cmEthernetTrafficPortOuterVlanLoopbackMask is a read-write bitmask (0-16) specifying which up to three outer VLANs are looped back when the port is in a VLAN-scoped loopback configuration. An administrator sets specific bits in this mask to selectively loop back only the traffic of chosen outer VLANs rather than looping the entire port, letting one service be tested without disturbing others sharing the same physical port. For example, testing only the customer traffic on outer VLAN 1 while leaving other VLANs on the same port passing normal traffic would be done by setting only the bit corresponding to Outer VLAN 1 in this mask.