When each edge device interface on a backbone network connects to more than one user network and Layer 2 protocol data units (PDUs) from the user networks carry a single VLAN tag, configure VLAN-based Layer 2 protocol tunneling to allow the Layer 2 PDUs from the user networks to be tunneled across the backbone network. Layer 2 PDUs from the user networks then travel through different Layer 2 tunnels to reach the destinations to perform Layer 2 protocol calculation.
In Figure 1, PEs on the backbone network edge must tunnel tagged Layer 2 PDUs from VLAN 100 and VLAN 200 across the backbone network.
In most circumstances, PEs serve as aggregation devices on a backbone network. PE1, PE2, and PE3 constitute a backbone network, the aggregation interfaces on PE1 and PE2 receive Layer 2 PDUs from both LAN-A and LAN-B. To differentiate the Layer 2 PDUs from the two LANs, the PEs must identify tagged Layer 2 PDUs from CEs, with Layer 2 PDUs from LAN-A carrying VLAN 200 and those from LAN-B carrying VLAN 100
To tunnel Layer 2 PDUs from the user network across the backbone network, configure VLAN-based Layer 2 protocol tunneling on user-side interfaces on PE1, PE2, and PE3.
CE1 sends Layer 2 PDUs with specified VLAN Tag to the backbone network.
Configure Layer 2 forwarding on the aggregation device PE1 to allow BPDUs that carry specific VLAN Tags to pass through.
PE1 receives Layer 2 PDUs from the user networks and identifies that the Layer 2 PDUs carry a single VLAN tag. PE1 then replaces the multicast destination MAC address in the Layer 2 PDUs with a specified multicast MAC address and sends the PDUs onto the backbone network.
The internal devices on the backbone network forward the Layer 2 PDUs with a specified multicast MAC address to the egress devices.
The egress devices PE2 and PE3 restore the original destination MAC address in the Layer 2 PDUs based on the configured mapping between the multicast destination MAC address and the specified multicast address and send the Layer 2 PDUs to the user networks. The Layer 2 PDUs are transparently transmitted.