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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

A network switch running a distance-vector dynamic routing protocol detects that a directly connected link to a remote network segment has failed. To immediately inform neighbouring routers that the route is down and prevent routing loops during convergence, the switch broadcasts a routing update advertising the metric for that failed route as infinity (unreachable). Which dynamic routing mechanism is being demonstrated in this scenario?

  1. Route poisoningAnswer
  2. B
    Split horizon
  3. C
    Holddown timers
  4. D
    Poison reverse

Answer

Route poisoning is the mechanism used by distance-vector protocols to immediately advertise a failed network path with an infinite metric value, signaling to neighboring routers that the route is unreachable.
Route poisoning is a distance-vector protocol feature designed to speed up convergence and eliminate routing loops. When an interface goes down, the router immediately sets the metric for that network to an infinite value (such as 16 hops in RIP) and advertises it to neighboring nodes so they remove the bad path instantly.

Step-by-Step Solution

1
Analyze the network failure event and notification behavior
The router detects a link failure and immediately sends out a routing update containing a metric set to infinity.
Distance-vector routing protocols rely on metric limits (e.g., maximum hop count) to indicate network unreachability.
2
Identify the dynamic routing stability control mechanism
Advertising a metric of infinity to invalidate a route upon failure is defined as route poisoning.
This mechanism ensures that neighbor routers invalidate their routing table entries for the destination without waiting for hop counts to slowly count to infinity.

Key Concept

Distance-Vector Loop Prevention Mechanics (Route Poisoning)
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Topics

Routing Concepts and Dynamic Routing Protocols

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