[c-nsp] BGP load sharing accross 2 different providers

[Justin M. Streiner]

On Tue, 22 Jan 2008, Mohamed Ahmad wrote:

> I was wondering what's the best way of load sharing traffic across 2 bgp
> sessions with 2 different isp's (different AS numbers) but same bandwidth (1
> Gbps). I found this article but not sure if anyone has tried this before:
>
> http://www.ccnalab.net/load_sharing_bgp.php

Maybe I'm just old-school, but I think configuring speed and duplex
negotiation on fast ethernet interfaces on routers, if they're connecting 
to other routers and switches, is a bad idea, but I digress...

The article does make two important points:
1. you will need your own AS number, registered with whatever Internet 
Registry serves your area.
2. you will need at least a /24 of routable address space to advertise to 
your upstreams.  This can be assigned from an upstream, or if you have 
your own provider independent assignments already, that will work too.

> I have also found this Cisco article which uses a different method:
>
> http://www.cisco.com/warp/public/459/40.html#conf4

You can do load sharing to two different upstreams using BGP - this is a 
pretty common design.  It's important to note that "load sharing" will 
in all likelihood not result in a perfect 50-50 distribution of inbound 
and outbound traffic across both links, nor should you expect this.  Also 
note that to achieve some level of load sharing, you will need to take 
more than just a default route from your upstreams.  You don't 
necessarily need to take full routes from your upstreams, unless you're 
providing full BGP routes to downstream customers.

jms