Multiple BGP Routes in FIB
matt at overloaded.net
Sun Sep 10 01:15:45 UTC 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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