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List:       nanog
Subject:    Re: Multi-homed clients and BGP timers
From:       Steve Bertrand <steve () ibctech ! ca>
Date:       2009-05-26 12:32:47
Message-ID: 4A1BE16F.7010007 () ibctech ! ca
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Steve Bertrand wrote:

> 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.

Thanks to all for the great feedback. In summary, I've learnt:

- Even though BFD would be a fantastic solution and would require only
minimal changes (to my strict uRPF setup), it's a non-starter, as I
don't fit all of the requirements that Ivan pointed out

- fast-external-fallover is already enabled by default, but in order for
this to be effective, the interface has to physically go into down
state. In my case, although not impossible, it is extremely unlikely

- adjusting BGP timers is the best option given it's really the only one
left. Although I generally try to keep consistency among all equipment
(if I set the timers at one end, I would set them the same at the
other). Iljitsch recommended to leave the CPE end alone, so if something
bad happens, access to the CPE would not be necessary to revert the change

- I'm going to set the timers to 5/16. I like the idea of the extra
second on top of being divisible by three. That will ensure that at
least three keepalives have a chance to make it before the session hold
timer is reached

Cheers!

Steve


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