PVST is per vlan spanning-tree

with pvst each vlan will has its own spaning tree instances like the rooot bridge, secondary root and so on

for eaxmple one switch can be a root for vlan 10 for example and secondary or non-root for vlan 20 and so on

but when u have al of vlans for example u have 100 vlan in this case each vlan will has its own STP instances then will be very high CPU intensive work on the switch.. not scaleable

on the other hand MSTP multiple spaning tree

this way u gonna make for example on each switch two instances and then u can assign each vlan to one of the instances lets say u have 100 vlan 50 of then in one instance and other 50 will be on the other in stances

and instance one use switch1 as root and instnce two use switch2 as a root so that u can divid the vlans and at the same time u have only two in stances which is much better than with PVST when u had 100 instances

good luck

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