BGP Dampening when Primary/Backup on same router?

[jason.henderson]

Hi all,

I need to setup a primary and backup ADSL link to the same ISP, from the same router.  Primary is ethernet, backup is ADSL.  I will setup different weights and route maps to prefer the primary link inbound and outbound.

Customer would like to be able to make sure that if the primary goes down and the router flips over to the ADSL, that it stays on ADSL for at least 15 minutes before flipping back to the primary (when it comes back up and has been stable for that period).

Will BGP dampening work?  All the routes advertised over both the primary and backup from the ISP will be identical (the next hops will be different obviously).  I can't work out whether dampening will suppress the route irrespective of the different next hop, or whether it can distinguish and provide what i'm looking to achieve.

Thanks in advance,

Jason

[saeedkhalid]

Dear Dampening will work by default unless other wise config by the ISP.

The best thing to do is to establish the peering b/w loopback IPs(both customer and ISP) and then the route to the loopback can be influenced accordingly for both ethernet and ADSL as primary and backup.

The ADSL connection should be configured to always be up (not dial on demand). This will ensure no BGP blackouts for your prifixes.

Hope I got your requirements right.

[Amit Aneja]

Saeed,

I don't agree with you on following statement.

"Dear Dampening will work by default unless other wise config by the ISP."

AFAIK, you have to enable dampening, it is off by default.

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfbgp.html#wp1002414

Regards,

Amit

[Amit Aneja]

Jason,

The answer to your question is that BGP dampening is neighbor specific, it will dampen the route from one path & still have the same route working fine via the stable neighbor.

I didn't find any document clearly stating that, so I replicated it on GNS.

Take a look below:

R1#sh ip bgp 1.1.1.0

BGP routing table entry for 1.1.1.0/24, version 2

Paths: (2 available, best #2, table Default-IP-Routing-Table)

  Advertised to update-groups:

     1

  200, (suppressed due to dampening)

    192.168.12.2 from 192.168.12.2 (192.168.20.2)

      Origin IGP, metric 0, localpref 100, valid, external

      Dampinfo: penalty 2614, flapped 3 times in 00:04:11, reuse in 00:05:09

  200

    192.168.10.10 from 192.168.10.10 (192.168.20.10)

      Origin IGP, metric 0, localpref 100, valid, external, best

R1#

The two EBGP neighbors 192.168.12.2 & 192.168.10.10 are on one router only & I could route to 1.1.1.1 just fine.

Regards,

Amit