BGP - How to control Ingress Traffic When using Multihoming

Answered Question
Apr 30th, 2009
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Hi everyone,

Network Design: We are using BGP with two ISP peers, and we are using AS Path prepending to somewhat control how traffic comes into our network.

Problem: For the most part AS path prepending is working as intended, but in some cases some networks ingress from the non-preferred ISP. There is one network in particular that we would really like to ingress through a particular ISP.

Proposed Solution: I am thinking of advertising this network to both ISPs, but with different subnet masks. Right now we are advertising the network with 24 bit mask to both ISPs. And I would like to advertise this network to the "non preferred" ISP with a 23 bit mask.


Any advise or suggestions?


Thanks,

Mohamad

Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Mohamad,


you should be fine if the whole /23 block is under your control (if the company owns it as a whole).

Clearly ip addresses will be only on the most specific /24 space.


A /24 should be seen in the internet and as most specific route should be preferred


Hope to help

Giuseppe


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you could just add more to your prepend to assure that the network in question is coming to your preferred isp.


other than that you could do as you proposed but that would lack redundancy if either went down. you could do a /23 on both sides including the /24


ISP-A

10.1.1.0/23

10.1.1.0/24 < use a route-map to set prepending on this advertisement


ISP-B

10.1.1.128/23

10.1.1.0.24 < use a route-map to set prepending on this advertisement

Correct Answer
Giuseppe Larosa Thu, 04/30/2009 - 10:17
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Hello Mohamad,


you should be fine if the whole /23 block is under your control (if the company owns it as a whole).

Clearly ip addresses will be only on the most specific /24 space.


A /24 should be seen in the internet and as most specific route should be preferred


Hope to help

Giuseppe


dgroscost Wed, 05/06/2009 - 11:39
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Do either of your ISPs support BGP communities?

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