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Figure 8.9. Route poisoning example
To avoid the Count to Infinity, routers implement split horizon and
poison reverse. Both methods achieve the same goal by different
means. By implementing split horizon, the router deletes the network
information contained in its routing update, and that it has learned
from the interface through which it is transmitting the routing table
update. In other words, an interface's route message to other routers
after split horizon will not contain the routes it learned from them. So,
when Router C sends its update to Router E, it deletes the vector
distance information for the 192.124.39.0 network.
Poison reverse does just what its name indicates. When the router
sends out an update, it sets the hop count to 16 (because RIP only
supports a maximum network diameter of 15) for all the routes its has
learned from the interface through which it is transmitting the routing
table update. Both methods achieve the same goal by different means.
Both of these route poisoning strategies work well with RIP because
its vector distance algorithm is interested in only one thing: finding
the lowest hop count path.