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Spanning TreeMST Config

instance [id] vlan [range]

Creates or modifies an MST instance and maps VLANs to it, enabling per-instance spanning tree behavior in MSTP.

Syntax·MST Config
instance [id] vlan [range]

When to Use This Command

  • Load balancing traffic across multiple uplinks by assigning different VLANs to different MST instances.
  • Reducing CPU and bandwidth overhead by grouping multiple VLANs under a single spanning tree instance.
  • Isolating STP topology changes to a subset of VLANs to improve network stability.
  • Implementing a redundant Layer 2 design with multiple MST instances for failover granularity.

Command Examples

Map VLANs 10-20 to MST instance 1

instance 1 vlan 10-20

No output is generated on success. The command silently maps VLANs 10 through 20 to MST instance 1. Use 'show spanning-tree mst configuration' to verify.

Map VLANs 30,40,50 to MST instance 2

instance 2 vlan 30,40,50

Maps VLANs 30, 40, and 50 to MST instance 2. VLANs can be specified as a list separated by commas.

Understanding the Output

This command produces no direct output. To verify the mapping, use 'show spanning-tree mst configuration'. The output displays the MST region configuration, including instance-to-VLAN mappings. Each instance is listed with its ID and the VLANs assigned. Ensure that VLANs are not mapped to multiple instances; overlapping VLANs cause errors. Also verify that all VLANs used in the network are mapped to some instance (default instance 0 handles unmapped VLANs).

CCNA Exam Tips

1.

Remember that MST instance 0 is the default and always exists; VLANs not explicitly mapped belong to instance 0.

2.

The 'instance' command is configured in MST configuration mode, not global config. Enter with 'spanning-tree mst configuration'.

3.

CCNA may test that MST instances must be consistent across all switches in the same MST region.

4.

Know that the range can be a hyphen (e.g., 10-20) or a comma-separated list (e.g., 10,20,30).

Common Mistakes

Mapping the same VLAN to multiple MST instances, which is rejected by IOS.

Forgetting to enter MST configuration mode before using the 'instance' command.

Not applying the configuration with 'exit' or 'abort' to activate the changes.

Assuming the command takes effect immediately without verifying with 'show spanning-tree mst'.

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