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BFD: Bidirectional Forwarding Detection

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The problem: slow failure detection

Routing protocols like OSPF and BGP use hello timers to detect neighbor failures. OSPF's dead interval is 40 seconds by default. BGP's hold-down is 90 seconds. During that window, traffic black-holes.

BFD is a lightweight protocol whose only job is detecting forwarding path failures — fast. It's independent of any routing protocol but integrates with all of them.

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