[isp-bgp] Re: outbound load balancing with BGP?

[Pete Templin <petelists@...>]

The routes that you receive from the two ISPs (and the best routes that 
your router chooses from the two sets) dictate how your packets leave 
your network.  You can adjust this with several variables: weight, local 
preference, AS path prepending, and MED (metric) adjustments.  If you 
have one router, weight has the same impact as local preference, and 
would be difficult to use effectively, at least at a global level.  MED 
(metric) is normally only effective if you have two or more links to the 
same upstream provider, but might be useful (slim chance though) if the 
two providers can supply visibility into their network through the MED 
value.  That leaves AS path prepending, if you're looking for a global knob.

You may want to prepend the provider's AS one or more times to the 
routes you RECEIVE from the link being saturated outbound.  Most likely, 
that provider has fewer intermediate AS "hops" between you and your 
destinations, so it's winning the battle at that stage of the BGP path 
selection algorithm.

If you can't reach a good balance with just this knob globally, you may 
want to research what ASes each ISP connects to, and consider applying 
AS path prepending to the routes you learn from a subset of your 
saturated link's upstream to attempt to fine-tune the balancing.

Hope this helps.

pt

Michael W. Lucas wrote:
> Hi,
>
> I'm running a data centre with a Cisco 3640 router and two sets of
> three bonded T1s, for 4.5MB to each ISP.  Everything is monitored via
> MRTG, so I know exactly how much in & out bandwidth is used on each
> connection.
>
> While both sides of the connection are used in both direction, we are
> seeing a heavy preference on one ISP for outbound traffic.  At times,
> one ISP is pushing the full 4.5MB outbound while the other is pushing
> less than 1MB.  
> While it's obvious that one ISP has better connectivity than the
> other, I'd like to balance the load somewhat.  Is there any way I can
> force my local router to choose to give one peer a "bonus", weighting
> it more heavily for outbound traffic?
>
> All the docs I've found talk about load-balancing inbound traffic with
> prefix manipulation and AS-path prepending, but I want to balance it
> leaving the network.  Surely someone's dealt with this before...
>
> Any help would be appreciated!
>
> Regards,
> ==ml





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