routing update messages at regular intervals and when the network topology changes.
Each router "advertises" routing information by sending a routing information update every 30
seconds. If a router does not receive an update from another router for 180 seconds, the routes
provided by that router are declared invalid. After another 120 seconds without receiving an update
for those routes, the routes are removed from the routing table and respective regular updates.
