From: Geert Nijs (geert.nijs@simac.be)
Date: Wed Jul 20 2005 - 04:33:42 GMT-3
Hi group,
I wonder how service providers prevent routing loops in the following
situation:
R1 is part of AS1
R2 and R3 are part of AS2
R2 and R3 are also running an IGP like ie EIGRP
R4 is part of AS4
**** On router R2 and R3 redistribution of BGP into EIGRP is configured
!
R1 (AS1) ---- EBGP ----- R2 (AS2) ----------- IBGP + EIGRP
----------- R3 (AS2) --------- EBGP ---------- R4 (AS4)
On router R3 someone forgot to remove a network statement, so router R3
contains the statement:
router bgp 2
network x.x.x.x
but network x.x.x.x is not in the routing table so it is not advertised
(no problems yet).
Now, the problems begin when AS1 (R1) starts advertising this network
x.x.x.x
The network x.x.x.x enters AS2 through router R2, this router
redistributes the route into EIGRP, and now,
the network x.x.x.x is part of the routing table on router R3, so R3
suddenly starts advertising network x.x.x.x back to R2 over IBGP
R2 prefers this route (AS path is empty). R2 starts advertising network
x.x.x.x back out AS2 to R1 and stops redistributing x.x.x.x into EIGRP
(by default, no redistribution of IBGP routes). So the route is removed
from the routing table on R3, so that R3 stops advertising the route
again.
When R2 removes the route from BGP table, it prefers back the route
received from R1 and starts advertising the route again to R3 over
EIGRP....
and so we have a beautifull routing loop and lots of constant BGP
advertisements going on forever.....
I am looking for a method to prevent this from happening.
Any ideas ?
Geert Nijs
CCIE #13729 <http://www.cisco.com/en/US/learning/le3/ccie/index.html>
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