BGP smallest segment

[velimirmkd]

I cant find info on what is the smallest segment that router can advertise in BGP. I've done a C class but is it possible to advertise just one address or it has to be a whole class?

For example can I advertise host 129.45.6.78 to one ISP peer and the rest of the class 129.45.6.0 to the other ISP peer.

Thanks

Velimir

[reinke]

Hi Velimir,

BGP-4 is able to advertise every prefix/mask combination, because it supports CIDR (even a host route is possible).

Most multi-homed people are splitting their address space for load-sharing. Be aware that prefixes greater than /20 are not advertised throughout the internet, because this is actually the smallest allocation size of the Internet Registries and there is no need for the global routing to know e. g. a /22.

Remember that all routes you which to advertise have to be in the routing table of your router.

Edgar

[james.feger]

That used to be the case. It is not uncommon to see /24's in the full routes table of the internet now. To answer the original question, you can advertise any block size you want, its up to the peer to accept, reject, or even readvertise. If you are dealing with a major ISP, chances are you wont get anything longer than a /24 advertised out to the net, and even then I think a few of the large carriers still require a /23 or /22 unless you talk nice to them.

[ruwhite]

Beyond this, even if your ISP accepts the longer the /24 prefix, their peer may not, which could cut your connectivity off pretty severly.

Russ

[xavierchang]

Like my colleague pointed out, your information is outdated. The fact is, the global Internet BGP table is now filled with prefixes much longer than /20. I've seen a few /26 and even /28's from time to time.

[velimirmkd]

Thank you all for your answers. From the conversations I see that it is not advisable to advertise anything smaller than /24. That is acctually what I suspected it would be, and I will try to split my 32 C classes block.

Thanks again

Velimir