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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Layer 2 Protocol Tunneling

Spanning tree bridge protocol data units (BPDUs) use a reserved destination MAC address called the bridge group address, which is 01-80-C2-00-00-00.

Only spanning-tree bridges on the local area network (LAN) recognize this address and process the BPDU. When you use VLAN stacking to connect physically separate regions of a network, BPDUs attempting to traverse the intermediate network might be consumed and later dropped because the intermediate network itself might be using spanning tree (shown in the following illustration).
Figure 1. VLAN Stacking without L2PT Illustration of VLAN stacking without L2PT.

You might need to transport control traffic transparently through the intermediate network to the other region. Layer 2 protocol tunneling enables BPDUs to traverse the intermediate network by identifying frames with the Bridge Group Address, rewriting the destination MAC to a user-configured non-reserved address, and forwarding the frames. Because the frames now use a unique MAC address, BPDUs are treated as normal data frames by the switches in the intermediate network core. On egress edge of the intermediate network, the MAC address rewritten to the original MAC address and forwarded to the opposing network region (shown in the following illustration).

Dell Networking OS Behavior: In the Dell Networking OS versions prior to 8.2.1.0, the MAC address that Dell Networking systems use to overwrite the Bridge Group Address on ingress was non-configurable. The value of the L2PT MAC address was the Dell Networking-unique MAC address, 01-01-e8-00-00-00. As such, with these versions, Dell Networking systems are required at the egress edge of the intermediate network because only the Dell Networking OS could recognize the significance of the destination MAC address and rewrite it to the original Bridge Group Address. In the Dell Networking OS version 8.2.1.0 and later, the L2PT MAC address is user-configurable, so you can specify an address that non-Dell Networking systems can recognize and rewrite the address at egress edge.

Figure 2. VLAN Stacking with L2PT Illustration of VLAN stacking with L2PT.

Implementation Information

  • L2PT is available for STP, RSTP, MSTP, and PVST+ BPDUs.
  • No protocol packets are tunneled when you enable VLAN stacking.
  • L2PT requires the default CAM profile.

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