Figure 1 Interface-based transparent transmission of Layer 2 protocol packets
As shown in Figure 1, each interface on a PE connects to one user network. The user networks belong to different LANs, that is, LAN-A and LAN-B. BPDUs sent from user networks to the PE are untagged. The PE, however, needs to identify that LAN from which the BPDUs come. BPDUs of a user network in LAN-A must be sent to other user networks in LAN-A rather than the user networks in LAN-B. In addition, BPDUs must not be processed by PEs.
In this application scenario, the following processing methods are available:
Change the default multicast MAC address of the Layer 2 BPDU that can be identified by the devices on the ISP network into another multicast MAC address.
Set the roles of all devices in the ISP network to provider. Thus, the destination MAC addresses of the BPDUs sent by the devices on the ISP network are changed to 01-80-C2-00-00-08 instead of the original 01-80-C2-00-00-00.
Set the roles of all devices in a user network to customer. Thus, the destination MAC addresses of the BPDUs sent by the user network are still 01-80-C2-00-00-00.
On the device of the ISP network, add the interfaces that connect to the same user network to the same VLAN. After receiving the Layer 2 protocol packet from the user network, the device on the ISP network adds the default VLAN ID of the interface to the packet.
The devices (of the provider type) on the ISP network do not take the BPDU as the Layer 2 BPDU and do not send the BPDU to the CPU for processing. Instead, the devices select a corresponding Layer 2 tunnel according to the default VLAN ID of the interface to forward the BPDU.
The BPDU is normally forwarded by the devices on the ISP network and normally traverses the ISP network.
When reaching the egress on the ISP network, the Layer 2 BPDU is forwarded to the UPE without being changed.
This method applies to only the STP, RSTP, or MSTP protocol, and the associated configuration command is bpdu-tunnel stp bridge role provider.
Replace the original multicast MAC address of the Layer 2 BPDU with a specified multicast MAC address.
This method applies to all types of transparent transmission of Layer 2 protocol packets.
After receiving and identifying the Layer 2 protocol packet (such as a BPDU of the STP protocol) from the user network, the device on the ISP network adds the default VLAN ID of the interface to the Layer 2 protocol packet.
According to the mapping between the special destination multicast MAC addresses and Layer 2 protocols, the device on the ISP network changes the standard destination multicast MAC address of the Layer 2 BPDU into the specified destination multicast MAC address.
The Layer 2 BPDU is normally forwarded by the devices on the ISP network, thus successfully traversing the ISP network.
When the Layer 2 BPDU reaches the egress, the egress restores the destination multicast MAC address to the standard destination multicast MAC address of the Layer 2 BPDU according to the mapping between the special destination multicast MAC addresses and Layer 2 protocols, and then forwards the BPDU to the UPE.
