What does poison reverse do in distance-vector routing?

• A. It black-holes the route by dropping updates.
• B. It immediately forwards the best route to all neighbors.
• C. It uses a route's actual distance rather than infinity when advertising.
• D. It advertises an infinite metric for a route back to the neighbor that reported it in order to prevent loops.

Answer: (D)

Poison reverse is a loop-prevention technique used in distance-vector routing. When a router learns a route to a destination via a neighbor, it answers that neighbor by advertising an infinite metric for that destination back to the same neighbor. This signals, on a per-neighbor basis, that the route to that destination should not be used through this router, which helps break potential routing loops that can form if the topology changes. It’s different from simply broadcasting the best route to all neighbors, and it’s not about dropping updates or advertising the real distance to everyone. The essence is telling the neighbor who reported the route that, for that destination, the path via this router is unusable.