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solution for prefix-splitting in eBGP

gaurav_thapar79
Level 1
Level 1

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 192.168.0.0/22 on three ISPs as per PBR.

Requirement:

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.

3 Replies 3

kerek
Level 4
Level 4

Hello,

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

Krisztian

Mohamed Sobair
Level 7
Level 7

Hi,

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.

Regards,

Mohamed Sobair

Hi Mohamed,

As long as MED is not advertised after your peer's AS it will not influence the reverse path.

Krisztian

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