I'm playing around with MST and PVST in a lab and I'm seeing some behavior that seems confusing to me, so I'm looking for a bit of clarification.
For PVST, every VLAN does it's own STP, thus each STP is it's own CST for other PVST switches.
In an MST region, the IST instance 0 is the CIST for that region and that IST instance 0/CIST acts as the CST when speaking to non MST devices.
In a Cisco MST/PVST scenario, the MST switch detects a PVST neighbor so the MST IST instance 0 CIST (CST) sends BPDUs for each of the vlans that it knows about in it's CIST to those PVST switches.
The part that I'm not understanding is when you add an instance inside an MST region (where that region is connected to a PVST device), that VLAN is then removed from instance 0. If instance 0 is what sends the BPDUs to a PVST device, how can my PVST switch possibly be seeing a BPDU for the VLAN that is no longer in instance 0?
Along the same lines, what I'm seeing on my PVST switch doesn't help with that confusion :) It sees the MST switch as type P2p Peer(STP) where the MST switch sees the PVST switch as type P2p Peer(PVST). If the PVST switch sees the MST switch as a STP device, how can it receive BPDUs for multiple VLANs? I suppose if the MST switch is sending BPDUs on VLAN1, that sorta makes sense, but again, if that's the case, I don't understand how the PVST switch can see a BPDU for VLAN900 if the MST switch is sending STP BPDUs on VLAN1. It's my understanding that the old-style STP doesn't send any VLAN information.