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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

A network engineer is reviewing routing loop prevention mechanisms across dynamic routing protocols deployed within an enterprise network. Which of the following mechanisms are specifically utilized by distance-vector dynamic routing protocols to prevent routing loops? (Select TWO.)

  1. Split horizon, which stops a router from advertising a route back out the same physical interface from which it learned that route.Answer
  2. Route poisoning, which sets the metric of a failed route to an infinite/unreachable value to immediately invalidate it across neighbor routers.Answer
  3. C
    Using 802.1Q encapsulation headers to tag Layer 3 routing updates with VLAN identification markers.
  4. D
    Deploying Port Address Translation tables to translate hop count metrics into public IPv4 addresses.

Answer

The correct mechanisms are split horizon (preventing route re-advertisement back out the receiving interface) and route poisoning (broadcasting an unreachable metric when a link fails).
The statements defining split horizon and route poisoning correctly identify distance-vector loop prevention techniques. Split horizon prevents a router from transmitting route updates back out the interface where the route originated, breaking two-node routing loops. Route poisoning explicitly sets the metric of a down link to an unreachable value (e.g., 16 hops in RIP) so surrounding neighbors rapidly drop the bad route.

Step-by-Step Solution

1
Identify the protocol operational category specified in the stem
The scenario focuses specifically on distance-vector dynamic routing protocols.
Distance-vector protocols (such as RIP) rely on neighbor updates and hop counts, making them susceptible to routing loops like count-to-infinity.
2
Evaluate candidate loop prevention features for distance-vector protocols
Split horizon and route poisoning are classic distance-vector mechanisms designed to prevent count-to-infinity and routing loops.
Split horizon ensures updates are not echoed back to the source node, while route poisoning marks invalid routes with infinite metric values immediately upon failure.
3
Analyze and eliminate invalid non-routing mechanisms
802.1Q trunking operates at Layer 2 for VLAN isolation, and PAT is a Layer 3/4 address translation technique.
Neither technology plays a role in dynamic routing protocol convergence or loop mitigation.

Key Concept

Distance-Vector Routing Loop Prevention Mechanisms
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Topics

Routing Concepts and Dynamic Routing Protocols

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