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load balancing incoming traffic w/ BGP

Right now we have 3 backbone providers. We're running pretty much the same amount of bandwidth to each. The problem is one provider has VERY good connectivity, AT&T, and the other 2 providers we have have less then good BGP connectivity.

We NEED to load balance incoming traffic. Basically its better to take a longer AS path on a link w/ little traffic VS a shorter AS on a link in near saturation.

Obviously the best solution would get more bandwidth from your best backbone provider. But I have to deal with what I have.

In looking for a way to do this I came across two main plans of attach. Just want to get the pros/cons of each plus some feedback from others.

1st Would be to pre-pend our ASN to all outgoing BGP updates to the best provider - AS-path being a manditory attrib - would get passed to the global BGP mesh and would make our AT&T link artificially less desireable - hense incoming traffic would take another route via the other 2 provders.

2nd Confiure a high Med to the good provider - this will artificially sour our best link BUT as MEDs are non-transitive optional attributes they will not get passed along to other AS's.

Right now I'm mulling over which to do. I *believe* they will generate the desired effect BUT by different methods. The AS path prepend seams the better option as the world will see the AT&T link has the longer path - as opposed to a high MED which will only tell incoming traffic thats already gotten to AT&T to say - connect through our other two providers.


Community Member

Re: load balancing incoming traffic w/ BGP

The first solution is the "best" and only one you really have. The catch with it (and any other solution designed to load share Internet traffic) is that the traffic is not static...while the configuration is! In other words, increasing the AS_PATH length for a set of prefixes (so that they take the alternate path) may help you for the current set of traffic levels...but if the traffic characteristics change, then you'll have to adjust the policy.

Currently, there's no ideal solution (released!) that adjusts to the load. In the meantime, take a look at the OER slides I presented at Networkers this year:

As far as you second solution, it won't work as you wrote above. The MED is only considered (by default) on paths received from the same AS neighbor. In other words, it would help yoo discriminate between two links between you and AT&T (in this case), not between paths directly to you and through a different provider. Note that the AS_PATH length is considered first in the selection process, so the MED would most likely never be compared.



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