PVST+ uses the spanning tree instance for VLAN 1 to join the CST in the network to build the CST, PVST+ processes and sends standard IEEE Bridge protocol data units (BPDUs) on all the ports in VLAN 1 (access/trunk).
Across IEEE 802.1q trunks, Extreme switches run PVST+. The goal is to interoperate with standard IEEE STP (or RSTP or MSTP), while transparently tunneling PVST+ instance BPDUs across the MST region to potentially connect to other Extreme switches across the MST region.
On trunk ports that allow VLAN 1, PVST+ also sends PVST+ BPDUs to a Cisco-proprietary multicast MAC address (0100.0ccc.cccd) or Extreme-proprietary multicast MAC address (0304.0800.0700) depending on the configuration. By default, the PVST+ BPDUs are sent to Extreme-proprietary multicast MAC address on Extreme switches. These BPDUs are tunneled across an MST region. The PVST+ BPDUs for VLAN 1 are only used for the purpose of consistency checks and that it is only the IEEE BPDUs that are used for building the VLAN 1 spanning tree. So in order to connect to the CST, it is necessary to allow VLAN 1 on all trunk ports.
For all other VLANs, PVST+ BPDUs are sent on a per-VLAN basis on the trunk ports. These BPDUs are tunneled across an MST region. Consequently, for all other VLANs, MST region appears as a logical hub. The spanning tree instances for each VLAN in one PVST+ region map directly to the corresponding instances in another PVST+ region and the spanning trees are calculated using the per-VLAN PVST+ BPDUs.
Similarly, when a PVST+ region connects to a MSTP region, from the point of view of MSTP region, the boundary bridge thinks it is connected to a standard IEEE compliant bridge sending STP BPDUs. So it joins the CIST of the MSTP region to the CST of the PVST+ region (corresponding to VLAN 1). The PVST+ BPDUs are tunneled transparently through the MSTP region. So from the Extreme bridge point of view, the MSTP region looks like a virtual hub for all VLANs except VLAN 1.
The PVST+ BPDUs are sent untagged for the native VLAN and tagged for all other VLANs on the trunk port.
On access ports, Extreme switches run classic version of IEEE STP/RSTP protocol, where the BPDUs are sent to the standard IEEE multicast address "0180.C200.0000". So if we connect a standard IEEE switch to an access port on the Extreme switch, the spanning tree instance (corresponding to the access VLAN on that port) of the Extreme switch is joined with the IEEE STP instance on the adjacent switch.
For introductory information about STP BPDUs, see the section BPDUs.