Symptom

--Ports on a 9400 switch may stay in loopback state after testphyloopback or testunusedportloopback diagnostic test, when native vlan is configured but not present in the vlan database. --Affected port will also see itself as its own cdp neighbor during the loopback condition. --If STP is in use then the port may also go into the STP backup role / blocking state

Conditions

When native vlan is configured at the interface level but not present in the vlan database. if the interface is shut state/admin down. The Diagnostic health monitoring test (TestUnusedPortLoopback) will be triggered every 90sec leading to this issue. or if dynamically run (TestPhyLoopback) also with native vlan config without having global VLAN entry.

Workaround

Work Around 1: --Add the native VLAN ID to the VLAN database. Workaround 2: If the native VLAN cannot be created --Remove the native VLAN configuration from ports on the shutdown state --Run the diagnostic again for "TestUnusedPortLoopback" by using the "diagnostic start module test TestUnusedPortLoopback" command Workaround 3: --Disable the TestUnusedPortLoopback diagnostic test on the module by using the "no diagnostic monitor module test TestUnusedPortLoopback" command The best solution for moving forward is: --Keep unused ports in their default state. --Unshut the unused interface before applying any configuration if planning to use the interface. Workaround 4: -- Fake remove the SFP if its SFP port or physically remove SFP and insert back it back to clear the loopback. Make sure to add this step along with any other above workaround to avoid being hit by the issue again.

Further Problem Description