Dell S6100 Configuration Manual page 800

If the next-hop system's TPID does not match the outer-tag TPID of the incoming frame, the system drops the frame. For example, as
shown in the following, the frame originating from Building A is tagged VLAN RED, and then double-tagged VLAN PURPLE on egress at
R4. The TPID on the outer tag is 0x9100. R2's TPID must also be 0x9100, and it is, so R2 forwards the frame.
Given the matching-TPID requirement, there are limitations when you employ Dell Networking systems at network edges, at which, frames
are either double tagged on ingress (R4) or the outer tag is removed on egress (R3).
VLAN Stacking
The default TPID for the outer VLAN tag is 0x9100. The system allows you to configure both bytes of the 2 byte TPID.
Previous versions allowed you to configure the first byte only, and thus, the systems did not differentiate between TPIDs with a common
first byte. For example, 0x8100 and any other TPID beginning with 0x81 were treated as the same TPID, as shown in the following
illustration. Dell Networking OS Versions 8.2.1.0 and later differentiate between 0x9100 and 0x91XY, also shown in the following illustration.
You can configure the first 8 bits of the TPID using the vlan-stack protocol-type command.
The TPID is global. Ingress frames that do not match the system TPID are treated as untagged. This rule applies for both the outer tag
TPID of a double-tagged frame and the TPID of a single-tagged frame.
For example, if you configure TPID 0x9100, the system treats 0x8100 and untagged traffic the same and maps both types to the default
VLAN, as shown by the frame originating from Building C. For the same traffic types, if you configure TPID 0x8100, the system is able to
differentiate between 0x8100 and untagged traffic and maps each to the appropriate VLAN, as shown by the packet originating from
Building A.
Therefore, a mismatched TPID results in the port not differentiating between tagged and untagged traffic.
800 Service Provider Bridging