Calling all my Cisco heros,
I recently implemented a Cisco 3850-12XS to be our core switch in our environment. We receive our uplink from another department in the building over fiber and currently the configuration is working on our 2960x switches.
Tonight I attempted to switch over to our 3850 switch which should have been straight forward. Unplug fiber from 2960x and plug into port 1 on 3850. However whenever I do that the 3850 is not seeing any connection. It does recognize the sfp module and I confirmed that the port is not in an err-disabled state.
I then attempted to plug the 3850 directly into the 2960 using the exact same sfp module, port and fiber patch cable. It recognizes the connection and I am then able to access the 3850 anywhere on my network. Plug the 2960x back into the uplink port and it immediately sees the uplink and I have my full network again.
Move the uplink back into the 3850 gi1/0/1 again. However it does not recognize the connection at all. I verified that the fiber tx and rx are not interfering and even intentionally crossed them to attempt a err-disabled message on that port but no avail. I replaced the sfp module and fiber cable. At this point I'm a little confused on what would even cause this. The configuration is working on the 2960 in the exact same room. However the 3850 is acting like it can't see anything coming from the uplink at all. It acts like the fiber is not connected but as soon as I plug into the 2960 it communicates as it should.
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