OSI Layer 3: Routed and Routing Protocols
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F I G U R E 2 . 1 2
Route types recognized by IGRP
Routing Stability
Three methods are used to help stabilize the routing environment:
Holddown timers
Split horizon
Poison reverse updates
Holddown timers prevent an invalid route from being injected back into
the route table. The timers allow routing updates to be communicated
throughout the network, thus avoiding advertisement of the invalid route
while the new valid route is being propagated.
With split horizon, route information learned from a given interface isn't
sent back out the same interface. Using Figure 2.13 as a reference, we'll
explain split horizon updates.
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