[bess] A question regarding draft-wang-bess-evepn-control-word

Alexander Vainshtein <Alexander.Vainshtein@ecitele.com> Tue, 23 October 2018 08:46 UTC

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From: Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
To: "draft-wang-bess-evpn-control-word.authors@ietf.org" <draft-wang-bess-evpn-control-word.authors@ietf.org>
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Thread-Topic: A question regarding draft-wang-bess-evepn-control-word
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Date: Tue, 23 Oct 2018 08:46:15 +0000
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Subject: [bess] A question regarding draft-wang-bess-evepn-control-word
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Dear authors of draft-wang-bess-evpn-control-word<https://tools.ietf.org/html/draft-wang-bess-evpn-control-word-00>00>,
I have doubts regarding at least one of the approaches for negotiating the CW usage in the EVPN encapsulation between egress and ingress PE that is defined in the draft.

In the case when the egress PE can receive EVPN-encapsulated packets both with and without CW, the draft seems to propose (as one of the possibilities) advertisement of two EVPN routes for each ES or MAC/IP pair:

-          One of these routes would use the CW Capability to indicate that it refers to the EVPN encapsulation that uses the CW, and would carry the appropriate label in its NLRI

-          The other route would not use the CW Capability to indicate that it refers to the EVPN encapsulation that does not use the CW, and carry a different label in its NLRI

The ingress PE that accepts these routes would then use one of them based on its own ability to use the CW (or lack thereof), and use the corresponding label it its EVPN encapsulation, while  the DP in the egress PW would infer presence or absence of the CW from the received EVPN application label.

Unfortunately, I do not think that this can work because, as per RFC 7432<https://tools.ietf.org/html/rfc7432>32>, labels in the labeled NLRI of EVPN routes are not part of the route key for the purpose of the BGP route key processing, while the label is treated just as the BGP attribute. This means that, unless some form of BGP multi-path is enabled in the ingress PE (and in all RRs on the way between the egress PE and ingress PE) for the L2VPN/EVPN  AFI/SAFI, only one of these routes will be selected by the BGP selection process.

Did I miss something substantial here?

Regards, and lots of thanks in advance,
Sasha

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