Routing Information Protocol (RIP) version 1 is defined in RFC 1058. This is a distance vector routing protocol that uses hop count as a metric. The maximum hop count is 15, with 16 registering as unreachable. RIP is a classfull protocol and does not support VLSM or authentication. RIP routers send updates to their neighbors every 30 seconds. RIP was designed for smaller networks, usually implemented in UNIX environments.
RIP routers expect to see routes from their neighbors every 30 seconds. An expiration timer is implemented to set a route as an invalid route. For RIP, the invalid timer is 180 seconds (six times the update timer). Another timer is used to remove the route from the routing table. This is the flush timer and it is set to 240 seconds, by default. Although not in RFC 1058, Cisco implements a hold-down timer for RIP. When a change occurs on a route, a triggered update is sent and the route is placed in holddown—no changes are accepted until the hold-down timer expires. The default hold-down timer is 180 seconds (six times the update timer).
RIP employs algorithms to prevent routing loops. These algorithms include triggered updates and split horizon with poison reverse.
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