Editing IEEE 802.1Q (section)

===Double tagging===
[[IEEE 802.1ad]] introduced the concept of double tagging. Double tagging can be useful for [[Internet service provider]]s (ISPs), allowing them to use their VLANs internally while carrying traffic from clients that is already VLAN tagged.  The outer (next to source MAC and representing ISP VLAN) S-TAG (service tag) comes first, followed by the inner C-TAG (customer tag).  In such cases, 802.1ad specifies a TPID of 0x88a8 for service-provider outer S-TAG.
{{wide image|TCPIP 802.1ad DoubleTag.svg|1328px|Insertion of 802.1ad double tag in an Ethernet frame}}