The consulting company I work for does a fair number of projects where we remove older Cisco switches and replace them with Juniper switches. I'm labbing with physical switches and am running into some interesting STP behavior that I am trying to fully understand.
With default STP configs (rapid pvst+ on the 3750 & RSTP on the EX2300), nothing unusual happens. However, add a second EX2300, and give it a redundant connection to the other EX2300 (see the attached image), and suddenly the switch labeled 3750-01 goes into STP blocking mode for all vlans on the trunk (fa0/0/47) to the EX2300s. Apart from removing the redundant link, I can resolve the issue by converting the EX switches to VSTP. I believe switching to MSTP will also do the trick, but I've yet to lab that out.
I know how to fix the problem, but I want to know why the Cisco switch blocks the uplink to the Juniper switches. The Cisco is the root bridge for all vlans. I've run several debugs on the Cisco switch, but I have been unable to determine exactly why this happens. Perhaps that is the point. Mix STP protocols and expect unexpected results.