Two tags with different jobs
With Q-in-Q, a provider edge can add an outer service tag (S-tag) to an already tagged customer frame. The original customer tag (C-tag) stays inside it. IEEE 802.1ad introduced provider bridging, which is now covered by IEEE 802.1Q. The standard tag identifiers are 0x88a8 for the S-tag and 0x8100 for the C-tag. Some equipment also supports other outer tag identifiers.
The provider can forward by service VLAN without coordinating customer VLAN numbers across customers. Two customers can both send VLAN 10 if their traffic enters different service VLANs. The outer service tag distinguishes the two services across the shared provider network.
At the provider edges
An ingress edge pushes the outer tag before forwarding the frame across the provider network. At the remote customer-facing edge, the provider removes the outer tag and sends the frame with its original customer tag. The extra tag adds four bytes, so the links carrying both tags must accept the larger frame.
Try it in a lab
- On equipment that supports provider bridging, send a VLAN 10 frame through a service configured for VLAN 200. Observe the frame on the customer side and then on a provider link.
- On the provider link, find outer VLAN 200 and inner VLAN 10 in that order. At the far customer-facing edge, confirm that VLAN 10 is still present and VLAN 200 has been removed.
- Send an identical customer VLAN ID into a second service. Verify that the different outer VLAN separates the services on the provider link.