Concept 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 124. VLAN Stacking without L2PT
You might need to transport control trac 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-congured 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).
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Service Provider Bridging