IGRP Convergence

The steps for IGRP convergence are as follows:

Step 1 When the local router sees a connected route disappear, it sends a flash update and removes the route entry from its table. This is called a triggered update with poison reverse.

Step 2 The receiving routers send flash updates and put the affected route in holddown.

Step 3 The originating router queries its neighbor for alternative routes. If the neighbor has an alternative route, it is sent; otherwise, the poisoned route is sent.

Step 4 The originating router installs the best alternative route that it hears because it has purged the original routes. It sends a new flash update. This is either the routing table, with or without the network available, stating the higher metric.

Step 5 Routers that are in holddown ignore the alternative route.

Step 6 When the routers come out of holddown, they accept the alternative route.

When the other routers emerge from holddown, they will accept the alternative route.

Step 7 Convergence takes the time for detection, plus holddown, plus the number of routing updates (equal to the hop-count diameter of the network). Because the update timer is 90 seconds, this could take a very long time.

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