Dell Force10 Z9000 Configuration Manual page 677

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 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 105. VLAN Stacking with L2PT
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Service Provider Bridging