Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 107
RIP Fixes

To overcome the limitations, a few rules were added to the IP RIP algorithm:

Split horizon. Implemented by every protocol that uses a variation of RIP (AppleTalk, IPX, XNS, and IP), this states that a router will not broadcast a learned route back through a port from which it was received. Therefore, router B will not broadcast the entry of network z back to router A. This keeps router B from broadcasting the reachability of network z back to router A, thereby eliminating the possibility of a lower hop count being introduced when network z becomes disabled. The entry in router B’s update to router A would not include an entry for network z.

Hold-down timer. This rule states that once a router receives information about a network that claims a known network is unreachable, it must ignore all future updates that include an entry (a path) to that network (typically, for 60 seconds). Not all vendors support this in their routers. If one vendor does support it and another does not, routing loops may occur.

Poison reverse and triggered updates. These are the last two rules that help to eliminate the slow convergence problem. They state that once the router detects a disabled network connection, the router should keep the present entry in its routing table and then broadcast “network unreachable” (metric of 16) in its updates. These rules become efficient when all routers in the internet participate using triggered updates, which allow a router to broadcast its routing table immediately following receipt of this “network down” information. The two most common are split horizon and poison reverse.