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Re: Multiple ISP Load Balancing
From: "Justin M. Streiner" <streiner () cluebyfour org>
Date: Wed, 14 Dec 2011 15:10:10 -0500 (EST)

On Wed, 14 Dec 2011, Holmes,David A wrote:

From time to time some have posted questions asking if BGP load balancers such as the old Routescience Pathcontrol device are still around, and if not what have others found to replace that function. I have used the Routescience device with much success 10 years ago when it first came on the market, but since then a full BGP feed has become much larger, Routescience has been bought by Avaya, then discontinued, and other competitors such as Sockeye, Netvmg have been acquired by other companies.

It's important to keep in mind what load-balancing is and isn't in this case. The terminology gap can be important because load-balancing (more
accurately, load-sharing) in the context of internetwork traffic
engineering is very different from load-balancing pools of servers in a data center. Some people can (and sometimes do) confuse the two, which can cause unrealistic expectations to be set :)

Achieving a perfect split of network traffic across two or more upstream links rarely happens in the real world. General good practice is to put bandwidth where the network traffic wants to go, but that can be a moving target, and executives and accountants don't like those :) Traffic engineering still has a place on many networks, for a veriety of reasons (technical, financial, political, some combination of these), but as other posters have mentioned, it's often done manually, i.e. looking at Netflow reports, seeing where your traffic is going/coming from, adjusting BGP policies accordingly. Repeat as needed. Since traffic profiles can change over time, any policy tweaks that are put in place need to be reviewed periodically.

Depending on how much prep work and planning you're willing to do, you can put a fairly rich set of internal BGP communities in place to control things like localpref, MEDs, selective prepends, and tagging outbound advertisements with provider-specific communities. With that kind of structure, you could control many aspects of your traffic engineering from a route server, rather than having to tinker with route policies on each outside-facing router.

One caveat: If your route server crashes or has to be taken down for maintenance, the traffic patterns that were being tweaked by your policy framework could start to revert to the way the traffic would flow in its un-altered state, which could cause you some unintended headaches.


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