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North American Network Operators Group

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Re: Multiple BGP Routes in FIB

  • From: Matt Buford
  • Date: Sat Sep 09 21:24:18 2006

Do providers really do this? Would they install multiple BGP Paths
with different AS Paths (but same length) in their FIB, and yet
advertise only one?

Is the the right thing to do?
What you see in BGP is not necessarily what you get for actual routing. This isn't the only situation where advertisements do not match actual routing. Consider traffic engineering systems such as the Internap FCP (old NetVMG). Imagine I have two upstreams (A and B) and you advertise a /20. I might prefer path A for your /20. However, my traffic engineering system may inject a no-export /24 route into my network to shift a portion of your traffic to go out my upstream B.

This is quite interesting/confusing from the customer perspective, where you only see the BGP path through upstream A advertised, yet in reality a /24 out of that /20 is going through a completely different path that you do not see via BGP.

Is this wrong/evil? I guess that is up to each network to decide.

Disclaimer: I use such a system and have many /24 no-export routes in my table. It works great and keeps my 7 upstream providers all nicely balanced with no manual intervention ever. However, I have no BGP downstreams so the above misrepresentation of BGP advertisements is not an issue since it never leaves my network.

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