Limitations of BGP AS confederations?

[Erik Zscheile (Software Development)]

Hello,

Am 06.11.20 um 04:56 schrieb Ondrej Zajicek:
> On Fri, Nov 06, 2020 at 12:01:20AM +0100, Erik Zscheile (Software Development) wrote:
>> in an article [1] (which was one of very few articles which I found about BIRD + BGP AS confederations)
>> it is stated that currently, BGP AS confederations have the following limitation:
>>
>>> Bird won't consider the confederation part while calculating distance, which leads to weird routing results.
> 
> That is expected behavior per RFC 5065 5.3 (3).
> 
> The idea is that inside confederation BGP next hops are preserved when
> paths are propagated from one internal ASN to another, therefore paths
> are compared based on IGP cost to confederation border (assuming shared
> IGP routes / metrics within whole confederation).
> 
>>> Bird neither provides a variable for the filter to calculate confederation length and make manual adjustments.
>>>   * bgp_path.len in Bird doesn't contain the length of Confederation, as stated above;
> 
> It is true that bgp_path.len returns AS PATH length without confederation
> ASNs, to be consistent with AS_PATH length used for best route selection.
> 
> Seems to me that the best tool for your case might be AIGP option (RFC
> 7311, implemented in version 2.0.7).
> 
This might be a good fit, yes. But from the documentation I wasn't able to
conclude how it should be used (e.g. how to set or increment the AIGP value)...

> BIRD uses AS_CONFED_SEQUENCE, treats AS_SEQUENCE as length n, AS_SET as length 1
> and AS_CONFED_SEQUENCE / AS_CONFED_SET as length 0.
Thanks for that information. Maybe this should be mentioned in the documentation,
as it is non-obvious.

Regards,
Erik Zscheile