SupV 10 GE IOS version: 12.2(31)SGA8 IPBase-M.
Problem: Standby Sup engine reboot continuously. No service impact as active sup is taking care of the chassis.
Resolution: Reload active sup engine. redundancy forced-switchover command in privilege exec mode will not work because standby sup kept rebooting hence no chance for active sup to detect there’s a standby sup.
Replicating the problem:
Switch#redundancy forced-switchover command is executed, IOS might ask if you want to save your configuration, choose Yes. After active sup save and compressed the configuration the problem will occur.
An invalid command will be inserted by the switch on gbic port randomly. For my case, switchport trunk encapsulation dot1q was inserted by the switch on gi5/12, gbic ports only support dot1q and not ISL hence this command is not valid, you cannot negate the command by issuing no switchport trunk encapsulation dot1q, the switch will say it is invalid.
Prevention: while you do forced switchover, choose No when switch asked if you want to save the configuration. You should save the configuration first before doing redundancy forced-switchover.
When the command redundancy forced-switchover is executed the active switch will be reloaded, however this command can only be used when a standby sup engine is detected by the active sup engine. When you have done configuring the 4500 series switch it is a good practice to save your configuration first before reloading or doing redundancy forced-switchover.