• U — 802.1Q access port• NU — Native VLAN (untagged)Dell# debug member vlan 603vlan id : 603ports : Te 2/4/1 (MT), Te 3/1/1(MU), Te 3/25/1(MT), Te 3/26/1(MT), Te3/27/1(MU)Dell#debug member port tengigabitethernet 2/4/1vlan id : 603 (MT), 100(T), 101(NU)Dell#VLAN Stacking in Multi-Vendor NetworksThe first field in the VLAN tag is the tag protocol identifier (TPID), which is 2 bytes. In a VLAN-stackingnetwork, after the frame is double tagged, the outer tag TPID must match the TPID of the next-hopsystem.While 802.1Q requires that the inner tag TPID is 0x8100, it does not require a specific value for the outertag TPID. Systems may use any 2-byte value; Dell Networking OS uses 0x9100 (shown in the following)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 systemdrops the frame. For example, as shown in the following, the frame originating from Building A is taggedVLAN 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 systemsat network edges, at which, frames are either double tagged on ingress (R4) or the outer tag is removedon egress (R3).VLAN StackingThe default TPID for the outer VLAN tag is 0x9100. The system allows you to configure both bytes of the2 byte TPID.Previous versions allowed you to configure the first byte only, and thus, the systems did not differentiatebetween TPIDs with a common first byte. For example, 0x8100 and any other TPID beginning with 0x81were treated as the same TPID, as shown in the following illustration. Dell Networking OS Versions 8.2.1.0and later differentiate between 0x9100 and 0x91XY, also 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 ruleapplies 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 andmaps both types to the default VLAN, as shown by the frame originating from Building C. For the sametraffic types, if you configure TPID 0x8100, the system is able to differentiate between 0x8100 anduntagged traffic and maps each to the appropriate VLAN, as shown by the packet originating fromBuilding A.Therefore, a mismatched TPID results in the port not differentiating between tagged and untagged traffic.Service Provider Bridging 817
