Concept of route poisoning: In distance-vector routing, what does the term “route poisoning” specifically describe when a link or network goes down?

Introduction / Context:
Distance-vector protocols such as RIP exchange periodic updates that can suffer from slow convergence and counting-to-infinity problems. To speed failure notification and avoid routing loops, they use techniques like route poisoning and hold-down timers.

Given Data / Assumptions:

• A previously reachable route has failed (link down or network unreachable).
• Neighbors must be informed promptly to stop forwarding traffic along the bad path.
• We are using a classic distance-vector protocol model.

Concept / Approach:

With route poisoning, the detecting router advertises the failed network with an infinite metric (for RIP, hop count 16). This “poisons” the route so neighbors immediately mark it unreachable and avoid using that path. Combined with poison reverse and hold-downs, this reduces loops during convergence.

Step-by-Step Solution:

Verification / Alternative check:

Use debug ip rip to observe updates marking the route unreachable; show ip route confirms removal of the prefix after propagation.

Why Other Options Are Wrong:

Options A and B are unclear and not standard definitions.

Option C describes a hold-down timer's purpose, not route poisoning.

Common Pitfalls:

Confusing route poisoning with split horizon or hold-downs; forgetting that “infinity” is protocol-specific (16 for RIP but different or conceptual for others).

Final Answer:

It describes when a router sets the metric for a downed link to infinity.