Are you using F5 BigIP v11.x platform (not Viprion) ?
- The device with the highest management IP address will be selected as the active device by default, unless the peering unit is explicitly configured in forced offline state.
- If you force your active unit to standby mode, it will retain it's standby status, until next system failure of the active unit. This behavior can be changed by enabling failback configuration - not recommended.
While you are doing any activities which will break the active-standby cluster, I recommend issuing the forced offline status on your standby unit. Alternatively, just disable all network interfaces besides the mgmt interface. You'll know you can't screw things up this way :).