If each user-side interface on a PE is connected to multiple CEs, BPDUs sent from user networks must carry VLAN tags to identify these user networks. To transparently transmit BPDUs from user networks over the carrier network, deploy VLAN-based BPDU tunneling.
If each user-side interface on a provider edge (PE) is connected to multiple customer edges (CEs), BPDUs sent from user networks must contain VLAN tags so that the PE device can identify BPDUs from different user networks. To transparently transmit BPDUs from user networks over the carrier network, you can configure VLAN-based BPDU tunneling. With VLAN-based BPDU tunneling configured, BPDUs from user networks are transparently transmitted through different BPDU tunnels over the Layer 2 network of the carrier network to perform the spanning tree calculation.