Multiple BGP Routes in FIB

Sun Sep 10 01:58:41 UTC 2006

> 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.

This situation subverts BGP's basic loop prevention mechanism.  If the
/20 is ever deaggragated into more specifics, a forwarding loop may
result.

If you want to put rounds in the chamber before pointing the muzzle at
your temple, you're free to do so.  However, some of us would prefer to
stand a long way away.

Tony