Re: [bess] Signaling Control Word in EVPN

James Bensley <> Tue, 09 October 2018 14:35 UTC

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From: James Bensley <>
Date: Tue, 09 Oct 2018 15:34:52 +0100
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To: "Andrew G. Malis" <>
Cc: Alexander Vainshtein <>, Muthu Arul Mozhi Perumal <>,,,,,,, Rotem Cohen <>
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Subject: Re: [bess] Signaling Control Word in EVPN
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On Tue, 9 Oct 2018 at 15:16, Andrew G. Malis <> wrote:
> James,

Hi Andy,

> It's much harder to mandate use of EL than the CW for several reasons:

I didn't say it should be mandated, but recommended.

> - CW implementation is much more common than EL implementation
> - PWs and/or EVPN are rarely the only traffic in an MPLS traffic tunnel, rather, they will be multiplexed with other MPLS-based applications that are using the traffic tunnel to reach a common destination. Thus, by using the CW, you can disable ECMP only for those MPLS packets that cannot tolerate reordering.

The CW does not disable ECMP. Any LSR on the path between ingress and
egress LER is free to look beyond the MPLS label stack and
misinterpret the 0x00 0x00 at the start of a control-word as a valid
MAC that starts 00:00:XX:XX:XX:XX and try to hash on Ethernet headers
starting directly after the MPLS label stack, and not label stack + 4
bytes. This is my point. The PWMCW doesn't stop re-ording in all
cases, but it does in most. So yes, not all devices support EL, but CW
doesn't stop re-ordering in all cases, so?

> That said, I'm also concerned that because of the existing text in 7432, implementations may not be using the CW even for P2P EVPN.
> And we still don't have a good answer for Muthu's original question. :-)

Sorry my intention is not to send this thread off-topic.