Multiple Spanning Tree Protocol (MSTP) is an extension of the original Spanning Tree Protocol (STP) used in computer networks to prevent loops in bridged or switched networks. The purpose of STP is to create a loop-free logical topology by blocking redundant paths in the network. However, the original STP operates on a per-VLAN basis, which can be inefficient in networks with multiple VLANs.

PVST or Rapid PVST+ is default STP mode in cisco Catalyst Switches. So that each VLAN is operating to a single spanning tree instance. When you have 15 VLANs, it means there are 15 instances of spanning tree is running. 

Per VLAN Spanning Tree creates a separate STP instances for each VLAN so sometimes it’s can create overhead in switches. So, to reduce the extra load on switches MST is used as a prevention mechanism.

MSTP was introduced to address this limitation by allowing multiple VLANs to be mapped to the same spanning tree instance. This means that a single spanning tree instance can cover multiple VLANs, providing more efficient use of network resources.

Example:

Instance 1: VLAN 2 – 7

Instance 2: VLAN 8 – 16

If we have 2 to 16 VLANs so we can easily map those all in just two MSTP instances

Only two spanning tree instances are required for all these VLANs.

Problems with normal STP & RSTP:

• Single Root Bridge: In STP, there is only one root bridge for the entire network. If the root bridge fails or becomes unreachable, the network must undergo a convergence process to elect a new root bridge, causing potential downtime and disruptions.
• Resource Inefficiency: STP creates a separate spanning tree for each VLAN in the network. This can lead to inefficient use of network resources, as each VLAN may have its own set of blocked links, resulting in underutilization of available bandwidth.
• Limited Scalability: In large networks with many switches and VLANs, STP can become complex to manage and scale. The need to configure and maintain multiple spanning tree instances for each VLAN adds to the administrative overhead and can increase the likelihood of misconfigurations.

Benefits of MSTP:

• Instance Mapping: MSTP allows network administrators to map multiple VLANs to a specific spanning tree instance. This mapping is based on the VLAN-to-instance mapping configured on each switch in the network.
• Multiple Instances: MSTP supports the creation of multiple spanning tree instances, each with its own independent spanning tree topology. This allows for different VLANs to be grouped and treated as a single entity for loop prevention purposes.
• Reduced CPU and Bandwidth Usage: By allowing multiple VLANs to share the same spanning tree instance, MSTP reduces the CPU and bandwidth overhead associated with running separate spanning tree instances for each VLAN.
• Backward Compatibility: MSTP is backward compatible with the original IEEE 802.1D Spanning Tree Protocol (STP) and the Rapid Spanning Tree Protocol (RSTP). This means that MSTP-enabled switches can coexist with non-MSTP switches in the same network.

MSTP is defined in the IEEE 802.1s standard, and it enhances the scalability and efficiency of loop prevention mechanisms in modern networks, especially those with a large number of VLANs.