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Cisco Bug: CSCvv18112 - Ports on a 9400 switch may stay in loopback state if native vlan is not present in the vlan database

Last Modified

Sep 23, 2020

Products (1)

  • Cisco Catalyst 9400 Series Switches

Known Affected Releases

16.12.3

Description (partial)

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.
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