For reference, http://www.velvet.org/~mrz/stp-two-root.png

The 6509s are all connected to core1s and core2s on dot1q trunks. The two core switches are also connected to each other.

The new installation I'm working on involves the 2970s and 2950s. The 2970s are single homed off a 6905 running pvst+. The 2950s are multihomed on each of the 2970s. Due the limit of STP instances a 2970 can run in pvst, I changed the mode to mst (because I wanted to continue running vtp and we have more than 240 some vlans).

When a 2950 is connected to both 2970s, both trunk ports show FWD and some servers in the server cloud are unreachable. If I shutdown one trunk port, those servers become reachable again and stay reachable while the trunk is in BLK and LRN. Often the 2950 appears to get into a confused state and all devices are unreachable until I reload the switch (this is often after a lot of shut/no shuts on the trunks).

How do I resolve this? Do I need to goto pvst? I suspect the 2950s think they are roots but that's not the desired setup (and it's not working that way either).

Any help is most appreciated.