RIP v1 and IGRP


Many distance vector protocols employ periodic updates to exchange routing information with their neighbors and to maintain up-to-date routing information in the routing table. RIP and IGRP are examples of two such protocols.

In the animation, the routers are periodically sending the routing table to neighbors. The term periodic updates refers to the fact that a router sends the complete routing table to its neighbors at a predefined interval. For RIP, these updates are sent every 30 seconds as a broadcast (255.255.255.255) whether or not there has been a topology change. This 30-second interval is a route update timer that also aids in tracking the age of routing information in the routing table.

The age of routing information in a routing table is refreshed each time an update is received. This way information in the routing table can be maintained when there is a topology change. Changes may occur for several reasons, including:
Failure of a link
Introduction of a new link
Failure of a router
Change of link parameters
RIP Timers

In addition to the update timer, the IOS implements three additional timers for RIP:
Invalid
Flush
Holddown

Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires.

Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.
Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. By default, the holddown timer is set for 180 seconds. The holddown timer is discussed in more detail later in this chapter.
Unlike other distance vector routing protocols, EIGRP does not send periodic updates. Instead, EIGRP sends bounded updates about a route when a path changes or the metric for that route changes. When a new route becomes available or when a route needs to be removed, EIGRP sends an update only about that network instead of the entire table. This information is sent only to those routers that need it.

EIGRP uses updates that are:
Non-periodic because they are not sent out on a regular basis.
Partial updates sent only when there is a change in topology that influences routing information.
Bounded, meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated.
The timer values can be verified with two commands: show ip route and show ip protocols. Notice in the output from show ip route that each route learned through RIP shows the elapsed time since the last update, expressed in seconds.
This information is also repeated in the show ip protocols output under the heading Last Update. The show ip protocols command details when this router, R1, is due to send out its next round of updates. It also lists the invalid, holddown, and flush timer default values.

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