Multi-homed clients and BGP timers

Zaid Ali zaid at zaidali.com
Fri May 22 17:58:40 CDT 2009


>From experience I found that you need to keep all the timers in sync with all your peers. Something like this for every peer in your bgp config.

neighbor xxx.xx.xx.x timers 30 60

Make sure that this is communicated to your peer as well so that their timer setting are reflected the same.

Zaid
----- Original Message -----
From: "Steve Bertrand" <steve at ibctech.ca>
To: "nanog list" <nanog at nanog.org>
Sent: Friday, May 22, 2009 3:45:20 PM GMT -08:00 US/Canada Pacific
Subject: Multi-homed clients and BGP timers

Hi all,

I've got numerous single-site 100Mb fibre clients who have backup SDSL
links to my PoP. The two services terminate on separate
distribution/access routers.

The CPE that peers to my fibre router sets a community, and my end sets
the pref to 150 based on it. The CPE also sets a higher pref for
prefixes from the fibre router. The SDSL router to CPE leaves the
default preference in place. Both of my PE gear sends default-originate
to the CPE. There is (generally) no traffic that should ever be on the
SDSL link while the fibre is up.

Both of the PE routers then advertise the learnt client route up into
the core:

*>i208.70.107.128/28
                    172.16.104.22             0    150      0 64762 i
* i                 172.16.104.23             0    100      0 64762 i

My problem is the noticeable delay for switchover when the fibre happens
to go down (God forbid).

I would like to know if BGP timer adjustment is the way to adjust this,
or if there is a better/different way. It's fair to say that the fibre
doesn't 'flap'. Based on operational experience, if there is a problem
with the fibre network, it's down for the count.

While I'm at it, I've got another couple of questions:

- whatever technique you might recommend to reduce the convergence
throughout the network, can the same principles be applied to iBGP as well?

- if I need to down core2, what is the quickest and easiest way to
ensure that all gear connected to the cores will *quickly* switch to
preferring core1?

Steve





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