This attribute indicates the number of multicast branches used in
switched Vccs, including Vpt Vccs, that are currently active on this
interface.
The sum of this attribute for all AtmIf components linked to an Lp
added to the sum of permanentMulticastBranches for all AtmIf
components linked to the same Lp gives the value of
multiCastBranchesUsage in the Arc component.