Mostly what Matt said. ( I should have also said 'ride the 0/0 train INTO the DFZ, my mistake.)

Essentially, if ASN X is announcing a prefix with an excessive number of prepends, they are saying to the world 'This path exists , but it is hot garbage.' I'm more than happy to oblige those instructions and just drop that announcement completely. If ASN X announces that prefix with a reasonable number of prepends, happy to take it and use it.

If I get a prefix with an as-path longer than 10, (regardless of the state of prepends), I interpret that as :

1. They have made a mistake.

2. Someone else made a mistake.

3. They think that's a good idea, and have some things yet to learn. ( The old clue by four as Matt put it.)

4. It's malicious.

5. They actually are somehow more than 10 ASNs away from me. ( I don't even know if this is possible anymore without extreme effort. )

In any of those scenarios , I don't really care about optimized routing to that destination. Perfectly happy to just follow 0/0 to a DFZ upstream and let it go how it's going to go, or not. If there is a traffic impact such that an exception HAS to be made, that can be addressed as needed, but I can't think of a single circumstance I have hit where the correction involved accepting an obnoxiously long as_path announcement. ( I don't mean to imply none exist ; I'm sure someone has had to work though that. )

Maybe a length of 10 doesn't work for some environments and use cases, but I still am a firm believer that at a certain point, there is a length that becomes straight 'really don't care'. I'd rather not discover a BGP implementation problem or acid trip memory pointer party by accepting announcements that are useless.

On Fri, Mar 25, 2022 at 5:56 PM Adam Thompson <athompson@merlin.mb.ca> wrote:

Tom, how exactly does someone “ride the 0/0” train in the DFZ?

I’m connected to both commercial internet and NREN, and unfortunately-long paths are not uncommon in this scenario, in order to do traffic steering. If there’s another solution that affects global inbound traffic distributions, I’d love to hear about it (and so would a lot of my peers in edu).

If there were a usable way to “dump” the excessively-long path only as long as a better path was already known by at least one edge router, that might be workable, but you’d have to keep track of it somewhere to reinstall it if the primary route went away… at which point you may as well have not dropped it in the first place.

-Adam

Adam Thompson
Consultant, Infrastructure Services

100 - 135 Innovation Drive
Winnipeg, MB, R3T 6A8
(204) 977-6824 or 1-800-430-6404 (MB only)
athompson@merlin.mb.ca
www.merlin.mb.ca

From: NANOG <nanog-bounces+athompson=merlin.mb.ca@nanog.org> On Behalf Of Tom Beecher
Sent: Friday, March 25, 2022 4:13 PM
To: Paschal Masha <paschal.masha@ke.wananchi.com>
Cc: nanog <nanog@nanog.org>
Subject: Re: DMARC ViolationAS21299 - 46.42.196.0/24 ASN prepending 255 times

The best practice with regards to as_path length is to have an edge filter that dumps any prefix with a length longer than say 10. Depending on the situation, might even be able to go smaller.

At a certain point, keeping that route around does nothing for you, just shoot it and ride the 0/0 train.

On Fri, Mar 25, 2022 at 4:09 AM Paschal Masha <paschal.masha@ke.wananchi.com> wrote:

:) probably the longest prepend in the world.

A thought though, is it breaking any standard or best practice procedures?

Regards
Paschal Masha | Engineering
Skype ID: paschal.masha

----- Original Message -----
From: "Erik Sundberg" <ESundberg@nitelusa.com>
To: "nanog" <nanog@nanog.org>
Sent: Friday, March 25, 2022 6:43:38 AM
Subject: DMARC ViolationAS21299 - 46.42.196.0/24 ASN prepending 255 times

If anyone from AS21299 is lurking on Nanog. Please reduce your AS prepends for 46.42.196.0/24 from 255 prepends to a more reasonable number of prepends let's say 20. Thanks!

This is a Kazakhstan register IP Block and ASN

Network Next Hop Metric LocPrf Weight Path

*> 46.42.196.0/24 x.x.x.x 0 100 0 2914 174 3216 3216 35168 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21 299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 21299 i

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