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MST can interoperate with other Spanning Tree protocols and even other MST regions. When an MST region is connected to switches in another MST region, or simply not running MST at all, it will make the MST region appear as a single RPVST+ or PVST+ topology, depending on what the peer switch is running. This is called PVST simulation. To illustrate, let's change SW2 back to RPVST+ mode:

SW2(config)#spanning-tree mode rapid-pvst SW2(config)#do show spanning-tree vlan 1-10 summary Switch is in rapid-pvst mode Root bridge for VLAN0001 is 32768.0015.f9fb.1e80. Root bridge for VLAN0002 is 32768.0015.f9fb.1e80. Root bridge for VLAN0003 is 32768.0015.f9fb.1e80. Root bridge for VLAN0004 is 32768.0015.f9fb.1e80. Root bridge for VLAN0005 is 32768.0015.f9fb.1e80. Root bridge for VLAN0006 is 32768.0015.f9fb.1e80. Root bridge for VLAN0007 is 32768.0015.f9fb.1e80. Root bridge for VLAN0008 is 32768.0015.f9fb.1e80. Root bridge for VLAN0009 is 32768.0015.f9fb.1e80. Root bridge for VLAN0010 is 32768.0015.f9fb.1e80. ! Output truncated

SW1 advertises itself as the root for all VLANs, even though in the MST topology it's only the root for the MST1 instance. If a switch outside of the MST topology attempts to become root, the MST switches will block the ports, placing them into a PVST Simulation Inconsistent state. For example, let's try to make SW2 the root for VLAN 1:

SW2(config)#spanning-tree vlan 1 priority 0 ! SW1 will block the port that's connected to SW2 and place it into a broken state, resulting in no traffic traversing the link: *Sep 15 01:01:21.412: %SPANTREE-2-PVSTSIM_FAIL: Blocking root port Gi0/0: Inconsistent inferior PVST BPDU received on VLAN 7, claiming root 32775:0015.fa88.4e80 SW1#show spanning-tree int gi0/0 Mst Instance Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- MST0 Root BKN*20000 128.1 P2p Bound(PVST) *PVST_Inc MST1 Mstr BKN*20000 128.1 P2p Bound(PVST) *PVST_Inc MST2 Mstr BKN*20000 128.1 P2p Bound(PVST) *PVST_Inc SW1#show spanning-tree inconsistentports Name Interface Inconsistency -------------------- ------------------------ ------------------ MST0 GigabitEthernet0/0 PVST Sim. Inconsistent MST1 GigabitEthernet0/0 PVST Sim. Inconsistent MST2 GigabitEthernet0/0 PVST Sim. Inconsistent Number of inconsistent ports (segments) in the system : 3