Nortel Networks NN47200-503 manual An Introduction to IP Routing Protocols

24An Introduction to IP Routing Protocols

3.The neighbors will send their routing tables and the new router will update its routing table based on the advertisements received.

4.From now on periodic updates are send by each router in the network to ensure a correct routing database.

If a router does not receive an update from another router within a timeout period, it deletes the routes served by the nonupdating router from its routing table. However, it keeps these routes temporarily in a garbage list and continues to advertise them with a metric of 16 for a holddown period, so that neighbors know that the routes are unreachable. If a valid update for a garbage route is received within the holddown period, the router adds the route back into its routing table. If no update is received, the router completely deletes all garbage list entries for the nonupdating router.

To prevent routing loops and to promote fast convergence, RIP uses the mechanisms of split horizon, with or without poisoned reverse, and triggered updates. Simple split horizon means that IP routes learned from a neighbor are not advertised back in updates to that neighbor. Split horizon with poisoned reverse means that these routes are advertised back to the neighbor, but they are “poisoned” with a metric of 16, which represents infinite hops in the network. The receiver neighbor therefore ignores this route. Triggered updates means that a router is required to send update messages whenever it changes the metric for a route, even if it is not yet time for a regular update message.

RIP sends routing information updates every 30 seconds. These updates contain information about known networks and the distances (hop count) associated with each. For RIP version 1, no mask information is exchanged; the natural mask is always applied by the router receiving the update. Mask information is always included for RIP version 2.

If information about a network is not received for within the allotted timeout period (180 seconds by default), it is removed from the routing table and the route is moved to the garbage list . From the garbage list it will be advertised for the allotted holdown period (120 seconds by default) with metric set to infinity (16). These timers can be changed by configuring the RIP Interface Timeout Timer and Holddown Timer parameters.

RIP supports the following standard behavior:

•periodic RIP updates about effective best routes

•garbage collection

•split horizon with or without poisoned reverse

•triggered update for changed RIP routes

•unicast to the specific query requestor

•broadcast/multicast of regular and triggered updates

Nortel Ethernet Routing Switch 5500 Series

Configuration-IP Routing Protocols

NN47200-503 03.01 Standard

5.127 August 2007

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