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Multiple Spanning Tree - MST

 MST (Multiple Spanning Tree) – 802.1s Overview:


  • Purpose: MST reduces the number of STP instances by mapping multiple VLANs to a single STP instance, improving efficiency and minimizing CPU cycles.
  • Compatibility: MST is backward-compatible with 802.1D (CST), 802.1w (RSTP), and Cisco’s proprietary PVST+.
  • MST Region: A group of interconnected switches with the same MST configuration. Switches in the same region appear as a single virtual switch to external devices.

MST Features:

  • MSTI (MST Instance): Maps one or more VLANs to a single STP instance, reducing the number of required STP instances.
  • Load Balancing: VLANs are distributed across multiple MST instances, allowing better load distribution across the network.
  • IST (Internal Spanning Tree): The default instance (MSTI 0) runs on all switch ports and is responsible for managing VLANs not assigned to other instances.

MST Configuration Key Points:

  • Region Consistency: Switches must have the same MST configuration (VLAN-to-instance mapping, region name, and revision number) to belong to the same MST region.
  • BPDUs: MST only sends BPDUs for the IST instance (MSTI 0), embedding information about other MST instances. MST minimizes BPDU traffic by consolidating them into a single set of BPDUs.
  • Digest Comparison: Instead of sending the entire VLAN mapping, MST uses a digest (hash code) of the configuration to compare with other switches, ensuring consistency.

MST Operation:

  • MST Instances: MST supports up to 16 instances per switch (MSTI 0–15). VLANs are assigned to MST instances, and a VLAN can only belong to one instance at a time.
  • Load Balancing: MST allows traffic to be load-balanced by assigning VLANs to different MST instances and adjusting port priorities.

MST Troubleshooting:

  • Common Misconfigurations:
    • Incorrect VLAN assignment to the IST.
    • Trunk link pruning issues, where VLANs are incorrectly pruned from trunk links.

MST in Practice:

  • MST Region: In most networks, a single MST region is sufficient, though multiple regions can be configured.
  • MSTI Boundary: When interacting with external switches or regions, the IST presents the MST region as a single virtual bridge, allowing seamless communication between MST regions and non-MST devices.

Important Considerations:

  • VLAN Mapping: Changing the VLAN-to-MSTI mapping requires restarting the MST process.
  • Path Cost Calculation: MST uses the long path cost method, ensuring more accurate spanning tree calculations for larger networks.
  • MST with VTPv3: MST configurations can be propagated using VTPv3 across the network, simplifying configuration management.

Summary:

MST allows the consolidation of VLANs into fewer STP instances, optimizing network resources and enabling load balancing across multiple VLANs. MST regions must be consistent across switches, and the IST handles external communication, ensuring interoperability with non-MST switches.

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