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Using example IPv4 addresses for discussion, assume I have used BGP to announce to the Internet the prefix 192.0.0.0/16 from datacenter A, and I have also announced a longer prefix 192.0.2.0/24 (a subset) from a different datacenter B.

In practice when datacenter B withdraws the longer /24 prefix, a period of service disruption (between 10 seconds to 2 minutes) has been observed where traffic to the /24 is effectively black-holed even though the datacenter A route continued to be announced.

The intent is that both datacenter A and B are capable of handling traffic for the same /24 but datacenter B is preferred when available.

Is this service disruption avoidable, or minimizable, and how?

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Just as it takes time for the more specific announcement to propagate, it takes time for the withdrawal to propagate. But unlike for the former, the later results in traffic heading into a dead end as the route-removal traverses the tree.

In the chain of "A - B - C - D - E", where A is datacenter B, the instant B processes the withdrawal, it will stop routing traffic towards A, yet C, D, & E will still have the more specific path in their route tables. When C hands that traffic to B, B no longer wants it. Eventually the more specific will be withdrawn by B, and then likewise C will withdraw it, etc., etc. While this should, in theory, take a few seconds, the internet is a very big, very busy place.

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  • That helps me understand why, thanks. For a planned withdrawal could this be mitigated by first re-announcing the /24 with path prepending (or some other attribute to give it less priority than the /16) and then withdrawing after the internet has had time to propagate the new priority? – Jason Stangroome Jul 23 '20 at 10:21
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    There's no comparison of "priority" between a /16 and /24. Announcing a /24 from your DR site as part of a planned switch-over process most likely won't reduce blackholing/transition interval; you will end up with two different periods of blackholing (once when you announce the new DR /24, second when you withdraw the primary /24) but measuring the affect is difficult. In general, adding complexity to your BGP announcements to the Internet is a bad idea. When in doubt, don't add complexity. – Jeff Wheeler Jul 23 '20 at 11:07

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