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Chapter 2
Protocol Attributes
F I G U R E 2 . 1 3
Poison reverse and holddown
Router D advertises network 10.2.1.0/24 to Routers A and B. After
Router B receives the update and recalculates its own route table, a broad-
cast update is sent. Specific route information learned from Router D isn't
advertised back to Router D, however.
Poison reverse updates use the holddown function within IGRP, which
stops the router from being confused about multiple routes to a given net-
work. When a route's associated metric increases by a factor of 1.1, IGRP
sends out a poison reverse update that removes the affected route from a
router's route table and places it in holddown.
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