Returns the ifIndex of the phy which is the preferred
active phy. A value of 0 indicates that no phy is
currently active.
Setting this object to a value different from its current
value causes the ATM module to make the indicated phy active,
and the active phy inactive.
Setting this object to its current value has no effect.
After performing the phy change, the agent will update
all phy information (including cadpStatOperActivePhy)
and send a ciscoAtmDualPhyChange trap.