If an IS-IS device is configured on a device, the minimum time it takes to sense a neighbor failure is seconds. However, after being associated with BFD, BFD can replace the hello mechanism of the…

Question

Options

• ATRUE
• BFALSE

• A
  72% (36)
• B
  28% (14)

Explanation

Option A is correct because IS-IS natively detects neighbor failures through hello packet exchanges, where the default hold time (typically 3× the hello interval, so ~30 seconds) must expire before a failure is recognized - a relatively slow process. BFD (Bidirectional Forwarding Detection) is a dedicated, lightweight protocol designed specifically for rapid link-failure detection, capable of detecting failures in milliseconds (e.g., 50ms or less) by operating independently of the routing protocol's own hello timers. When IS-IS is associated with BFD, BFD takes over the neighbor-liveness role, so the routing protocol no longer needs to wait for its own hello timer to expire. Option B is wrong because it incorrectly implies BFD does not improve IS-IS failure detection - this contradicts how BFD is designed and deployed in modern networks.

Memory tip: Think of IS-IS hellos as a slow postal letter (seconds to arrive) and BFD as a real-time heartbeat monitor (milliseconds) - BFD is always watching the line, so it alarms the moment the pulse drops, without waiting for the next scheduled hello.

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