IP Routing

Speeding Up Convergence

Like RIP, IGRP implements hold-downs, split horizon, triggered updates, and poison reverse (see Chapter 2 for details on these convergence methods). Like RIP, IGRP also maintains an update timer, an invalid timer, a hold-down timer, and a flush timer for every route in the routing table:

Update timer (default value: 90 seconds)

After sending a routing update, IGRP sets the update timer to 0. When the timer expires, IGRP issues another routing update.

Invalid timer (default value: 270 seconds)

Every time a router receives an update for a route, it sets the invalid timer to 0. The expiration of the invalid timer indicates that the source of the routing information is suspect. Even though the route is declared invalid, packets are still forwarded to the next hop specified in the routing table. Note that prior to the expiration of the invalid timer, IGRP would process any updates received by updating the route’s timers.

Hold-down timer (default value: 280 seconds)

When the invalid timer expires, the route automatically enters the hold-down phase. During hold-down all updates regarding the route are disregarded -- it is assumed that the network may not have converged and that there may be bad routing information circulating in the network. The hold-down timer is started when the invalid timer expires.

Flush timer (default value: 630 seconds)

Every time a router receives an update for a route, it sets the flush timer to 0. When the flush timer expires, the route is removed ...