3Com 5500-EI PWR Install Manual page 974

Figure 3-1 BPDU Tunnel network hierarchy
When a BPDU packet coming from a customer network reaches an edge device in the service
provider network, the edge device changes the destination MAC address carried in the packet from
a protocol-specific MAC address to a private multicast MAC address, which can be defined using a
command. A packet with this multicast address as its destination address is called a tunnel packet.
In the service provider network, the tunnel packet can be forwarded as a normal data packet.
Before the device in the service provider network forwards the packet to the destination customer
network, the edge device will identify the tunnel packet, determine the packet type based on the
type field in the packet, restore its destination MAC address to the original protocol-specific MAC
address and then forward the packet to the access device on the user side. This ensures the
packet to be forwarded is consistent with the packet before entering the tunnel. So, a tunnel here
acts as a local link for user devices. It enables Layer 2 protocols to run on a virtual local network.
Figure 3-2 and Figure 3-3
Figure 3-2 The structure of a BPDU packet before it enters a BPDU tunnel
Figure 3-3 The structure of a BPDU packet after it enters a BPDU tunnel
show the structure of a BPDU packet before and after it enter a BPDU tunnel.
3-2