From: Alexander Zubkov

Date: 2025-12-09 03:11

To: zx@zxinc.org

CC: bird-users

Subject: Re: Writing filters that detect intersection between a prefix and a prefix set

Hello,

If you want to match any prefix that intersects (less or more specific) with some prefix of interest - you can use a prefix set like:

  define filter1 = [ 192.168.1.0/24{0,32}, 192.168.2.0/24{0,32} ];

I.e. you need to set the mask range from minimum to maximum - from 0 to 32.

Then you can accept or reject depending on the match result. For example:

  if net ~ filter1 then reject;

Regards,

Alexander

On Sat, Dec 6, 2025 at 6:42 AM zx@zxinc.org <zx@zxinc.org> wrote:

Hello,

I have a prefix set like [ 192.168.1.0/24+, 192.168.2.0/24+ ] and I want to write a filter that

filters all prefixes intersects with the prefix set and accepts all others, that is,

accepts 192.168.0.0/24, 192.168.3.0/24, 10.0.0.0/8,

rejects 192.168.1.1/32, 192.168.1.0/24, 192.168.2.0/24, 192.168.2.0/23, 192.168.0.0/16, 192.0.0.0/8,

Anyone who knows how to write such a filter? Is it possible?

Thanks.

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zx