Hello, The mask range works magically! Thank you very much! zx 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. ---- zx