Dell C9000 Series Networking Configuration Manual page 1028

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 126. 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).
Service Provider Bridging 1028