The way it works when connecting MST and PVST domain is that the MST switch at the edge of the domain will simulate PVST+ by sending same BPDU for all VLANs configured on the trunk towards the PVST+ switch.
The CIST replicates BPDU for every VLAN. Because there is not one instance per VLAN for MST the sys ID is set to 0 on these BPDUs. When connecting MST and PVST+ domain VLAN 1 is used to connect the domains.
When the PVST+ side sends a BPDU with priority 0 for VLAN 1 it is actually 1 because of extended system ID. If VLAN 2 is also configured then PVST+ side sends BPDU with ID of 2 because of the extended system ID.
The way PVST simulation works on MST switch is that the port can only have one role because there is not one instance running per VLAN. The CIST is now sending BPDU that it knows a root which has a priority of 1. This means that MST switch should be root forwarding but then it sees a BPDU with a priority of 2. Because the MST switch has better root information it should become designated forwarding on the port but the port can only have one role so the PVST simulation fails.
To make this work the PVST+ side must have worse priority on VLAN 1 than all the other VLANs on the trunk. The following configuration will work:
spanning-tree vlan 1 priority 4096
spanning-tree vlan 2-4094 priority 0
This way the PVST simulation will work and the following log message should be seen:
%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared
More information can be found in the following link.
I recently went through a similar migration that eradicated proprietary protocols (i.e., PVST+, HSRP, etc.); our transition was focused on posturing us for a 'vendor neutral' network. Since you're running Cisco devices, you're likely running PVST+.
Cisco covers this in more detail, but the crux of it is fairly simple: establish a hierarchy and, contrary to what common sense would tell you, start from the core and move outward.
Set your priorities in a manner that cascades down. Base this hierarchy on what is closest to a central node. All nodes participating in this STP setup should be on a common structure, so for simplicities sake, manually set one root bridge with the spanning-tree vlan xx,xx root primary.
Once you're ready for the migration to begin, start with the core. The reason for this is because MST is backwards compatible with PVST, not the other way around. A PVST core won't be able to communicate with BPDUs from MST nodes; those trunk links will drop. If everything is going right, you'll see a small drop while the new STP protocol comes online and figures out the current lay of the land.
Then just trickle these protocol changes down all the way to your furthest nodes.
End-users might not even notice the small blip while MST elects a root bridge.
Best Answer
Rapid Spanning Tree Protocol (RSTP) is an enhancement of the original STP 802.1D protocol. The RSTP 802.1w protocol is an IEEE open implementation. Cisco has its own proprietary implementation of RSTP, that includes the benefits of its Per-VLAN spanning tree protocols, called Rapid-PVST+. Rapid-PVST+ and RSTP are important enhancements to the original STP protocol because they can switch ports from blocking to forwarding without relying on timers, execute spanning tree calculations and converge the network faster than STP. In STP, network convergence can take up to 50 seconds, with RSTP and Rapid-PVST+ network convergence can happen in just over 6 seconds. In RSTP and Rapid-PVST+, switchports transition from discarding (blocking) to forwarding based solely on a switch-to-switch proposal and agreement process, using BID priority, instead of having to also wait for timers.
RSTP includes many of Cisco’s earlier proprietary 802.1D enhancements. It is also backwards compatible with STP and can work with legacy switches running STP. RSTP and its variants are widely implemented so it is beneficial to understand a little about how RSTP works.
STP Port Roles and Port States
Root Port – port that is closest to the root bridge. It goes directly to the forwarding state.
Designated Port – only one designated port allowed per network segment (switch-to-switch link), this can either be an alternate port and a designated port, or a root port and a designated port. When there are two designated ports in the same segment the proposal and agreement process will result in one of them becoming an alternate (discarding) port. In this way designated ports make the most use of the proposal and agreement process. A designated port will eventually be a forwarding port though it can temporarily transition to the listening (discarding) state during the proposal and agreement process.
Alternate Port – available to transition to designated and forwarding if needed, otherwise discarding (blocking)
Backup Port – used with a redundant switch-to-switch port link. Is in the discarding state.
Source: http://danscourses.com/category/courses/ccna-3/
Rapid PVST+ Port Roles
Rapid PVST+ provides rapid convergence of the spanning tree by assigning port roles and learning the active topology. Rapid PVST+ builds upon the 802.1D STP to select the switch with the highest priority (lowest numerical priority value) as the root bridge as described in the “Election of the Root Bridge” section. Rapid PVST+ then assigns one of these port roles to individual ports:
Root port— Provides the best path (lowest cost) when the switch forwards packets to the root bridge.
Designated port— Connects to the designated switch, which incurs the lowest path cost when forwarding packets from that LAN to the root bridge. The port through which the designated switch is attached to the LAN is called the designated port.
Alternate port— Offers an alternate path toward the root bridge to the path provided by the current root port. An alternate port provides a path to another switch in the topology.
Backup port— Acts as a backup for the path provided by a designated port toward the leaves of the spanning tree. A backup port can exist only when two ports are connected in a loopback by a point-to-point link or when a switch has two or more connections to a shared LAN segment. A backup port provides another path in the topology to the switch.
Disabled port— Has no role within the operation of the spanning tree. In a stable topology with consistent port roles throughout the network, Rapid PVST+ ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the blocking state. Designated ports start in the blocking state. The port state controls the operation of the forwarding and learning processes.
Rapid PVST+ Port State Overview
Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When a LAN port transitions directly from nonparticipation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames.
Each LAN port on a software using Rapid PVST+ or MST exists in one of the following four states:
Blocking— The LAN port does not participate in frame forwarding.
Learning— The LAN port prepares to participate in frame forwarding.
Forwarding— The LAN port forwards frames.
Disabled— The LAN port does not participate in STP and is not forwarding frames.
When you enable Rapid PVST+, every port in the software, VLAN, and network goes through the blocking state and the transitory states of learning at power up. If properly configured, each LAN port stabilizes to the forwarding or blocking state.
When the STP algorithm places a LAN port in the forwarding state, the following process occurs:
The LAN port is put into the blocking state while it waits for protocol information that suggests it should go to the learning state.
The LAN port waits for the forward delay timer to expire, moves the LAN port to the learning state, and restarts the forward delay timer.
In the learning state, the LAN port continues to block frame forwarding as it learns the end station location information for the forwarding database.
The LAN port waits for the forward delay timer to expire and then moves the LAN port to the forwarding state, where both learning and frame forwarding are enabled.
Blocking State
A LAN port in the blocking state does not participate in frame forwarding.
A LAN port in the blocking state performs as follows:
Learning State
A LAN port in the learning state prepares to participate in frame forwarding by learning the MAC addresses for the frames. The LAN port enters the learning state from the blocking state.
A LAN port in the learning state performs as follows:
Forwarding State
A LAN port in the forwarding state forwards frames. The LAN port enters the forwarding state from the learning state.
A LAN port in the forwarding state performs as follows:
Disabled State
A LAN port in the disabled state does not participate in frame forwarding or STP. A LAN port in the disabled state is virtually nonoperational.
A disabled LAN port performs as follows:
Rapid PVST+ whitepaper: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/CLIConfigurationGuide/RPVSpanningTree.html