[ixpmanager] IXP Manager Route Server Behaviors

[Nick Hilliard]

Hi Richard,

Richard Laager via ixpmanager wrote on 16/01/2024 01:10:
> RFC 7947, 2.3.2.2.2 discusses BGP ADD-PATH. This is what I was looking 
> into which lead me here. I am looking at adding "add paths tx;" to the 
> route server template. However, this thread about a potential BIRD bug 
> gives me pause:
> https://bird.network.cz/pipermail/bird-users/2024-January/017296.html

at the time those templates were written, add-path support was 
universally poor and there were no compelling reasons to start deploying 
it at route servers because multiple ribs worked well. Some of the 
limitations of the era still persist in some routing stacks, e.g. 
support for add-path on ibgp only (huawei), or no support at all (lots 
of stacks). Also, it adds complexity to an already complex system.  I'd 
argue that at the point that this makes a difference to you, bilateral 
peering is likely to be a better option.

> However, if the IXP participant is filtering on receive, ADD-PATH can 
> still provide value. That is, if the participant would filter out the 
> best path (from the route server's perspective) but would not filter 
> out an alternate path, then ADD-PATH would allow that participant to 
> use that alternate path.

Yep, there are edge cases where add-path would have a better outcome 
than not having it. The cost of this is additional complexity, i.e. more 
likely to go wrong, and more difficult to debug when it goes wrong.

> My reasoning about ADD-PATH largely assumes that receive-side 
> filtering could not be moved to the route server. I was also given 
> this link (see "IRRDB based Filtering"):
> https://www.ams-ix.net/ams/documentation/ams-ix-route-servers
>
> They have apparently implemented a script that honors:
> https://www.ietf.org/archive/id/draft-ietf-grow-rpsl-via-01.txt
>
> That would be a lot more work to implement. Does anyone here know 
> AMS-IX people? Would they be willing to share their script?

Their contact details are here: https://bgpview.io/asn/52462

The export-via / import-via hacks to rpsl never made it to rfc status. 
Also, out of the tiny number of people who use rpsl at all, most of 
these don't publish usable routing policy using export/import / 
mp-export/mp-import.

> RFC 7947, 2.2.3 says to preserve the MULTI_EXIT_DISC. Is BIRD doing 
> that (with the IXP Manager template config)?

yep bird does this by default in transparent RS mode.

Nick

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