In-Service Software Upgrade (ISSU) is the first, comprehensive, transparent software upgrade capability in the industry for the IP/Multiprotocol Label Switching edge router. Full image ISSU allows you to upgrade the complete Cisco IOS software image without the need to take the switch or network out of service.
The active reports bulk sync failure due to Mismatch Command List (MCL) when standby reboots. MCL contains the commands that were successfully configured into the running-config of the active supervisor engine, yet failed at the standby supervisor engine during bulk sync.
This MCL error occurs only when speed command is configured on port-channel interface. The standby automatically resets and comes back in RPR mode.
The MCL error can be fixed if you remove the configuration lines reported in the MCL list from the running config on the active, and then reboot the standby. Once standby is up and the switch reaches the STANDBY HOT state, reconfigure the original command.
In other words, after the upgrade, if an unsupported CLI command is present in the newer version, the switch goes to RPR. Hence, the best way to proceed is to unconfigure the offending command and then do the upgrade.