1-23) The encapsulated Layer 2 protocol packet (called bridge protocol data unit, BPDU) is forwarded toPE 2 at the other end of the service provider network, which decapsulates the packet, restores theoriginal 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 thesetypes of Layer 2 protocol packets:z Cisco Discovery Protocol (CDP)z Device Link Detection Protocol (DLDP)z Ethernet Operation, Administration and Maintenance (EOAM)z GARP VLAN Registration Protocol (GVRP)z HW Group Management Protocol (HGMP)z Link Aggregation Control Protocol (LACP)z Link Layer Discovery Protocol (LLDP)z Port Aggregation Protocol (PAGP)z Per VLAN Spanning Tree (PVST)z Spanning tree protocol (STP)z Uni-directional Link Direction (UDLD)z VLAN Trunking Protocol (VTP)BPDU Tunneling ImplementationThe BPDU tunneling implementations for different protocols are all similar. This section describes howBPDU tunneling is implemented by taking the Spanning Tree Protocol (STP) as an example.z The term STP in this document is in a broad sense. It includes STP, RSTP, and MSTP.z 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 atone side of the customer network, the devices at this side of the customer network send BPDUs todevices on the other side of the customer network to ensure consistent spanning tree calculation in thewhole 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 theseBPDUs. In this case, neither the service provider network nor the customer network can correctlycalculate its independent spanning tree.
