This read-write object reports, and lets an operator set, the ifIndex of the phy that should be the preferred active one; writing a different ifIndex here causes the ATM module to actually make that phy active and the other inactive. Because this triggers a real hardware switchover, it's how an operator manually forces failover between the two redundant phys, for instance ahead of planned maintenance on whichever phy is currently active, and traffic will briefly interrupt while the module reprograms itself. An admin changing this from ifIndex 5, the working fiber, to ifIndex 6, the protect fiber, before maintenance on port 5 should then see cadpStatOperActivePhy read back 6.