What is MSTP Multiple Spanning Tree Protocol

Technical details of the Multiple Spanning Tree Protocol (MSTP).

1. Background Information:
   • MSTP is an IEEE standard inspired by the Cisco proprietary Multiple Instances Spanning Tree Protocol (MISTP) implementation.
   • It provides a way to create multiple spanning trees (instances) for each Virtual LAN (VLAN) on a single physical network.
   • MSTP allows each VLAN to have its own configured root bridge and forwarding topology.
   • The goal is to optimize network performance, enhance redundancy, and prevent loops in complex network designs.
2. How MSTP Works:
   • MSTP divides the network into regions, each with its own spanning tree instance.
   • Within a region, multiple VLANs share the same spanning tree instance.
   • The Common Spanning Tree (CST) covers the entire network and interacts with external switches.
   • MSTP uses the concept of regions, MST instances, and Internal Spanning Tree (IST).
3. Regions:
   • A region is a group of switches that share the same MST configuration.
   • Each region has a unique region name (a 32-character alphanumeric string).
   • All switches within a region must have the same region name and revision number.
4. MST Instances:
   • Each region can have multiple MST instances (usually one per VLAN).
   • MST instances map VLANs to spanning tree instances.
   • The IST is the primary instance that handles CST interactions.
   • Other instances are called MSTIs (MST instances).
5. IST (Internal Spanning Tree):
   • The IST is the common spanning tree for all VLANs within a region.
   • It ensures connectivity between MST regions and the CST.
   • IST parameters (root bridge, designated ports, etc.) are consistent across all switches in the region.
6. MSTIs (MST Instances):
   • MSTIs are separate spanning trees for specific VLANs.
   • Each MSTI has its own root bridge, designated ports, and topology.
   • MSTIs are identified by instance numbers (0 to 4095).
   • VLAN-to-instance mapping is defined in the MST configuration.
7. Configuration:
   • To configure MSTP:
     • Set the region name and revision number consistently across switches.
     • Define VLAN-to-instance mapping.
     • Adjust IST parameters (priority, cost, etc.) if needed.
   • MSTP uses the RSTP (802.1w) algorithm for rapid convergence.
8. Use Cases:
   • MSTP is useful in large networks with multiple VLANs and complex topologies.
   • It provides better load balancing and faster convergence than traditional STP.
   • Common scenarios include data centers, campus networks, and service provider networks.
9. Cisco Catalyst Switch Support:
   • MSTP is supported on various Cisco Catalyst switches, with compatibility varying by platform and IOS version.