Routing Information Protocol (RIP) is an IP route exchange protocol that uses a distance vector (a
number representing distance) to measure the cost of a given route. The cost is a distance vector
because the cost often is equivalent to the number of router hops between the Layer 3 Switch and
the destination network.

A Layer 3 Switch can receive multiple paths to a destination. The software evaluates the paths,
selects the best path, and saves the path in the IP route table as the route to the destination.
Typically, the best path is the path with the fewest hops. A hop is another router through which
packets must travel to reach the destination. If the Layer 3 Switch receives a RIP update from
another router that contains a path with fewer hops than the path stored in the Layer 3 Switch
route table, the Layer 3 Switch replaces the older route with the newer one. The Layer 3 Switch
then includes the new path in the updates it sends to other RIP routers, including Layer 3 Switches.

RIP routers, including the Layer 3 Switch, also can modify a route cost, generally by adding to it, to
bias the selection of a route for a given destination. In this case, the actual number of router hops
may be the same, but the route has an administratively higher cost and is thus less likely to be
used than other, lower-cost routes.

A RIP route can have a maximum cost of 15. Any destination with a higher cost is considered
unreachable. Although limiting to larger networks, the low maximum hop count prevents endless
loops in the network.

Layer 3 Switches support the following RIP versions:

•

Version 1

•

V1 compatible with V2

•

Version 2 (the default)

ICMP host unreachable message for undeliverable ARPs

If the router receives an ARP request packet that it is unable to deliver to the final destination
because of the ARP timeout and no ARP response is received (router knows of no route to the
destination address), the router sends an ICMP Host Unreachable message to the source.

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