I have replaced old Cisco 1900 switches with 3850's in my plant process areas.
Added Fiber links between 3850's for increased performance. I can't get data transfer through the fiber uplinks.

I have assigned some of the LAN ports to the different VLANS and they are communicating correctly. The Gib Fiber ports when assigned to the VLAN do not pass data.

As a "Band-Aid" I installed Fiber media convertors to connect the fiber backbone to the VLANs. Data does pass through the convertors to the VLAN ports.

Not sure what I'm missing as why the fiber ports aren't passing data as the LAN ports are.

Gone through several Cisco manuals and checked multiple options with no success.

My process is operational so I am now limited in my access to the switches until the next shutdown.

Any suggestions would be appreciated.

  • 6
    Do you have the correct GBICs for the fiber type? What speed are you trying to run? Which GBICs are you using? What is the fiber type? You need to edit your question provide more information. Include any relevant information.
    – Ron Maupin
    Jun 9 '16 at 18:09
  • 2
    Configuration posted here can help, some one could see if you missed something Jun 9 '16 at 20:32
  • 1
    What kinds of connectors on the ends of the fiber? If the connectors at the end of each strand aren't married together you could have the strands reversed at one end. What's the interface status when the fiber is connected? Did you check for errdisabled status? Jun 9 '16 at 21:55
  • 1
    As others have mentioned, it sounds like the fibre and GBIC/SFP are incompatible. e.g multimode fibre and a single mode GBIC/SFP. Are the fibre ports up/up, up/down, or down/down when the fibre is patched into the switches?
    – OzNetNerd
    Jun 10 '16 at 2:01
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 14 '17 at 14:22

Make sure the module is recognized by the switch.

Issue: show inventory

Stack1#sh inv

NAME: "Switch 1 FRU Uplink Module 1", DESCR: "2x1G 2x10G Uplink Module" PID: C3850-NM-2-10G , VID: V01 , SN: XXX

NAME: "Te2/1/1", DESCR: "SFP-10GBase-SR" PID: SFP-10G-SR , VID: V03 , SN: XXX

If the Uplink module is not recognized, contact your sales rep. By the way, yours may vary slightly. The second named item is the GBIC.


Cisco 3850 Series Switch is a 10-Gbps network switch, which means you need to use 10G SFP+ optical module or under 10G optical module, such as 1G SFP. And make sure the optical modules is correctly connected with the corresponding port on the switch. Hope this would help.


The 1900 switch has 100-fx ports, which as the name implies, is 100 Mb. The Gigabit fiber ports can't operate at that speed.

  • I'm not sure the 1900s are still there. He was unclear if he replaced them all, or only some of them. "Added Fiber links between 3850's for increased performance." Connections from 3850 to 3850 should work, if he has all his ducks in a row. I was trying to get him to give us the scoop.
    – Ron Maupin
    Jun 10 '16 at 0:15
  • Yes, all the 1900's are gone. This is a direct link from 3850 to 3850. I have the Cisco SFP - GLC-SX-MMD.
    – fairchdm
    Jun 10 '16 at 14:21
  • My plant install is MMF for the backbone, I'll have to double check the cable jacket for exact details. The original ends were terminated SC, I added an adapter to change them to LC for the SFP. The fiber ports are 1GB. The LC's are married, the SC's were not. I tried the swapping ends of the SC's thinking they may have gotten reversed in the transition. I'll pull the configuration files for switches and post them next.
    – fairchdm
    Jun 10 '16 at 14:32

You may be up against a distance limitation. If the old switches were uplinked using 100Base-FX (100Mbps) realize that signal can propagate over 1 mile over FDDI-grade multimode fiber. Newer gigabit uplinks can propogate 1804 feet (550 m).

There are 100Base-FX transceivers for the 3850, so you might need to get those, or upgrade your fiber. There may also be a way for you to use a long haul gigabit optic paired with mode conditioning patch cords.


Verify distance limitations of the GBICs.

Make sure the speed commands (if any) match at both ends.

Verify it's a compatible GBIC for Cisco. (There are unofficial commands to use non-Cisco GBICs but I'll let you research that and use at your own risk)

Verify you have fiber transmit and receive flipped on one end. If fiber A is on the left make sure fiber B is on the left at the other end. If you're not sure just try flipping one end. It'll light up if that's the issue.

Try the switchport trunk nonegotiate command on the interface. I've had to do this with 3750's and 3850's in the past.


the nm modules you have is 2x1G and 2x10G. as per data sheet, the first 2 ports are 1G and the last 2 ports are 10G. so ideally yyou should use the te2/1/3 or 2/1/4 with the 10 g SFP as the first 2 ports will not support the SFP-10-R modules.

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