Multiple BGP Routes in FIB

Matt Buford matt at
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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