[Idr] Re: revision idr-5g-edge-service-metadata to address Route selection issues #40 on the GitHub

[Jeffrey Haas <[e-mail]>]

Linda,

Thanks for your patience for this reply.

On Wed, Dec 04, 2024 at 07:08:59AM +0000, Linda Dunbar wrote:
> Jeff,
> 
> We added a new section (6. Policy Based Metadata Integration) to describes how the information carried in the Metadata Path Attribute is integrated into the BGP route selection process to address the Route selection issues #40 on the GitHub.
> 
> https://datatracker.ietf.org/doc/draft-ietf-idr-5g-edge-service-metadata/
> 
> Can you please check if the new section description is enough?

The contents in the new section 6 are an excellent step in the right
direction.  Thanks.

In particular, this is the first time it's been clearly articulated that:
1. The input to the decision process for the metadata mechanism is at the
top of the BGP route selection process.
2. That tie-breaking for it subsequently follows to normal BGP tie-breaking
rules.

(Recall that I've asked a few times "where in the process does this happen?"
This was the needed answer.)

This roughly corresponds to RFC 4271, §9.1.1.  You're asking for the
metadata preference to be calculated before this spot, right?  And does such
calculation differ if it's coming from eBGP vs. iBGP?

The procedure for selection at the reflector is partially correct.  What's
implied here is that the reflector may act as the deployment's "server" for
the best paths.  This works fine if the server is the only way routers in
the AS receive the routes mediated by the metadata attribute.

Where it may not work properly is if the reflector is receiving sets of
routes from the rest of the AS, but the rest of the AS may be partially
meshed on its own.  I.e., some routers can directly exchange iBGP routes
without the reflector.  If it's the case that the reflector, acting as a
"server" can come to a different answer than individual routers, you may end
up with inconsistent routing.

Possible fixes:
1. If centralized RR server is a model, recommend that routers ONLY peer
through such server RRs.  Or,
2. Consistent procedure means the RR is just another node.

-- Jeff