Re: BGP Session Flapping with dual AS

Jorge,

When you do no-prepend, you can pass routes to your neighbor that might not
have your "local-as" first and so another peer may drop this and kill the
session. Example:

R1---R2---R3

R1 (AS10) and R3 (AS30) both treat R2 as in AS100.
R2 is AS200.

If R2 uses the no-prepend option, routes from R3 look like this to R1:

route: 10.1.1.3
as-path: 20 30

R1, by default, expects all routes from R2 to have 100 as the first AS in
the path. You can use "no bgp enforce-first-as" to allow this.

You can read more details in RFC 4271:

6.3. UPDATE Message Error Handling

"the local system MAY check whether the leftmost (with respect to the
position
   of octets in the protocol message) AS in the AS_PATH attribute is
   equal to the autonomous system number of the peer that sent the
   message. If the check determines this is not the case, the Error
   Subcode MUST be set to Malformed AS_PATH."

It might be worth noting that if you want dual-as and replace-as, you must
use no-prepend. Looks like you can have a lot of fun with this command.
Interesting scenario :)

On Thu, Dec 17, 2009 at 6:44 PM, gokul krishna <gokulbruno_at_gmail.com> wrote:

> Hi Jorge,
>
> Did you check the IGP routing table to reach the neighbour? Is it steady?
> Is
> the BGP route flapping every 30 sec?
>
> Kris
>
>
>
>
> On Thu, Dec 17, 2009 at 1:39 PM, Jorge Cortes
> <jorge.cortes.cano_at_gmail.com>wrote:
>
> > Hi Team,
> >
> > Working on the lab for BGP Dual AS I am running into a problem where
> > the BGP session between the eBGP peers is constantly flapping and I
> > just can't understand where the problem could be. Following the
> > relevant configurations on both peers:
> >
> >
> > ======Peer 155.1.45.4=======
> >
> > router bgp 146
> > no synchronization
> > bgp log-neighbor-changes
> > network 155.1.146.0 mask 255.255.255.0
> > aggregate-address 155.1.0.0 255.255.0.0 summary-only
> > neighbor 155.1.45.5 remote-as 200
> > neighbor 155.1.45.5 local-as 100 no-prepend replace-as dual-as
> > neighbor 155.1.45.5 remove-private-as
> > neighbor 155.1.146.1 remote-as 146
> > neighbor 204.12.1.254 remote-as 54
> > neighbor 204.12.1.254 local-as 100 no-prepend replace-as
> > neighbor 204.12.1.254 remove-private-as
> > no auto-summary
> >
> >
> > ======Peer 155.1.45.5=========
> >
> > router bgp 200
> > no synchronization
> > bgp log-neighbor-changes
> > bgp dampening 4
> > neighbor 155.1.0.2 remote-as 200
> > neighbor 155.1.0.2 route-reflector-client
> > neighbor 155.1.23.3 remote-as 200
> > neighbor 155.1.45.4 remote-as 146
> > neighbor 155.1.45.4 remove-private-as
> > neighbor 155.1.58.8 remote-as 200
> > neighbor 155.1.58.8 route-reflector-client
> > neighbor 155.1.108.10 remote-as 200
> > neighbor 155.1.108.10 route-reflector-client
> > no auto-summary
> >
> >
> > Here the log messages and the debug TCP on peer 45.5
> >
> > Rack1R5#
> > *Dec 17 19:03:23.330: tcp0: O SYNRCVD 155.1.45.4:11528 155.1.45.5:179
> > seq 1925862638
> > OPTS 4 ACK 2264873293 SYN WIN 16384
> > *Dec 17 19:03:23.358: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925862639
> > DATA 64 ACK 2264873338 PSH WIN 16339
> > *Dec 17 19:03:23.386: %BGP-5-ADJCHANGE: neighbor 155.1.45.4 Up
> > *Dec 17 19:03:23.386: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925862703
> > DATA 366 ACK 2264873357 PSH WIN 16320
> > *Dec 17 19:03:23.450: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925863069
> > DATA 54 ACK 2264873771 PSH WIN 15906
> > *Dec 17 19:03:23.470: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925863123
> > DATA 38 ACK 2264873877 PSH WIN 15800
> > *Dec 17 19:03:23.690: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925863161
> > ACK 2264874151 WIN 15526
> > Rack1R5#
> > *Dec 17 19:03:54.082: %BGP-5-ADJCHANGE: neighbor 155.1.45.4 Down BGP
> > Notification sent
> > *Dec 17 19:03:54.082: %BGP-3-NOTIFICATION: sent to neighbor 155.1.45.4
> > 3/11 (invalid or corrupt AS path) 13 bytes 40020A02 04006400 36003200
> > 3C
> > Rack1R5#
> > *Dec 17 19:03:54.082: BGP: 155.1.45.4 Bad attributes FFFF FFFF FFFF
> > FFFF FFFF FFFF FFFF FFFF 0033 0200 0000 1840 0101 0040 020A 0204 0064
> > 0036 0032 003C 4003 049B 012D 0408 7108 70
> > *Dec 17 19:03:54.082: tcp0: O ESTAB 155.1.45.4:11528 155.1.45.5:179
> > seq 1925863161
> > DATA 34 ACK 2264874351 PSH WIN 15326
> > *Dec 17 19:03:54.102: tcp0: O CLOSEWAIT 155.1.45.4:11528
> > 155.1.45.5:179 seq 1925863195
> > ACK 2264874352 WIN 15326
> > *Dec 17 19:03:54.182: tcp0: O LASTACK 155.1.45.4:11528 155.1.45.5:179
> > seq 1925863195
> > ACK 2264874352 FIN PSH WIN 15326
> >
> > I would appreciate if someone could provide any pointers on what I
> > could be missing here.
> >
> > Thanks and regards,
> >
> > Jorge
> >
> >
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-- 
Bryan Bartik
CCIE #23707 (R&S, SP), CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com
Blogs and organic groups at http://www.ccie.net