Advertising Subnets smaller than /24 in BGP

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May 7th, 2009
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We are planning our BGP implementation and creating our subnets from our provided /22 IP block. We are aware of the fact that most ISP's will not advertise any subnets smaller than /24. But we would like to split our /22 IP block between our various sites without using more IP addresses than we have to. Our plan was to split the /22 block into /25 blocks and then split a couple of the /25 blocks into smaller blocks. But even if our ISP's agree to advertise the smaller blocks, they will most likely not get advertised throughout the Internet. Is there some way to split the /22 block into blocks smaller than /24 and still get them advertised throughout the Internet? We have five sites and we do not want to use /24 blocks at each site and waste unused IP addresses.


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Giuseppe Larosa Thu, 05/07/2009 - 10:27
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Hello Ken,

the risk is that some ISP will aggregate to /24 boundary and this will create the idea that a single /24 is reachable via two sites.

You can do this only if all your sites are using the same provider and making agreements with him.

The provider will advertise an aggregate for you.

If multiple providers are involved unless all of them make an agreement on passing the /25 between them you will have troubles.

I would suggest to join two sites with any means (an EoMPLS link for example) and using /24 prefixes out the four resulting locations.

Hope to help



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