On 9/25/14 6:40 AM, David Deutsch wrote:
Hello all,
Please excuse my disorganized posting/question, as this is my first
posting to the mailing list.
Currently I am attempting to implement two ASR1002-X routers on my
network edge, each with diverse eBGP provider connections receiving
full routes. Everything up to a point has gone very well, including
reception of full routes, blocking of default routes, local prefs,
etc. However I am getting some odd results when iBGP connecting the
two together.
I appears that certain routes from each other are being filtered out,
so each router is in some cases aware of both routes to a prefix and
in some cases aware of only one. Examples:
My advice to "new BGP students" is to slow down, focus on one route at a
time, and "think like a router". So, pick one route, go onto one of your
routers, and do 'sh ip bgp a.b.c.d/e'.
What do you see? Likely an EBGP route, and possibly an IBGP route.
Which one is best? Why? (Hint: dig out the path selection algorithm, and
identify the exact step why)
Is it advertised to anyone else?
Now, hop on the other router, and do 'sh ip bgp a.b.c.d/e' for the same
route.
What do you see? Likely an EBGP route, and possibly an IBGP route.
Which one is best? Why?
Is it advertised to anyone else?
I suspect you have a situation where R1 might be getting a really "good"
path for route X (EBGP), so it advertises it to R2. R2 is getting a
really "bad" path for route X (EBGP) and the really "good" path from R1
(IBGP), so it prefers the route from R1 and therefore has no reason to
advertise its learned EBGP path to R1 (since it isn't 'best').
Meanwhile, for other routes, R1 might be getting a path for route Y
(EBGP), so it advertises it to R2. R2 is getting an "equally good" path
for route Y (EBGP), even when comparing to the IBGP route passed over
from R1, so it advertises its external path to R1. At this point, R1
recalculates, but it still likes its EBGP route, so R1/R2 "agree to
disagree" essentially.
Realize that for route X, if R2 happens to learn the EBGP path first,
it's the only path so it wins, and R2 advertises it to R1. Then when R1
happens to learn its EBGP path later, that one wins, so R1 picks that as
best path and advertises it to R2. At that point, R2 recalculates, and
decides the path from R1 wins, so it sends a withdrawal of the EBGP path
it was advertising earlier. So sequence is a factor, but the end result
is the same.
pt
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