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S-Tag is an abbreviation of Service VLAN tag.
In this double tagging, the S-Tag comes first followed by the C-Tag (customer tag). The standad 802.1ad specifies a TPID of 0x88a8 for the outer S-Tag.
The MEF Implementation Agreement for Ethernet Interconnect Points (EIP) MEF 54 makes reference to situation in which an Operator able to implement S-Tag encapsulation at its ENNIs is connecting to an Operator lacking this capability at the corresponding ENNI.