[GROW] Limiting AS path length?

[Iljitsch van Beijnum <[e-mail]>]

Dear Global Routing Operators,

I attended a presentation by someone from a tier-1 network who talked about BGP filtering. One thing he mentioned is filtering out prefixes with excessively long AS paths, in their case paths longer than 40 AS hops.

There are a few best practices style documents that suggest this:

http://bgpfilterguide.nlnog.net/guides/long_paths/ <http://bgpfilterguide.nlnog.net/guides/long_paths/>

https://nsrc.org/workshops/2018/linx103-bgp/networking/peering-ixp/en/presentations/05-BGP-BCP.pdf <https://nsrc.org/workshops/2018/linx103-bgp/networking/peering-ixp/en/presentations/05-BGP-BCP.pdf>

My question: is rejecting excessively long AS paths something we want to do?

If so, I think it's important to publish a best practices document that creates clear expectations, so we avoid the situation where people prepend their paths, and then those paths are filtered by some ASes but not others.

Similar how there's a clear expectation that any IPv4 prefix of /24 or shorter will be accepted by all ASes but ones longer than /24 will not, /48 for IPv6.

FYI: the number of IPv4 paths with AS paths with 20 - 45 hops (with 45 being the maximum currently seen by Routeviews) is 0.04% of all 32 million paths.

Iljitsch