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Routing & Switching · Intermediate

Poison Reverse

A rule where a router advertises a route back toward its source with an infinite metric, overriding split horizon.

Definition

Instead of staying silent toward the neighbor that taught it a route, the router actively tells that neighbor the route is unreachable through it. This guarantees the neighbor will not use the reverse path. It is used together with route poisoning in RIP implementations.

Example

R2 learned 10.1.0.0/16 from R1 and advertises it back to R1 with metric 16.

Exam tip

Poison reverse sends the route back with infinite metric; split horizon sends nothing back. Do not confuse them.

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