Switch User Manual

24 An 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 per iodic 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 per iod, 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
5.1 27 August 2007
Copyright © 2005-2007, Nortel Networks
.