The multicast group prefix length that, when combined
with hwPimBsrElectedBsrGrpMappingGrpAddr, gives the group
prefix for this mapping. The InetAddressType is given by
the hwPimBsrElectedBsrGrpMappingAddrType object. If
hwPimBsrElectedBsrGrpMappingAddrType is 'ipv4' or 'ipv4z',
this object must be in the range 4..32. If
hwPimBsrElectedBsrGrpMappingAddrType is 'ipv6' or 'ipv6z',
this object must be in the range 8..128.