Re: BGP Peers as basis of available routes

[William Herrin <bill () herrin us>]

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


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William D. Herrin ................ herrin () dirtside com  bill () herrin us
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