Reference Guide

806 | Service Provider Bridging
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Figure 45-5. Example of Output of debug member vlan and debug member port
VLAN Stacking in Multi-vendor Networks
The first field in the VLAN tag is the Tag Protocol Identifier (TPID), which is two bytes. In a
VLAN-stacking network, once 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 specific value for the outer
tag TPID. Systems may use any two-byte value; Dell Networking OS uses 0x9100 (Figure 45-6) while
non-Dell Networking systems might use a different 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, in Figure 45-6, 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 with the S5000
The default TPID for the outer VLAN tag is 0x9100. The S5000 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 Figure 45-6. Dell Networking OS differentiate
between 0x9100 and 0x91XY.
You can configure the first eight bits of the TPID using the command
vlan-stack protocol-type.
The TPID on the S5000 system 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, then 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 in
Figure 45-8. For the same traffic types, if you configure TPID 0x8100, then 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 in Figure 45-8.
Therefore, a mismatched TPID results in the port not differentiating between tagged and untagged traffic.
Dell# debug member vlan 603
vlan id : 603
ports : Te 2/47 (MT), Te 3/1(MU), Te 3/25(MT), Te 3/26(MT), Te 3/27(MU)
Dell#debug member port tengigabitethernet 2/47
vlan id : 603 (MT), 100(T), 101(NU)
Dell#