How to troubleshoot the link failure?

Hi Steelblue,
You can analyze failure causes based on the OSI reference model from bottom to top.
1. Check whether the indicators on switches are on. It is found that the indicators are off.
2. Check the interconnected interfaces on the two switches. Enter the corresponding interface view and run the display this interface command to check whether the interface is shut down. If so, run the undo shutdown interface command to enable the interface.
3. Check whether the optical fibers are damaged. You can replace the existing optical fibers with normal ones to check the connectivity. If an optical distribution frame (ODF) is used, check for faults between the ODF and switches.
4. Check whether the transmitter and receiver of the optical fiber are reversely inserted.
5. Check whether the types of optical modules at both ends are the same, including the transmission distance, wavelength, and interface mode. It is recommended that optical modules of the same brand be used.
6. Check whether the optical modules match the optical fibers. Single-mode fibers should be inserted into single-mode optical modules and the multi-mode fibers should be inserted into multi-mode optical modules.
7. Check whether the rates configured for the two interconnected interfaces are consistent.
8. Check whether the negotiation status of the two interconnected interfaces is consistent. The negotiation status of the interfaces at both ends must be either auto-negotiation or non-auto-negotiation. In addition, check whether the two interfaces can go Up when they work in either auto-negotiation or non-auto-negotiation mode.
9. Check whether the transmit and receive optical power of the optical interfaces is within the allowed range. If the receive power of optical interfaces is excessively low, interfaces cannot go Up. The excessively long transmission distance or low optical fiber quality may also be the reason why interfaces cannot go Up.
10. Check whether a transmission device exists between the two optical interfaces and whether the transmission device is misconfigured.
11. If the preceding items are all normal, perform internal loopback test on the interfaces. Connect the head and tail of an optical fiber to an optical module to check whether the interfaces can go Up.
12. If the fault cannot be located after the preceding operations are performed, replace the optical modules or fibers.
13. If the receive optical power of optical modules is excessively high, the interfaces may not go Up.