Concept Guide

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.
Therefore, a mismatched TPID results in the port not differentiating between tagged and untagged traffic.
Figure 130. Single and Double-Tag TPID Match
884
Service Provider Bridging