Re: Dual Homed BGP for failover

[William Herrin <bill () herrin us>]

On Tue, Jan 18, 2011 at 1:32 PM, Ahmed Yousuf <ayousuf0079 () gmail com> wrote:
>  It
> has now been requested to be able to distribute traffic across both links
> rather than preference traffic to the higher speed link.
> -          Is this really a good idea, as the BGP process won't care what
> the utilisation of the links are and you will see situations where the lower
> speed link gets used even though the high speed link utilisation is 0?

Hi Ahmed,

This really isn't an either/or situation. You can prefer the higher
speed link without excluding the lower speed link. One common way to
do this (there are better ones but this one is easy) is to prepend the
AS path you send and receive on the lower speed link so that it's
longer.


> -          If we are doing this, I don't want to take a full routing table,
> I would rather just take the ISPs routes and perhaps their connected
> customers.  One ISP has said they will only provide full routing table or
> default.  I really don't want to take a full table, is receiving default
> only going to be a problem for my setup?

IMO, that would be a mistake. Taking significantly less than a full
table severely limits your options for balancing traffic between the
links.


> -          Any advice on how to avoid situations where the low bandwidth
> link is being used even though there is 0 utilisation on the high bandwidth
> link?

Any particular communication is either going to go through one link or
the other. I'm generalizing here, ignoring some subtleties, but if
packets between two particular hosts have picked the low speed link,
they will take that one instead of the high speed link. So in a sense
it isn't possible to prevent that situation. However, you can adjust
the preferences for one path versus the other so that you're not
leaving either circuit underused overall and the disparity between
your circuits (30 and 10) is not enough to cause major performance
issues in and of itself.

Regards,
Bill Herrin



-- 
William D. Herrin ................ herrin () dirtside com  bill () herrin us
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