Here, the issue can be in multiple places your SFP module, your SFP slot, ISP SFP module / slot, cables, patch panels, etc. Try to troubleshoot this step by step until you isolate the issue.
1. Reseat SFP and Cable (at both ends). Clean and reconnect and check if it works. Proceed to next stop, if not luck.
2. SFP may be faulty - Do a loop test to check if SFP is good.
- Connect the Tx of the SFP to Rx of the same SFP. Check if the port comes back up, if yes, SPF is good. If not, SFP may be faulty.
- Why may be? Because SFP slot in C93108TC-FX chassis can also be faulty.
- So, connect the SFP and do a loop test in a known working SFP slot of C93108TC-FX chassis, to make sure issue is with SFP module or SFP slot.
- You can also check with a working spare SFP, if you have any
- Similarly do loop test in ISP device as well, just to validate no issues in their SFP or port.
3. If SFP is good at both ends, then continue the loop test hop by hop where the cable terminates, in the fiber patch panels until the ISP device to isolate the issue
- Check how the cable is connected, is it directly from N9K to ISP device or you have one or more patches running in between.
- If you have one or more fiber patch panels, you trace the patch panel and do loop test in every hop and check if the interface comes up or not.
- Let's say you have 4 hops, in first hop when you loop of Tx and Rx towards your N9K, if it comes up, then until that point, the cables are good. You proceed further, until you see the interface remaining down.
- Make sure you trace the proper cables, based on tags or labels.
- Involve any cable vendors if the building cabling is not under your control or if you do not have proper tools to check
