Note

Split horizon with poisoned reverse

In the scenario of Figure 4.4, router C would in fact advertise 10.1.4.0 and 10.1.5.0 to router D, but the network would be marked as unreachable. Figure 4.5 shows what the route tables from C to B and D would look like. Notice that a route is marked as unreachable by setting the metric to infinity; in other words, the network is an infinite distance away. Coverage of a routing protocol's concept of infinity continues in the next section.

Figure 4.5. Split horizon with poisoned reverse advertises reverse routes but with an unreachable (infinite)

metric.

Figure 4.5. Split horizon with poisoned reverse advertises reverse routes but with an unreachable (infinite)

metric.

Split horizon with poisoned reverse is considered safer and stronger than simple split horizon—a sort of "bad news is better than no news at all" approach. For example, suppose that router B in Figure 4.5 receives corrupted information causing it to believe that subnet 10.1.1.0 is reachable via router C. Simple split horizon would do nothing to correct this misperception, whereas a poisoned reverse update from router C would immediately stop the potential loop. For this reason, most modern distance vector implementations use split horizon with poisoned reverse. The trade-off is that routing update packets are larger, which may exacerbate any congestion problems on a link.

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