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Convergence

Router D advertises the new route to RouterC, but RouterC ignores
the new route listing because RouterC is in hold-down. RouterC sends
another poison reverse to RouterD.

The hold-down timers expire on Routers C, B, and A, which cause
their routing table entries to be updated.

The time required for RouterA to converge is the detection time, plus the

hold-down time, plus two update times, plus another update time. The com-
plete convergence to RouterA could be over 240 seconds.

IGRP Convergence

Using the same Figure 2.6 as an example, let's take a look at IGRP
convergence:

RouterD detects the failure on the link to RouterF. RouterD sends a
poisoned route update to Routers E and C. RouterC sends an update
to RouterB, and RouterB sends an update to RouterA. RouterD
purges the routing table of a link to the downed route.

RouterD sends a query broadcast, and RouterC sends back a poison-
reverse response. RouterD sends an update out all interfaces without
the failed link entry.

RouterE responds to the broadcast with an entry to the downed
link. RouterD puts this in the routing table and does not put a hold-
down on the link since it does not have an entry for this link. RouterD
sends a broadcast out all links, which includes this new entry.

RouterC receives this update but ignores the new route since it is in
hold-down. It responds again with a poison reverse.

Routers C, B, and A hold-down timers expire and their routing tables
will then be updated with the new route.

The time it takes for RouterA to converge is the detection time, plus the hold-
down time, plus two update times, plus another update time, which is over
490 seconds.

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