Exporting a larger prefix if a smaller prefix is being exported

[Nico Schottelius]

Hey Luke,

Lukas Haase via Bird-users <bird-users at network.cz> writes:

> Hi,
>
> Is is somehow possible to export a larger prefix if one or more sub-prefixes (subnets) are exported ... but also remove that prefix if no smaller subnet exist any more?

a short story about /24's in the Internet and how to handle them from my
perspective.

Some years ago I have been running into a very similar situation,
because of the /24 requirement in the DFZ. While in the beginning it
"feels strange" that you have to take care of it yourself, in the eyes
of the Internet whatever is smaller than a /24 is "your problem", as the
whole block belongs to you.

This is however NOT an eBGP requirement, if you are doing private
peering or non DFZ peerings, this requirement does not affect you.

Coming back on how to handle this: usually you will have one ASN that
announces the /24. If you are at multiple locations, you will announce
the /24 multiple times and now the question is: but how do you ensure
that parts of that /24 are routed to the right site?

Well, that is within your realm: within your ASN you have various ways
of internally routing to ensure that both sites know where to find the
smaller prefixes. This can be done via IGP/iBGP/VPNs/Mesh, whatever
comes to your mind to internally ensure that the routes are present.

In the IPv6 world this problem is way less enunciated, because you
usually have access to multiple /48s, which allow you to use location
specific announcements instead of having to "hack it".

So in short: you won't get the "smart logic" you described
yourself automatically, but you will need to announce the /24 on each
site and figure out how to route the traffic over to the other sites for
smaller prefixes on your own.

Or go with IPv6 and location specific /48's, which is much easier.

HTH and have a good weekend,

Nico

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