[c-nsp] SFP-10G-LRM in Nexus 5000

Nick Hilliard nick at foobar.org
Mon Sep 14 18:27:25 EDT 2015


On 14/09/2015 23:15, Crist J. Clark wrote:
> I am not exactly sure of the design decisions behind the choice to use LRM over
> SMF to orginally connect the distro to the core when the campus was built. But
> given that it was working, and we already had almost enough spare LRMs on hand
> to complete the project, It Seemed Like a Good Idea at the Time.

Right. :-(

> We also have OM3 MMF between the buildings so replacing this all with SR is
> the main plan right now, but whatever the problem is with the LRM modules
> looks more like software/firmware than a problem with the fiber paths (the links
> always do eventually come up with good light readings and no errors), and we
> were just curious if anyone had seen this issue with 5ks and LRM before and
> found a way to deal with it.

LRM transceivers are a bit of a curiosity tbh.  I've never seen anyone
using them in production.  Because they're uncommon, this has the
consequence that they will not be well debugged.  In many situation this is
not a problem, but if you have dispersion problems, it can be, because
dispersion compensation on 10G SFPs is handled on the switch / router
motherboard.  I.e. this can easily be a software problem.

Also, if you're using LRM out of spec (SMF cabling instead of MMF), you
should probably expect problems.

LR SFPs are cheap: ~$100 each for transceivers from reputable third
parties.  You will save yourself time, money and heartache by sticking with
LR over SMF for longer runs and not using MMF

Nick




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