Figure 1 VLAN-based transparent transmission of Layer 2 protocol packets
In most cases, a PE serves as a convergence device. As shown in Figure 1, the convergence interface on PE1 receives Layer 2 protocol packets from LAN-A and LAN-B. To differentiate BPDUs from two LANs, BPDUs sent from the CE to the PE must be tagged. The VLAN ID of a BPDU from LAN-A is 200 and the VLAN ID of a BPDU from LAN-B is 100.
Currently, some Layer 2 protocol packets, such as protocol packets of STP, RSTP, or MSTP, do not carry VLAN tags. When receiving Layer 2 protocol packets with VLAN tags, a device on the ISP network considers them as invalid protocol packets and discards them. To avoid this problem, you can configure VLAN-based transparent transmission of Layer 2 protocol packets on the devices on the ISP network. In this manner, the Layer 2 protocol packets can traverse the ISP network through Layer 2 tunnels.
Similar to the interface-based transparent transmission of Layer 2 protocol packets, there are two processing methods in this application scenario:
Change the default multicast MAC address of the Layer 2 protocol packet that can be identified by the device 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 in 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.
Set specific VLAN IDs for the Layer 2 protocol packets that are sent from user networks to the ISP network.
Configure the devices in the ISP network to identify the Layer 2 protocol packets with VLAN IDs and allow the packets to pass through.
The devices (of the provider type) on the ISP network do not take the packet as the BPDU and do not send the packet to the CPU for processing. Instead, the devices select a corresponding Layer 2 tunnel to forward the packet according to the VLAN IDs with which the packets are allowed to pass through.
The Layer 2 protocol packet is transmitted as an ordinary Layer 2 packet by the devices on the ISP network, thus successfully traversing the ISP network.
When reaching the egress on the ISP network, the Layer 2 protocol packet is forwarded to the CE without being changed.
This method applies to only the STP, RSTP, or MSTP protocol, and the related configuration command is bpdu-tunnel stp bridge role provider.
Replace the original multicast MAC address of the Layer 2 protocol packet with a specified multicast MAC address.
This method applies to transparent transmission of all types of Layer 2 protocol packets.
Set specific VLAN IDs for the Layer 2 protocol packets that are sent from user networks to the ISP network.
Configure the devices on the ISP network to identify the Layer 2 protocol packets with VLAN IDs and allow the packets to pass through.
According to the mapping between the specified destination multicast MAC address and the Layer 2 protocol, the device on the ISP network changes the standard destination multicast MAC address of the Layer 2 protocol packet into the specified destination multicast MAC address.
After the MAC address is changed, the Layer 2 protocol packet is transmitted as an ordinary Layer 2 packet by the devices on the ISP network, thus successfully traversing the ISP network.
When the Layer 2 protocol packet reaches the egress, the egress restores the destination multicast MAC address to the standard destination multicast MAC address according to the mapping between the specified destination multicast MAC addresses and Layer 2 protocols, and then forwards the Layer 2 protocol packet to the CE.
