This shouldn't work!

[Joel Cagle]

I was hired on to rebuild my companies neglected network. when I say neglected I mean it has been over a decade since it was documented. it has been patched up with bubblegum and bailing wire, and it is a complete mess. My current issue is I am documenting the core of the network, and seeing what needs to be upgraded. IT SHOULDN'T WORK! I have my core switch connected by OM4 single mode fiber with a ten gig SFP in the switch, and the other end going into the buildings patch panel. There is OM4 from the demark switch (and it is a ten gig SFP) and the other end going into the buildings patch panel. I just found out that the building is wired with OM1 multi-mode fiber between all of the internal patch panels. How is this even working? I have always been taught that you can't mix multi and single mode fiber. When I look at the switches the show ten gig bandwidth, and up up connections. With no bad packets, no data loss, no errors of any kind. I am afraid to touch anything because if I cause what ever, Voodoo networking miracle that's happening here, to stop working I am in big trouble. Does anyone have a recommendation of where to step next? or should I leave a sleeping dog lie, and pray it keeps working?

[Leo Laohoo]

You're probably have some stroke of luck. I've seen line errors when mixing OM4 with anything else. More so with it's on a 10 Gbps link.
I would probably get someone to rerun a new fibre run as "just in case" the existing one should break.

[chrihussey · Accepted solution]

Hello,

OM4 is a 50m multimode fiber. OM1 fiber is 62.5m also multimode. The difference between the two is the core size, purity and supported data rates. OM1 is not spec'ed for 10g at any great distances, but you might get away with it.

In my environment, I mix OM1 and OM3/4, mostly with 1G rates and have had little to no issues as long as I keep things within spec.

Running SM optics over OM1 and OM4 fibers may also provide better success than MM optics, so that may be why things are working for you. You should however,  check the error rates of the connections, as this may indicate the quality of the connections.

The bottom line is that, yes it is working, but is far from optimal and in order to support the 10Gig optics and future needs, upgrading the fiber infrastructure is definitely in order.

Hope this helps.

[Tom Randstrom]

OM1 and OM4 are both multi-mode fibers; OM1 has a 62.5um core and OM4 has a 50um core. 

What type of 10G SFP+ is installed in the switches?  The SFP-10G-LRM transceiver module will operate over OM1 fiber when a mode conditioning patch cord is used on each end of the link. This mode conditioning cable may be the fiber you are seeing coming off the 10G SFP.   

[Joseph W. Doherty]

I too was going to mention the possibility of a mode conditioning patch cord, but Tom beat me to it. ;)