Which protocol provides single-hop link failure detection within 50 ms?

• A. BGP
• B. RIP
• C. BFD
• D. IPv4

Answer: (C)

Fast detection of a failed link between two directly connected devices is what Bidirectional Forwarding Detection is designed for. BFD is a lightweight protocol that two endpoints run on the same link, exchanging small control packets at a very short interval and using a configurable detection time. If packets stop arriving, the session is considered down quickly—often in the low milliseconds range—so the forwarding plane can trigger fast reroute or failover. This rapid, single-hop failure detection is exactly what you need for quick reaction in SD-WAN scenarios.

In contrast, BGP relies on a TCP session between neighbors with its own keepalive/hold timers, which are typically much longer and geared toward routing convergence rather than ultra-fast link failure detection. RIP uses periodic updates (classic implementations around 30 seconds), so failure detection can take many seconds. IPv4 itself is just the addressing protocol and does not define a rapid link-failure detection mechanism. Therefore, the fastest, single-hop link failure detection within about 50 ms is provided by Bidirectional Forwarding Detection.