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BGP weighted routes

brad
Level 1
Level 1

Can I split my AS into two subnets and wieght each subnet such that a preference is given to one ISP for subnet A and the other ISP for subnet B.

Specifically, I have a small AS (globally routable) - call it 10.1.1.0/24 peered using two routers to two ISPs.

I have HSRP configured on the inside i/f of the two routers using two HSRP groups. For group 1, R1 is the active router and R2 is the standby router. For group 2, R2 is the active router and R1 is the standby router.

Traffic arriving at the R1 router is routed through ISP1 and traffic arriving at the R2 router is routed through ISP2.

Can I split my "10.1.1.0/24" subnet into two subnets say 10.1.1.0/28 and 10.1.1.128/28 and wieght the routes so that return traffic will prefer the path to the originating router?

Brad

P.S. Sorry, my first post was to the wrong group.

2 Replies 2

attrgautam
Level 5
Level 5

Yes you can do it not by using Weight which has local significance but by using AS Path Prepend. The Route which has the lowest AS Path will be preferred.

Check this link out for configuration

http://www.avici.com/documentation/HTMLDocs/02223-06_revBA/Routing_Pol30.html

mheusinger
Level 10
Level 10

Hi Brad,

As far as I understand you want to load balance the internet RETURN traffic.

You could split your address range and use AS path prepending. BUT it is highly unlikely that a prefix smaller than /24 will give you any connectivity at all, as many providers (f.e. MCI) will not accept anything smaller than /24. So even in case you could convince your two providers ISP1 and ISP2 to accept them, their next upstream AS would most likely not accept those prefixes. So in short:

If you cannot announce at least two /24 there is little hope to achieve load sharing for the return traffic.

If you have lets say 10.1.0.0/24 and 10.1.1.0/24 then announce to ISP1 through R1:

10.1.0.0/24 AS path 65000 65000 65000

10.1.1.0/24 AS path 65000

and to ISP2 through R2

10.1.0.0/24 AS path 65000

10.1.1.0/24 AS path 65000 65000 65000

assuming your AS is 65000.

Regards

Martin

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