Administrator Guide

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 104. 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 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 Dell Networking OS versions, Dell Networking
systems are required at the egress edge of the intermediate network because only Dell Networking OS could recognize the
significance of the destination MAC address and rewrite it to the original Bridge Group Address. In Dell Networking OS version
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Service Provider Bridging