Hi Silvio,
According to pure IEEE standards, MSTP is backwards compatible with RSTP and STP, and they should interoperate.
With regards to Cisco Catalyst switches, the cooperation may be problematic. Cisco switches implement their proprietary PVST+ and RPVST+ along normal STP, RSTP and MSTP. On a MSTP region boundary port, Cisco switches "translate" the MSTP into PVST+ or RPVST+ and vice versa. However, this translation - especially the direction from (R)PVST+ to MSTP - has certain prerequisities, and if they are not met, the port will be put into the discarding state, essentially isolating the region from the rest of the network. Unfortunately, this PVST Simulation - as Cisco calls it - can be deactivated only on 6500 and Nexus 7000 series switches.
In addition, for certain reasons, an MSTP boundary port configured as trunk reverts to 802.1D/PVST+ operation when speaking to another MSTP region or RSTP switch. That means that if the network is partitioned into MSTP regions or is heterogenous, no rapid convergence can be achieved on trunks between these regions even though the MSTP is based on RSTP and is capable of providing it. Again, this is not the problem of MSTP per se, rather, it is an implementation limitation on Cisco switches.
In any case, it is best not to mix versions of spanning tree in a single network if possible. If you absolutely need to run the MSTP in a part of your network make sure that the region becomes a root switch for all VLANs. That will make the PVST Simulation happy. For more information, see:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml
Best regards,
Peter
