Re: Internet Exchange Points and BGP paths

Hi Anukool,

> If two ISPs (A and B) decide to exchange traffic at a public exchange
> (C), would C show up in the ASpath between A and B?
>
> If not, then how does A know to route packets to B through exchange C?
> Is routing between A and B done statically?
>
> Typically, do public exchange points have their own AS numbers?  And
> if so, are these AS numbers advertised/appear in AS paths from BGP  
> tables?

Generally not - Most of the public exchanges I'm aware of place the
various peers on a broadcast domain of some sort.  A and B would extablish
a BGP session with each other, and traffic would flow between them over
the shared fabric.  The major exception to this would be in a route-server
scenario, where A and B (along with D, E, F etc) all establish a BGP
relationship with a route-server (AS C), in which case C would have a full
view, and would readvertise the best route across the shared fabric to all
neighbors.  In that case, a 'sho ip bgp x.x.x.x' from A would show C's as,
but a traceroute would go directly from A to B.  Very rarely will an
Exchange-point AS appear in a public environment - that's becuase very few
providers are willing to give free transit (which that would imply).  AS C
should only appear on routes learned from a peer rather than those learned
from a customer.  Ergo, the only way to see AS C in a path is to be a
direct customer of A, and request that they readvertise their full routing
table.

> 
> Finally, where can I find good references on the "inner-workings" of
> internet exchange points.  The only work I've found is Bill Norton's
> excellent papers but would greatly appreciate additonal pointers.

Take a look at telegeography.com - they're a Band-X subsidiary, and
they've done a lot of work in that space.

David Barak
Senior Network Engineer
UUNet/WorldCom
Voice: +1-703-886-5500
Fax: +1-703-886-0541

"Quis custodes ipsos custodiet?" - Juvenal