Re: [Idr] Bug in RFC 7911 (add-paths) and tie-breaking

bruno.decraene@orange.com Fri, 25 June 2021 15:54 UTC

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From: <bruno.decraene@orange.com>
To: Robert Raszuk <robert@raszuk.net>
CC: Claudio Jeker <cjeker@diehard.n-r-g.com>, John Scudder <jgs=40juniper.net@dmarc.ietf.org>, "idr@ietf. org" <idr@ietf.org>, "dwalton76@gmail.com" <dwalton76@gmail.com>
Thread-Topic: [Idr] Bug in RFC 7911 (add-paths) and tie-breaking
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Date: Fri, 25 Jun 2021 15:54:37 +0000
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Subject: Re: [Idr] Bug in RFC 7911 (add-paths) and tie-breaking
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From: Robert Raszuk [mailto:robert@raszuk.net]
Sent: Friday, June 25, 2021 5:40 PM
To: DECRAENE Bruno INNOV/NET <bruno.decraene@orange.com>
Cc: Claudio Jeker <cjeker@diehard.n-r-g.com>; John Scudder <jgs=40juniper.net@dmarc.ietf.org>; idr@ietf. org <idr@ietf.org>; dwalton76@gmail.com
Subject: Re: [Idr] Bug in RFC 7911 (add-paths) and tie-breaking

Bruno,

Don't agree. If a BGP speaker spends two paths with path_id they will be identical on both RRs.

[Bruno] You are assuming that your two “diverses RR” learn the paths from the same (set of) peers. I agree that this is typical, but this is not guaranteed. If not, the path-id would not be comparable.

If they are from different peers there is no problem to start with as best path already picks path from lowest neighbor address.

I meant two Add Path RR peers. Each peer/session advertising both paths in one session. Cf below topology


Add Path RR1a --- Diverse RR1
Add Path RR1b -----+



Add Path RR2a --- Diverse RR2
Add Path RR2b -----+

Notes:
- “—“ is a IBGP sessions advertising the two paths.
- I agree that the topology is debatable and does not match typical deployments. That’s all I found so far to find a counter example for your Diverse Path use case, so let’s call my example weak and close the Divers Path discussion

Thanks,
Bruno


Thx,
R.

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