Split Horizons

Another convergence-enhancing feature borrowed from RIP is the split horizon. Split horizons operate on the assumption that it is never a good idea to send routing information received from a given source back to that source. Figure 10-1 illustrates this point.

Figure 10-1: A split horizon scheme.

Figure 10-1: A split horizon scheme.

In Figure 10-1, the routers support the split-horizon logic. Therefore, Router C (which supports the only path to Router D) cannot receive updates from Router A about Network D. This is because Router A relies on Router C (and even Router B) for this route information. Router A must omit information about routes learned from Router C from its routing table updates to Router C. Advertising routing information about a specific route back to the source from which it was originally learned is known as reverse routing. This simple technique prevents routing loops by splitting the network's "horizon." In other words, advertising reverse routes is not permitted.

Although split horizons can be relatively effective, this technique does have a serious functional limitation. When you omit reverse routes from advertising, each node must wait for the route to the unreachable destination to expire. In IGRP, routes expire only after seven update messages fail to update a route. Therefore, there are six opportunities for a misinformed node to misinform other nodes about an unreachable destination. This time delay creates the opportunity for invalid routing information to start the loop. Using hold-downs, in conjunction with split horizons, should prevent the routing instability that can occur when neighboring routers misinform each other.

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