On page 69 of the CCIE Routing & Switching Exam Certification Guide 3rd edition, there is an interesting topology where there are 2 PVST+ regions of Cisco switches separated by a CST region of non Cisco Switches. PVST+ cannot be supported on non Cisco devices. To get around this the PVST+ regions treat the path through a CST region as a single link and tunnel through the region using multicast frames.

The 2 PVST+ regions tunnel by sending BPDUs to the mulitcast MAC of 0100.0CCC.CCCD, the non cisco switches treat the frame as multicast and not as a BPDU and forward it. The PVST+ devices on the edge of the CST region listen on that address to receive the frames and forward them into the PVST+ region. Hurrah the PVST+ regions can communicate!