BGP prefix filter list

James Jun james.jun at
Sat May 25 16:27:43 UTC 2019

On Fri, May 24, 2019 at 11:22:48AM -0700, William Herrin wrote:
> Get it? I announce the /24 via both so that you can reach me when there is
> a problem with one or the other. If you drop the /24, you break the
> Internet when my connection to CenturyLink is inoperable. Good job!

Or also likely, in the event of your CenturyLink circuit outage, the following
is likely to happen:

1. traffic comes into CenturyLink, dragged in by their /16 aggregate announcement
2. CenturyLink hears your more specific /24 from Verizon PX
3. CenturyLink sends traffic received from one peer, and out to another (Verizon),
   without touching a revenue side customer interface (temporary free transit
   situation, or temporary onintended hairpinning)

This assumes you're getting /24 allocation from an aggregate CenturyLink finds
acceptable to reassign to BGP multihomed customers, where they won't filter it out
right from their peers (for Level3, space is used for this typically).

I agree that this is very common.  I also found, not having LSP setup between
peering-only designated routers (who would've thought a peering router needs to
provide transit to another peering router that has no customers on it?!?) breaks
connectivity to customers that find themselves in this very situation, due to the
temporary hairpinning of traffic from one (peer|transit) interface out to another.


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