[j-nsp] 40G QSFP problems on QFX5100 after 16.1R6

[Sebastian Wiesinger]

Hello,

we've noticed problems with third party vendors QSFP 40G optics after
upgrading our JunOS on QFX5100. The problems manifest as a general
instablility on the QSFP links with symptoms like:

* Links take minutes to come up
* Links go down randomly
* Links show CRC/Align errors and packets get dropped

Further testing revealed that this is happening with 16.1R6 and higher
trains (tested 17.4 and 18.1).

After downgrading to 16.1R5-S2 the problems are gone IF you also
downgrade the host OS (force-host option). Only downgrading the JunOS
VM does not work.

Looking through release notes I noticed that PR1296011 got fixed in
16.1R6 and subsequent release trains. It reads:

    On QFX5100 platform, 40G interface might not come up if specific
    vendor direct attach copper (DAC) cable is used (eg: Molex cable 3m).

I wonder if some change for this PR caused these problems? Doe anyone
have the same problems and maybe already successfully communicated
this to Juniper? I have a case open right now but I'm not sure I'm
getting somewhere.

So in conclusion be wary about upgrading beyond 16.1R5-S2 when using
third party optics...

Regards

Sebastian

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