Split Horizon with Poison Reverse

Split Horizon with Poison Reverse

Another technique to avoid routing loop is split horizon with poison reverse. Using this technique, routes learned on an interface are advertised back on the same interface with an IGRP metric of infinity. This is called poison update. When the router receives the poison update, it considers the route as invalid. For example, in Figure 4-2, Router 1 receives an update for Network X from Router 2. With poison reverse, this specific route is advertised back to Router 2, but with an IGRP metric of 4,294,967,295, which indicates infinity. Because Router 2 receives the poison update from Router 1, Router 2 does not consider Router 1 as a valid path to reach Network X, thus preventing the possibility of a routing loop.

Figure 4-2. An Example of the Split Horizon Technique