Symptom

Link is continuously flapping if CTS is configured on the interface - link will go admin down, then back up again. Issue occurs on both copper and fiber links. If CTS configuration is removed, then the link stays up. First seen in 16.3.x code. Example of flapping: *Jul 14 18:15:04.308: %LINK-5-CHANGED: Interface TenGigabitEthernet2/0/1, changed state to administratively down *Jul 14 18:15:06.310: %LINK-3-UPDOWN: Interface TenGigabitEthernet2/0/1, changed state to down *Jul 14 18:15:09.433: %LINK-3-UPDOWN: Interface TenGigabitEthernet2/0/1, changed state to up *Jul 14 18:16:09.437: %LINK-5-CHANGED: Interface TenGigabitEthernet2/0/1, changed state to administratively down *Jul 14 18:16:11.439: %LINK-3-UPDOWN: Interface TenGigabitEthernet2/0/1, changed state to down *Jul 14 18:16:14.573: %LINK-3-UPDOWN: Interface TenGigabitEthernet2/0/1, changed state to up

Conditions

CTS must be configured both sides of the link. Example: interface TenGigabitEthernet2/0/1 switchport mode trunk cts manual sap pmk 0000000000000000000000000000000000000000000000000000000000001234 mode-list gcm-encrypt end

Workaround

Remove CTS configuration to stabilize link in the meantime.

Further Problem Description