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.
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