[92202] in North American Network Operators' Group
Re: Multiple BGP Routes in FIB
daemon@ATHENA.MIT.EDU (Matt Buford)
Sat Sep 9 23:18:59 2006
From: "Matt Buford" <matt@overloaded.net>
To: <tony.li@tony.li>, "'Glen Kent'" <glen.kent@gmail.com>,
"'NANOG list'" <nanog@merit.edu>
Date: Sat, 9 Sep 2006 22:18:02 -0500
Errors-To: owner-nanog@merit.edu
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.
It seems to me that this is only true if there is ever a possibility of one
of your next-hops believing the route to the destination is back through
you - or perhaps if the upstream has no route at all to the destination. In
the case of most non-tier-1 networks, any packet destined for anywhere
outside my own ASN (and customer ASNs if you have BGP customers - which I do
not) can be handed to any upstream transit provider without fear of looping.
So, the device injecting the traffic engineering route needs to be smart
enough to never inject a route that matches an announcement of you or your
customers. Beyond that, looping (should) never happen simply by definition
of the transit/customer relationship.
So, it subverts BGP's loop detection - but the transit/customer relationship
(hopefully combined with appropriate announcement filtering) avoids the
issue. Of course, if you leak one transit provider to another, and that
gets accepted, you might loop due to your traffic engineering routes - but
at that point you've got plenty of problems anyway.
