solution for prefix-splitting in eBGP

Unanswered Question
Sep 4th, 2007

Dear All,

I am looking forward to split a prefix for downlink traffic. Let me just put my thoughts in more specific case.

1. Running eBGP with 3-ISPs (Multihoming).

2. Advertised on three ISPs as per PBR.


1. I want to force the uplink via ISP-1.

2. I want to force the downlink traffic 6 class C networks via ISP2.

3. I want to force the downlink traffic for rest of 2 Class-C network via ISP-3.

please suggest the workaround with e-BGP.

I have this problem too.
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1. It can be achieved by either raise the weight and local preference for the ISP1 neighbor therefore favoring for outbound traffic.

2-3. You have to advertise the longer prefixes separately (C's) and use as prepending (longer the AS path therefore make less favorable) for the particular prefixes. For the rest you can either advertise the /22 block without prepending or the individual C's without prepending.

If your upstream ISP's are using more sophisticated routing policy based on communities you can also use that.

Hope it helps


Mohamed Sobair Tue, 09/04/2007 - 23:01


bellow are the suggestions:

1- setting a highest weight for a specific neighbor under the bgp neibor command would enforce all your upstream traffic.

2- to force the downlink, you either set the AS prpending option or the MED value, u can specify a route-map after the neighbors command & match certain prefixes then use the prepending option or manipulating the MED value.


Mohamed Sobair


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