How Are MSTP BPDUs Transparently Transmitted on a VPLS Network?

As shown in Figure 1, a Layer 2 network is connected to a VPLS network through PE1. PE1 can transparently transmit MSTP BPDUs on the VPLS network:

Figure 1 Example for configuring BPDUs to be transparently transmitted on a VPLS network

Transmitting untagged BPDUs transparently

If a BPDU from a Layer 2 network does not carry any tag, run the port default vlan vlan-id and stp disable commands on the user-side interface of PE1. After the commands are executed, PE1 adds a specific tag to the received BPDU from a Layer 2 network and transparently transmits the BPDU to PE2 over the VPLS network. After receiving the BPDU, PE2 removes the tag and sends the BPDU to a specific attached user.
Transmitting tagged BPDUs transparently

If the BPDU from a Layer 2 network carries tags, run the stp bpdu vlan vlan-id command on the user-side interface of a device on the Layer 2 network and the port trunk allow-pass vlan command on the user-side interface of PE1. After the commands are executed, PE1 transparently transmits the tagged BPDU from the Layer 2 network to PE2 over the VPLS network. After receiving the BPDU, PE2 directly sends the BPDU to a specific attached user.