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šŸŒ LAB 6.2: Multiple MSTI Load Balancing

Cisco CCNP 350-401 (ENCOR) & 300-410 (ENARSI) Hands-On Training

Multiple Spanning Tree Instances - Enterprise Load Balancing & Asymmetric Topology Design

šŸ”’ Protected
šŸŽÆ LAB OVERVIEW - Multiple MSTI Load Balancing
Enterprise Load Balancing using Asymmetric Root Bridge Selection Across Multiple MSTI Instances
šŸŽÆ Lab Duration: 90-120 minutes | šŸ“Š Difficulty Level: Advanced | šŸ”§ Equipment Required: 4 Cisco Switches (Extended Topology)
āœ… CCNP Exam Alignment: 350-401 (ENCOR) & 300-410 (ENARSI) - MST Load Balancing
šŸ“š Lab Version: 1.0 | āš” Real-World Performance: 70-80% CPU Reduction | 80% Memory Reduction | ~80% Faster Convergence
šŸ“š Learning Objectives
šŸ¢ Lab Scenario

Your enterprise has 20 VLANs distributed across 4 switches. To optimize bandwidth utilization and reduce CPU overhead, you need to create an MST region with multiple instances, each with different root bridges. This asymmetric topology enables traffic for different VLAN groups to follow different paths, maximizing redundant link usage.

ā„¹ļø Real-World Application: By implementing 4 MSTI instances instead of 20 PVST+ instances, you reduce CPU load by 70-80% while enabling intelligent traffic distribution across available paths.
šŸ”— Network Topology Diagram

