After receiving a Layer 2 protocol packet from User A network 1, PE 1 in the service provider
network encapsulates the packet, replaces its destination MAC address with a specific multicast
MAC address, and then forwards the packet in the service provider network;
The encapsulated Layer 2 protocol packet (called bridge protocol data unit, BPDU) is forwarded to
PE 2 at the other end of the service provider network, which decapsulates the packet, restores the
original destination MAC address of the packet, and then sends the packet to User A network 2.
Depending on the device models, BPDU tunneling may support the transparent transmission of these
types of Layer 2 protocol packets:
Cisco Discovery Protocol (CDP)
Device Link Detection Protocol (DLDP)
Ethernet Operation, Administration and Maintenance (EOAM)
GARP VLAN Registration Protocol (GVRP)
HW Group Management Protocol (HGMP)
Link Aggregation Control Protocol (LACP)
Link Layer Discovery Protocol (LLDP)
Port Aggregation Protocol (PAGP)
Per VLAN Spanning Tree (PVST)
Spanning tree protocol (STP)
Uni-directional Link Direction (UDLD)
VLAN Trunking Protocol (VTP)
BPDU Tunneling Implementation
The BPDU tunneling implementations for different protocols are all similar. This section describes how
BPDU tunneling is implemented by taking the Spanning Tree Protocol (STP) as an example.
The term STP in this document is in a broad sense. It includes STP, RSTP, and MSTP.
STP calculates the topology of a network by transmitting BPDUs among devices in the network.
For details, refer to MSTP Configuration in the Access Volume.
To avoid loops in your network, you can enable STP on your devices. When the topology changes at
one side of the customer network, the devices at this side of the customer network send BPDUs to
devices on the other side of the customer network to ensure consistent spanning tree calculation in the
whole customer network. However, because BPDUs are Layer 2 packets, all STP-enabled devices,
both in the customer network and in the service provider network, can receive and process these
BPDUs. In this case, neither the service provider network nor the customer network can correctly
calculate its independent spanning tree.