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When using AS prepending in a multi-homed BGP environment (primary/secondary routers, secondary prepends one more ASN), how fast is reconvergence of inbound traffic when the primary router goes down? How much time does it generally take for other routers to notice the primary router is down and it should not use this route?

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Default BGP keepalives are sent every 60 seconds. If a participating device does not receive a return response within 180 seconds, the default Hold Time, a reconvergence will occur. So by default, it will take 3 minutes for your devices to determine a neighbor is down before reconvergence.

Defaults
keepalive: 60 seconds
holdtime: 180 seconds

Here are some helpful commands for BGP link

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    Neighbouring routers aren't always the only problem, the change might need to ripple upstream through other peers. That takes more time to fully stabilize – Mike Pennington Feb 9 '15 at 23:34

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