TB - GNS3 Lab: Multiple Spanning Tree (MST)

by C. Casquatch

Lab from Renee Molenaar

Topology

SW1 <--> SW2 : Gi0/0, Gi0/1
SW1 <--> SW3 : Gi0/2, Gi0/3
SW2 <--> SW3 : Gi0/2, Gi0/3

Lab Goals

1. Configure all switches to use MST.
2. Configure VLANs 10, 20, 30, 40, 50, and 60 on all switches.
3. Assign:
   • VLANs 10, 20, 30 to Instance 1
   • VLANs 40, 50, 60 to Instance 2
4. Configure SW1 as the root bridge for CIST (instance 0).
5. Configure SW2 as the root bridge for instance 2.

Step-by-Step Configuration

1. Create VLANs (All Switches)

conf t
vlan 10,20,30,40,50,60
exit

2. Set MST Mode Globally (All Switches)

conf t
spanning-tree mode mst
exit

3. Configure MST Region and Instances (All Switches)

conf t
spanning-tree mst configuration
 name MSTRegion
 revision 1
 instance 1 vlan 10,20,30
 instance 2 vlan 40,50,60
exit

⚠️ All switches must use the exact same name, revision, and VLAN mappings.

4. Set Bridge Priorities

On SW1 (Root for CIST):

conf t
spanning-tree mst 0 priority 4096
exit

On SW2 (Root for Instance 2):

conf t
spanning-tree mst 2 priority 4096
exit

5. Enable Trunking on All Links

Repeat on each switch:

conf t
interface range gi0/0 - 3
 switchport trunk encapsulation dot1q
 switchport mode trunk
exit

💡 If you get an error like: Command rejected: An interface whose trunk encapsulation is "Auto"...
Use switchport trunk encapsulation dot1q before setting mode trunk.

Verification Commands

MST Region and VLAN mappings:

show spanning-tree mst configuration

MST active topology:

show spanning-tree mst

Root bridge details:

show spanning-tree mst 0
show spanning-tree mst 2

—

Notes

• CIST = Common and Internal Spanning Tree (MST Instance 0)
• MST is efficient for scaling VLANs across fewer spanning tree processes.
• Ensure consistency across all switches or MST regions won’t synchronise.

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