Dell C9000 Series Networking Configuration Manual page 1019

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. The system differentiates between 0x9100
and 0x91XY, as 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.
Service Provider Bridging 1019