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S- Tag

Service VLAN Tag

The IEEE standard 802.1ad provides for double-tagging by service providers so that they can use VLANs allocated internally together with traffic already tagged as VLANs by service provider customers.

In this double tagging, the S-Tag comes first followed by the C-Tag (customer tag). The standard 802.1ad specifies a TPID of 0x88a8 for the outer S-Tag.\