algorithm used by (RIPE region) ISPs to generate automatic BGP prefix filters

Jared Mauch jared at
Thu Feb 4 16:32:54 UTC 2016

> On Feb 4, 2016, at 6:58 AM, Henrik Thostrup Jensen <htj at> wrote:
>> In addition, in case of "as-set", an ISP needs to recursively find all the AS numbers from "members" attributes because "as-set" can include other "as-sets"?
> Some irrd servers, can expand this automatically (I think). But seriously, use a tool for this.
>> Quite a lot of question, but I would simply like to be sure that I understand this correctly.
> There are basically two abstractions:
> 1. as-set. Can contain other as-sets or as numbers.
> 2. prefixes are registered to an as-number.
> Remember that there are multiple IRR servers, and they mirror each other.
> Use to play around a bit :-).

Yes.  We record the customer ASN and the AS-SET for each AFI (v4|v6) and expand these and push updated lists to devices daily or on demand based on customer need.

You should be able to build off any of the mirrored IRRds out there as they all mirror each other, often with minimal lag (5-30 minutes).

The days of fetching via FTP once a day are long gone and a relic of the past.

I recommend using AS-PATH combined with prefix filters to keep your pants on.  Rejecting things like networks you may get transit from from customers, and peers helps avoid feeding my route leak system.

You should also not be using any IOS devices for BGP as documented in CSCuq14541 where they leak the full table.

- Jared

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