On Wed, Oct 19, 2011 at 2:46 AM, Nathanael C. Cariaga <nccariaga@stluke.com.ph> wrote:
In this regard, I would like to ask for your idea regarding this. Is it safe to conclude that the web hosting provider's available routes would would depend on the peers who are advertising their AS / network? (i.e if web hosting provider claims that they are peering with telco a, b, c but as seen from a third party looking glass, only C is seen advertising the web hosting provider network that would mean web hosting provider is effectively utilizing c as their upstream??)
Hi Nathan, BGP is a distance-vector protocol. In other words, when BGP is advertised on a link from one AS to another, only the "best" distance to a particular route is offered. If A and B both advertise the web hosting provider's (WHP's) route to D and A's distance is better then D won't advertise B's WHP route to A but it will advertise A's WHP route to B. Thus a looking glass at B will see both routes in the BGP RIB, but a looking glass at A will only see A's route. With more AS's between A and B than just D, it's possible (even likely) that neither A nor B will see the others' route during normal operation. Should WHP's connection to A drop, D will find that B's distance is now better and B's route to WHP will be newly advertised to A. A, then having no other connection to WHP, will accept the route via D to B, and pass it onward. When we talk about the BGP table "converging" after a change, this is the process we're talking about. Even if A and B are directly connected, if WHP sets its initial "distance" via B worse than via A, B will decide that A has a better distance to WHP and won't advertise its own version of the route to anyone else at all. And that's before you consider local prefs, communities and other mechanisms for fine-tuning route propagation. And, even if A, B and C have multiple routes in the BGP RIB, generally only one of those routes will be selected for the packet forwarding FIB. So, a traceroute from B to WHP may travel via A even though B is directly connected to WHP. Long story short, if WHP is connected to A, B and C then A, B and C should each see their own direct route to WHP in the BGP RIB, but there's no guarantee that anybody else will see more than one of the three at any given time. Regards, Bill Herrin -- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004