Concept Guide

VLAN Stacking in Multi-Vendor Networks
The rst eld in the VLAN tag is the tag protocol identier (TPID), which is 2 bytes. In a VLAN-stacking network, after the frame is double
tagged, the outer tag TPID must match the TPID of the next-hop system.
While 802.1Q requires that the inner tag TPID is 0x8100, it does not require a specic value for the outer tag TPID. Systems may use any 2-
byte value; the Dell Networking OS uses 0x9100 (shown in the following) while non-Dell Networking systems might use a dierent value.
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. Beginning with the Dell Networking OS version 8.2.1.0, the system allows you to
congure both bytes of the 2 byte TPID.
Previous versions allowed you to congure the rst byte only, and thus, the systems did not dierentiate between TPIDs with a common
rst byte. For example, 0x8100 and any other TPID beginning with 0x81 were treated as the same TPID, as shown in the following
illustration. The Dell Networking OS Versions 8.2.1.0 and later dierentiate between 0x9100 and 0x91XY, also shown in the following
illustration.
You can congure the rst 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 congure TPID 0x9100, the system treats 0x8100 and untagged trac the same and maps both types to the default
VLAN, as shown by the frame originating from Building C. For the same trac types, if you congure TPID 0x8100, the system is able to
dierentiate between 0x8100 and untagged trac 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 dierentiating between tagged and untagged trac.
764
Service Provider Bridging