When upgrading a BIG-IP redundant pair, what happens when one system has been updated but...

The F5 BIG-IP upgrade process for HA pairs requires a specific "staggered" approach to maintain uptime.

Version Mismatch:When one unit is upgraded to a newer version of TMOS (e.g., from 15.1 to 16.1), it enters a26"Version Mismatch"27state with its peer.

Configuration Sync:Because the configuration schemas between different versions are often incompatible,ConfigSync should not be performed. Attempting to sync a newer configuration to an older system (or vice-versa) can cause configuration corruption or system instability.

Failover Capability:Generally, a pair with a version mismatch can still fail over to ensure traffic continuity during the upgrade window, but administrative changes and syncs must be paused until both units are on the same version.