What is the purpose of poison reverse in distance-vector routing?

• A. Advertising a learned route with infinite metric on the interface it was learned from to prevent loops.
• B. Dropping the learned route from updates.
• C. Setting all route metrics to zero.
• D. Flooding all routes to all neighbors.

Answer: (B)

Poison reverse is used to prevent routing loops in distance-vector routing by telling the neighbor from which a route was learned that that route is unreachable via this router. When a router learns a route to a destination from a specific neighbor, it, in updates sent to that same neighbor, advertises that destination with an infinite metric (poison) instead of the real distance. This signals to the neighbor that the path through this router should not be used to reach that destination, so the neighbor won’t route traffic back through it in a way that could form a loop. It’s a targeted way of removing a potentially looping path without dropping all information or flooding updates; the route is still advertised to other neighbors, just not to the originator of that route.