Another technique used to avoid routing loops is split horizon with poison reverse. With this technique, routes learned on an interface are advertised back on the same interface, but they are poisoned, which means that they have a metric of 16 (unreachable). In Figure 2-1, Router 1 receives an update about Network X with a metric of 1 from neighboring Router 2. In the case of split horizon with poison reverse, Router 1 will advertise Network X back to Router 2, but with a metric of 16, which indicates infinity.
Split horizon with poison reverse is used only when a link failure occurs. It also can be used in a normal situation, but it is discouraged because it can potentially increase the size of the routing table.
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