setup where traffic is shared over two BGP sessions to ISP

How is such setup called where two local routers(customer routers) are connected with one ISP router and there is one BGP session between each local router and ISP router(in case of two local routers there are two BGP sessions in total)? As much as I understand, it isn't quite multihome setup because multihome requires at least two ISP routers while setup I'm thinking of has one ISP router and two local routers.
Local routers announce two /20 networks to ISP router and ISP router announces default route to both local routers. How to configure BGP in order to have traffic shared as equally as possible between two local routers(in other words between two BGP sessions)?

Find more posts tagged with

Comments

pham0329

dual-homed

DPG

maximum-paths 2

Forsaken_GA

It depends on whether or not you mean outgoing load balacing, or incoming.

In the case of outgoing, DPG's answer is correct, you need to allow more than one path to the external destinations so both routes get installed in the routing table, and it'll equal cost load balance across the circuits.

If you want to load balance the incoming traffic.. that's harder. You'll need to talk to the ISP and see what provisions they have in place for load balacing. Chances are, they're only going to have max paths set to 1, so whatever their routers select as the best path into your network is the one the traffic will take coming in.

The easiest way to do it is to break up your netblocks, assuming you have the space to do that (most providers won't accept anything longer than a /24 these days, so if a /24 is all you have, you're screwed) and announce them with different attributes, so that one ISP router prefers certain netblocks over others. You can do this by fiddling with the MED, or as path prepending, or using communities to influence local preference on the remote end if they're setup for that.

It's not a perfect solution, as you're essentially binding netblocks to specific connections, but while you have incredible control with how traffic leaves your system with BGP, your control with how it enters is very, very limited.