Layer 2 Protocol TunnelingSpanning 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 stackingto connect physically separate regions of a network, BPDUs attempting to traverse the intermediate network might be consumed and laterdropped because the intermediate network itself might be using spanning tree (shown in the following illustration).Figure 119. VLAN Stacking without L2PTYou might need to transport control traffic transparently through the intermediate network to the other region. Layer 2 protocol tunnelingenables BPDUs to traverse the intermediate network by identifying frames with the Bridge Group Address, rewriting the destination MACto a user-configured non-reserved address, and forwarding the frames. Because the frames now use a unique MAC address, BPDUs aretreated as normal data frames by the switches in the intermediate network core. On egress edge of the intermediate network, the MACaddress rewritten to the original MAC address and forwarded to the opposing network region (shown in the following illustration).848 Service Provider Bridging
