MST Instance to VLAN Mapping Mismatch Between Switches
Presenting Symptom
Some workstations in VLAN 10 cannot communicate with servers in VLAN 20 across the network, while other VLANs work fine.
Network Context
The network consists of three Cisco Catalyst 3750 switches in a small campus LAN. Switches are configured with Multiple Spanning Tree (MST) to load balance VLANs across redundant links. VLANs 10 and 20 are mapped to MST instance 1 on one switch, but to MST instance 2 on another switch, causing a spanning tree topology inconsistency.
Diagnostic Steps
Check MST configuration on each switch
show spanning-tree mst configurationName [default] Revision 0 Instance Vlans mapped -------- ----------------------------------------------------------------- 0 1-9,11-19,21-4094 1 10,20 2 none
Compare the VLAN-to-instance mapping across switches. If VLANs 10 and 20 are mapped to different instances on different switches, this indicates a mismatch.
Verify MST instance status
show spanning-tree mst 1MST1
Spanning tree enabled protocol mstp
Root ID Priority 4097
Address 0011.2233.4455
This bridge is the root
Bridge ID Priority 4097 (priority 4096 sys-id-ext 1)
Address 0011.2233.4455
Interface Role Sts Cost Prio.Nbr Type
---------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 20000 128.1 P2p
Gi1/0/2 Desg FWD 20000 128.2 P2pCheck if the MST instance is active and the port roles. If the instance is not present or ports are in blocking state due to inconsistency, it indicates a mapping problem.
Check for MST region configuration mismatch
show spanning-tree mstMST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address 0011.2233.4455
This bridge is the root
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 0011.2233.4455
Interface Role Sts Cost Prio.Nbr Type
---------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 20000 128.1 P2p
Gi1/0/2 Desg FWD 20000 128.2 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 4097
Address 0011.2233.4455
This bridge is the root
Bridge ID Priority 4097 (priority 4096 sys-id-ext 1)
Address 0011.2233.4455
Interface Role Sts Cost Prio.Nbr Type
---------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 20000 128.1 P2p
Gi1/0/2 Desg FWD 20000 128.2 P2pIf MST instances are missing or have different VLAN mappings, the region configuration is inconsistent. Also check that the MST region name and revision number match on all switches.
Verify VLANs on trunk ports
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi1/0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi1/0/1 1-1005 Port Vlans allowed and active in management domain Gi1/0/1 1,10,20,30 Port Vlans in spanning tree forwarding state and not pruned Gi1/0/1 1,10,20,30
Ensure VLANs 10 and 20 are allowed and active on the trunk. If they are missing, the issue may be VLAN pruning or allowed VLAN list mismatch.
Root Cause
The MST instance to VLAN mapping is inconsistent between switches. On SwitchA, VLANs 10 and 20 are mapped to MST instance 1, while on SwitchB they are mapped to MST instance 2. This causes the switches to treat the VLANs as belonging to different spanning tree instances, leading to a topology mismatch and potential blocking of ports for those VLANs.
Resolution
Verification
After applying the fix, run 'show spanning-tree mst configuration' on all switches to confirm VLANs 10 and 20 are mapped to the same MST instance. Then run 'show spanning-tree mst 1' to verify that the instance is active and ports are in forwarding state for those VLANs. Finally, test connectivity between devices in VLAN 10 and VLAN 20.
Prevention
1. Use a consistent MST configuration template across all switches in the region, including the same region name, revision number, and VLAN-to-instance mapping. 2. Implement configuration management tools (e.g., Ansible, Cisco DNA Center) to enforce standard MST configurations. 3. Before deploying changes, use 'show spanning-tree mst configuration' to verify consistency across all switches.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why certain VLANs are not communicating. The exam tests your ability to interpret 'show spanning-tree mst configuration' output and recognize a VLAN-to-instance mapping mismatch. A CCNA candidate must know that MST requires consistent region configuration (name, revision, mapping) across all switches in the region.
Exam Tips
Remember that MST region configuration must be identical on all switches in the region, including the revision number.
The exam may show output where VLANs are mapped to different instances; look for the 'Vlans mapped' line under each instance.
Know that 'show spanning-tree mst' displays all MST instances and their port states; a missing instance or blocked ports can indicate a mismatch.
Commands Used in This Scenario
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