How PVST+ communicates BPDU msg between CST and PVST.
PVST+ has a backward compatibility feature that helps it to communicate with CST and PVST. The only thing you lose is the inherent fast convergence benefits. So if you have a PVST-----CST network, on that shared segment, it behaves like CST. For a more in-depth reading, go to this link: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#comp802
Here is the wonderful article on PVST+ and how its interoperability with other versions of STP.
Hope this helps :-)
Here is another article that could help http://cciethebeginning.wordpress.com/2008/10/
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