[isp-bgp] Re: Outbound Balancing

[Iljitsch van Beijnum <iljitsch@...>]

On 23-apr-04, at 22:35, Serge Paquin wrote:

>     What are other options other than prepending inbound AS paths to 
> try to
> even out bandwidth usage between 2 links?  I'm connected to 2 ISP's 
> and my
> outbound traffic heavily prefered one ISP.  I prepended their AS 1 
> time and
> now my problem has almost reversed.  Most of my traffic flows down the 
> other
> ISP.
>     Do I have any other options?
You can set the local preference, as per James' example. The downside 
of that is that it completely overrides looking at the path so it's a 
bit severe. Or you can use the multi exit discriminator metric. But 
this has a pretty much identical effect to prepending, so you need to 
do it selectively, for instance:

!
router 123
 bgp always-compare-med
 neighbor 1.2.3.4 route-map transit-a-in in
 neighbor 5.6.7.8 route-map transit-b-in in
!
ip prefix-list pref-over-a permit 0.0.0.0/2
ip prefix-list pref-over-a permit 128.0.0.0/2
!
route-map transit-a-in permit 10
 match ip address prefix-list pref-over-a
 set metric 10
!
route-map transit-a-in permit 20
 set metric 30
!
route-map transit-b-in permit 10
 set metric 20
!

This gives all routes from B a metric of 20 and address space withing 
0.0.0.0 - 63.255.255.255 and 128.0.0.0 - 191.255.255.255 a metric of 10 
over A and all other address space a metric of 30 over A. The metric 
only gets looked at when the AS path is the same length so basically 
you're only overruling the tie breaker rules.







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