3Com 4500G Family Configuration Manual page 198

To allow each network to calculate an independent spanning tree with STP, BPDU tunneling was
introduced.
BPDU tunneling delivers the following benefits:
BPDUs can be transparently transmitted. BPDUs of the same customer network can be broadcast
in a specific VLAN across the service provider network, so that the geographically dispersed
networks of the same customer can implement consistent spanning tree calculation across the
service provider network.
BPDUs of different customer networks can be confined within different VLANs for transmission on
the service provider network. Thus, each customer network can perform independent spanning
tree calculation.
Figure 1-2 Network diagram for BPDU tunneling implementation
PE 1
CE 1
User A network 1
As shown in Figure 1-2, the upper part is the service provider network (ISP network), and the lower part
represents two different parts of a customer network: User A network 1 and User A network 2. Enabling
the BPDU tunneling function on the edge devices (PE 1 and PE 2) in the service provider network
allows BPDUs of the customer network to be transparently transmitted in the service provider network,
thus ensuring consistent spanning tree calculation of User A network, without affecting the spanning
tree calculation of the service provider network.
Assume a BPDU is sent from User A network 1 to User A network 2:
At the ingress of the service provider network, PE 1 changes the destination MAC address of the
BPDU from 0x0180-C200-0000 to a special multicast MAC address, 0x010F-E200-0003 (the
default multicast MAC address) for example. In the service provider network, the modified BPDU is
forwarded as a data packet in the VLAN assigned to User A.
At the egress of the service provider network, PE 2 recognizes the BPDU with the destination MAC
address 0x010F-E200-0003, restores its original destination MAC address 0x0180-C200-0000,
and then sends the BPDU to User A network 2.
Make sure, through configuration, that the VLAN tags carried in BPDUs are neither changed nor
removed during the transparent transmission in the service provider network; otherwise, the devices in
the service provider network will fail to transparently transmit the customer network BPDUs correctly.
PE 2
ISP network
BPDU tunnel
1-3
CE 2
User A network 2