BGP advertisement logic?

While doing some of the INE Vol1 BGP labs with 'debug ip bgp *neighbor' *turned
on, I noticed a strange behavior that I cannot explain.

I am doing a hard reset between two peers. R3 is in AS200, R1 is in AS100.
 From the debug of reset on R3, what is happening is that R3 is advertising
all of its prefixes to R1, then withdrawing all the prefixes that contain
AS 100 in their AS_PATH.

Full debug logs @ http://pastebin.com/raw.php?i=cRxBGbwm

I understand the logic behind not wanting to advertise a route into an AS
that is originated from, but doesn't R1 handle that too (DENIED due to:
AS-PATH contains our own AS)? And R1 knows what AS R3 is in -- why is it
sending the updates in the first place? Is there some other mechanism that
is making R3 withdraw the routes it JUST sent?

As I study for the lab, I'm trying to ensure I don't just know commands,
but understand the guts of how the protocols work. Your insight is
appreciated!

Keller Giacomarro
keller.g_at_gmail.com

Blogs and organic groups at http://www.ccie.net
Received on Sun Nov 20 2011 - 03:48:52 ART