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

Running config sync between two BIG-IP systems on different software versions risks corrupting or incompletely applying configuration due to potential differences in configuration schema, object syntax, or supported features between versions. F5 upgrade guidance explicitly states that sync should be suspended until both units are on the same version.

The first updated system does not automatically assume the active role; the active/standby state is governed by failover settings and traffic group priority, not by upgrade order.

BIG-IP redundant pairs support rolling upgrades where each unit is updated independently and sequentially, so a mixed-version state is a normal part of the upgrade process.

The older system does not generate SNMP traps specifically for a version mismatch with its partner; version mismatch detection is handled through HA status messaging, not SNMP traps.