Reference Guide

754 | Service Provider Bridging
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Debug VLAN Stacking
To debug the internal state and membership of a VLAN and its ports, use the debug member command, as
shown in Figure 42-5. The port notations in Figure 42-5 are as follows:
• MT — stacked trunk
• MU — stacked access port
• T— 802.1Q trunk port
• U— 802.1Q access port
• NU— Native VLAN (untagged)
Figure 42-5. 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; FTOS uses 0x9100 (Figure ) 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 , 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 S-Series
The default TPID for the outer VLAN tag is 0x9100. Beginning with FTOS version 8.2.1.0, the S-Series
allow 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 42-6. Versions 8.2.1.0 and later differentiate between 0x9100 and 0x91XY, as shown in
Figure 42-8.
FTOS# 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)
FTOS#debug member port Tengigabitethernet 2/47
vlan id : 603 (MT), 100(T), 101(NU)
FTOS#