RIPv2: Split Horizon and Poisoned Reverse

Split Horizon is a well known principle not specific to RIPv2 it states that a network should never be advertised back over the interface that is used to reach that network.  In Cisco’s RIPv2 implementation Split Horizon is turned on by default except for physical frame relay and ATM interfaces.

Another version of Split Horizon that’s used with RIPv2 is the addition of the Poisoned Reverse mechanism.  This principle states that a network should always be explicitly advertised as unreachable over the interface that is used to reach that network.  In addition to its normal function of advertising networks across other interfaces except the interface it learned the route from….it actually does send an advertisement to the next hop router with an infinite metric, immediately flushing itself from being considered a next hop router for that network.

poisonedreverse.PNG

 

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