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what really counts is the number of different Vlans involved.
With 50 Vlans you can use PVST without any concerns.
If you decide to use MST you need to provision some instances in addition to the IST (istance 0) if your VLans can be divided in some groups that will share the same topology (the same root bridge).
Configuration of region name, region revision number and what vlan ranges are associated to each STP instance have to be configured manually on every switch and must match.
So I agree that with 50 VLans and 2000 user ports PVST is much more manageable.
In a MAN context you would use MST for its scalability.
If you would have 130 Vlans not every Cisco switch can run 130 STP instances and here starts the problems with PVSTP.
Some old switches can have only 64 STP instances running at the same time.
These older switches cannot run MST.
Hope to help
Please refer to the following article
Multiple Spanning Tree (MST)
https://networklessons.com/spanning-tree/multiple-spanning-tree-mst/
Multiple Spanning Tree (MST) is an IEEE standard inspired from the Cisco proprietary Multiple Instances Spanning Tree Protocol (MISTP) implementation.
The Multiple Spanning Tree Protocol (MSTP) addresses the limitations in the previous spanning tree protocols i.e., STP and RSTP, within networks that use multiple VLANs with topologies that employ alternative physical links.
It supports multiple spanning tree instances on any given link within a network, and supports large networks by grouping bridges into regions that appear as a single bridge to other devices.
MSTP is defined in IEEE standard 802.1Q-2005. The protocol builds on, and remains compatible with, the previous IEEE standards defining STP and RSTP.
You can optimize your network so that each Vlan have it's own Root Bridge, you can assign each Vlan is root according to the network structure.
If we run multiple instance of STP switch go out of resource & lots of BPDU for every instance.
Switch can also not support more than 130 STP instances .
In MST we create Region and run one instance & associate multiple VLAN to that instance so that we can reduce no of control frame & switch overhead .
MSTP was designed to overcome one major problem with classic STP protocol – inability to use blocked links for traffic forwarding due to single STP instance present. This is accomplished by running multiple spanning trees in a topology and mapping VLANs to different trees for traffic forwarding. Even though this feature does not allow for precise and optimal traffic engineering it improves redundant link utilization. By using regions, MSTP allows for isolating different physical topologies from each other while maintaining Layer 2 connectivity between the regions.
normal spanning tree has one Bpdu message with each VLAN but with MST if group of VLANs have the same topology by MST only one Bpdu will send for all this group
MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning tree instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
The most common initial deployment of MST is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the kind of highly available network that is required in a service-provider environment.
MST provides rapid spanning tree convergence through explicit handshaking, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding state.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
•
Original 802.1D spanning tree
•Existing Cisco-proprietary Multiple Instance STP (MISTP)
•Existing Cisco per-VLAN spanning tree plus (PVST+)
•Rapid per-VLAN spanning tree plus (rapid PVST+)
