When upgrading a BIGIP redundant pair, what happens when one system has been updated but the other has not.

When one BIG-IP device in a redundant pair has been upgraded to a new software version while its partner has not, synchronization (config sync) should explicitly not be performed. Synchronizing configurations between devices running different software versions can lead to configuration corruption, unexpected behavior, or even system instability due to incompatible schema or features.

The active/standby role is determined by factors like priority, network status, and failover status, not simply by which system was updated first.

BIG-IP redundant pairs are typically upgraded in a "rolling" fashion, where one device is updated and rebooted while the other maintains traffic, making simultaneous updates an anti-pattern for high availability.

While an older system might experience communication issues or failover events due to version mismatch, issuing specific SNMP traps solely for "communication error with partner" due to a version difference is not the primary or universal outcome.