The PVX is a provider bridge/switch, whereas the ACX is an enterprise switch. This makes the spanning tree type between them incompatible. This article describes the difference between the two switches and suggests how the two can be made interoperable.
The BTI PVX will not participate in the spanning tree topology with the ACX, and will not forward the STP bridge protocol data units (BPDUs) from the ACX.
The PVX uses the provider bridge Spanning Tree Protocol (STP) destination MAC address 01:80:C2:00:00:08, whereas the ACX uses the standard enterprise STP destination MAC address 01-80-C2-00-00-00. The BTI PVX will not participate in the spanning tree topology with the ACX, and will not forward the STP bridge protocol data units (BPDUs) from the ACX unless the BPDUs have a VLAN field with a TPID of 88a8 and an S-VLAN tag (aka service VLAN), and the PVX is configured to forward that specific VLAN tag.
The PVX and ACX have different uses. The PVX should connect to other PVX devices or to other provider bridges. After the initial provider network is built and established, traffic from the ACX or any other Ethernet switch can be forwarded through the provider network by using Q-in-Q tagging.