What is poison reverse?

• 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 a loop-prevention technique in distance-vector routing. When a router learns a destination from a particular neighbor, it advertises that same destination back to that neighbor with an infinite metric. This signals to the neighbor that the route to that destination is not reachable via the router sending the update, preventing the neighbor from routing traffic back through it and creating a loop.

In practice, this means you explicitly “poison” the route back to the neighbor you learned it from, rather than simply dropping the route or manipulating all metrics. That’s why this option—advertising a learned route with an infinite metric to the interface it was learned from—best matches poison reverse.