The original Ethernet frame without tagging is shown below.
Original Ethernet frame without tagging
.png "GUID-FC85D69C-80EA-4625-A20E-9666A5EBA403-low.png")
In regular VLAN tagging, a tag/identifier is used to identify the traffic from VLANs.
Regular tagging
.png "GUID-26018DEC-82E1-4BF3-917D-0CC452E2E1AC-low.png")
In double VLAN tagging, when the frame enters the ISP network, a second VLAN tag is inserted into the frame. This tag is specific to the customer, and identifies the traffic from all the VLANs from that customer. This second tag is also known as the outer VLAN tag or Customer Identification tag (CID). The original VLAN tag is called the inner VLAN tag or VLAN identifier tag (VID).
Double VLAN tagging
.png "GUID-1D2029FD-80BA-47D8-B24E-D6700FD06E6B-low.png")