350-401 Question #430: Real Exam Question with Answer & Explanation

Question

What is an advantage of using BFD?

Options

• AIt local link failure at layer 1 and updates routing table
• BIt detects local link failure at layer 3 and updates routing protocols
• CIt has sub-second failure detection for layer 1 and layer 3 problems.
• DIt has sub-second failure detection for layer 1 and layer 2 problems.

Explanation

BFD (Bidirectional Forwarding Detection) Explained

Option B is correct because BFD is specifically designed to operate at Layer 3, detecting failures in the forwarding path between two neighboring routers and notifying routing protocols (such as OSPF, EIGRP, or BGP) to trigger rapid reconvergence - this is its core purpose and primary advantage.

Why the distractors are wrong:

• Option A is incorrect because BFD does not detect failures at Layer 1 (physical); it works at Layer 3, and it notifies routing protocols, not the routing table directly
• Option C is incorrect because BFD does not address Layer 3 problems in the way described - it detects path failures, not Layer 3 configuration issues, and the "sub-second" framing here is misleading
• Option D is incorrect because BFD does not operate at Layer 2; it functions between Layer 3 peers and has nothing to do with Layer 2 switching

Memory Tip: Think of BFD as a "heartbeat monitor for routers" - it sends rapid hello packets between Layer 3 neighbors, and when a heartbeat is missed, it immediately tells the routing protocol to find a new path. Remember: BFD = Layer 3 detection + routing protocol notification.

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