LAB 5.2: ACCESS LAYER REDUNDANCY - PORT PRIORITY CONFIGURATION

Network Topology
MSTI 0 (IST): All unassigned VLANs - Root: Core-SW-1
MSTI 1: VLANs 10-30 - Root: Core-SW-1 (Load Path 1)
MSTI 2: VLANs 40-60 - Root: Core-SW-2 (Load Path 2)
MSTI 3: VLANs 70-90 - Root: Core-SW-3 (Load Path 3)
āš™ļø Equipment Setup & Requirements
Device Hostname Role Key Interfaces MST Priority
Switch 1 Core-SW-1 IST Root + MSTI 1 Root Gi0/0-1 4096 (IST), 8192 (MSTI 1)
Switch 2 Core-SW-2 MSTI 2 Root Gi0/0-1,G1/1 8192 (MSTI 2)
Switch 3 Core-SW-3 MSTI 3 Root Gi0/1,G1/2 8192 (MSTI 3)
Switch 4 Dis-SW-1 Access/Distribution Gi1/1-2 Default Priority (32768)
šŸŒ VLAN Assignment & Instance Mapping
MST Instance Name/Purpose VLAN Range Primary Root Secondary Root
Instance 0 (IST) Management VLANs 1, 100, 200 Core-SW-1 (Pri: 4096) N/A
MSTI 1 Finance Production 10, 20, 30 Core-SW-1 (Pri: 8192) Core-SW-2 (Sec)
MSTI 2 Engineering Production 40, 50, 60 Core-SW-2 (Pri: 8192) Core-SW-3 (Sec)
MSTI 3 Development/Marketing 70, 80, 90 Core-SW-3 (Pri: 8192) Core-SW-1 (Sec)
āš ļø Pre-Lab Requirements
āš ļø Important Prerequisites:
  • ā€¢ Reset all switches: write erase then reload
  • ā€¢ Verify LAB 6.1 (basic MST) configuration is working before proceeding
  • ā€¢ Ensure all trunk links are properly configured
  • ā€¢ All switches must have compatible IOS versions with MST support
  • ā€¢ Physical connections must match the topology diagram
šŸ”§ Step-by-Step Configuration Guide
Phase 1: Baseline & VLAN Setup
Step 1: Initialize Switches & Create VLANs Execute on ALL switches:
! Reset switch to baseline Core-SW-1# write erase Core-SW-1# reload ! Proceed after switch boots ! On all switches: Switch# configure terminal Switch(config)# hostname SW-X ! Create all required VLANs Switch(config)# vlan 10 Switch(config-vlan)# name Finance-VLAN10 Switch(config-vlan)# exit Switch(config)# vlan 20 Switch(config-vlan)# name Finance-VLAN20 Switch(config-vlan)# exit Switch(config)# vlan 30 Switch(config-vlan)# name Finance-VLAN30 Switch(config-vlan)# exit Switch(config)# vlan 40 Switch(config-vlan)# name Engineering-VLAN40 Switch(config-vlan)# exit Switch(config)# vlan 50 Switch(config-vlan)# name Engineering-VLAN50 Switch(config-vlan)# exit Switch(config)# vlan 60 Switch(config-vlan)# name Engineering-VLAN60 Switch(config-vlan)# exit Switch(config)# vlan 70 Switch(config-vlan)# name Dev-Marketing-VLAN70 Switch(config-vlan)# exit Switch(config)# vlan 80 Switch(config-vlan)# name Dev-Marketing-VLAN80 Switch(config-vlan)# exit Switch(config)# vlan 90 Switch(config-vlan)# name Dev-Marketing-VLAN90 Switch(config-vlan)# exit Switch(config)# vlan 100 Switch(config-vlan)# name Management-VLAN100 Switch(config-vlan)# exit Switch(config)# vlan 200 Switch(config-vlan)# name Monitoring-VLAN200 Switch(config-vlan)# exit Switch(config)# end Switch# wr mem
āœ“ Verification Command: show vlan brief
Step 2: Configure Trunk Links Execute on ALL switches:
! Configure trunk links on all interfaces Switch# configure terminal ! Configure core trunk links (Gi0/1-3) Switch(config)# interface range gi0/1-3 Switch(config-if-range)# description Core-Trunk-Link Switch(config-if-range)# switchport mode trunk Switch(config-if-range)# switchport trunk allowed vlan 1,10-100,200 Switch(config-if-range)# switchport trunk native vlan 1 Switch(config-if-range)# no shutdown Switch(config-if-range)# exit ! Configure backup trunk links (Gi0/48-50) Switch(config)# interface range gi0/48-50 Switch(config-if-range)# description Access-Trunk-Link Switch(config-if-range)# switchport mode trunk Switch(config-if-range)# switchport trunk allowed vlan 1,10-100,200 Switch(config-if-range)# switchport trunk native vlan 1 Switch(config-if-range)# no shutdown Switch(config-if-range)# exit Switch(config)# end Switch# wr mem
āœ“ Verification Command: show interface trunk
Phase 2: Enable MST Mode & Configure Region
Step 3: Enable MST Mode on All Switches Execute on ALL switches:
! Enable MST spanning tree mode Switch# configure terminal Switch(config)# spanning-tree mode mst Switch(config)# end Switch# wr mem
āœ“ MST mode enabled. Ready for region configuration.
Step 4: Configure MST Region Attributes (IDENTICAL on all switches) Execute on ALL switches - Configuration MUST be IDENTICAL:
! CRITICAL: Region name and revision MUST match on all switches Switch# configure terminal Switch(config)# spanning-tree mst configuration ! Set region parameters Switch(config-mst)# name ENTERPRISE-MST Switch(config-mst)# revision 2 ! Map VLANs to instances Switch(config-mst)# instance 0 vlan 1,100,200 Switch(config-mst)# instance 1 vlan 10,20,30 Switch(config-mst)# instance 2 vlan 40,50,60 Switch(config-mst)# instance 3 vlan 70,80,90 ! Exit MST configuration mode Switch(config-mst)# exit Switch(config)# end Switch# wr mem ! Verify configuration Switch# show spanning-tree mst configuration
āœ… Expected Output:
Name [ENTERPRISE-MST]
Revision [2]
Instance VlanMapping
0 1,100,200
1 10,20,30
2 40,50,60
3 70,80,90
Phase 3: Configure Asymmetric Root Bridges per MSTI
Step 5: Configure Core-SW-1 as IST & MSTI 1 Root Execute ONLY on Core-SW-1:
Core-SW-1# configure terminal ! Configure Core-SW-1 as primary root for IST (Instance 0) Core-SW-1(config)# spanning-tree mst 0 root primary ! Priority automatically set to 4096 ! Configure Core-SW-1 as primary root for MSTI 1 Core-SW-1(config)# spanning-tree mst 1 root primary ! Priority automatically set to 8192 Core-SW-1(config)# end Core-SW-1# wr mem
āœ… Expected Result:
Core-SW-1 is now root for IST (Instance 0) and MSTI 1. Priority: 4096 (IST), 8192 (MSTI 1).
Step 6: Configure Core-SW-2 as MSTI 2 Root Execute ONLY on Core-SW-2:
Core-SW-2# configure terminal ! Configure Core-SW-2 as primary root for MSTI 2 Core-SW-2(config)# spanning-tree mst 2 root primary ! Priority automatically set to 8192 Core-SW-2(config)# end Core-SW-2# wr mem
ā„¹ļø Load Balancing Benefit:
VLANs 40-60 traffic follows path through Core-SW-2
VLANs 10-30 traffic follows path through Core-SW-1
This distributes load across multiple paths
Step 7: Configure Core-SW-3 as MSTI 3 Root Execute ONLY on Core-SW-3:
Core-SW-3# configure terminal ! Configure Core-SW-3 as primary root for MSTI 3 Core-SW-3(config)# spanning-tree mst 3 root primary ! Priority automatically set to 8192 Core-SW-3(config)# end Core-SW-3# wr mem
āœ… Expected Result:
Core-SW-3 is now root for MSTI 3. Priority: 8192.
Step 8: Set Secondary Root Bridges for Redundancy Execute on respective switches:
! On Core-SW-1 (backup for MSTI 2 & 3): Core-SW-1# configure terminal Core-SW-1(config)# spanning-tree mst 2 root secondary Core-SW-1(config)# spanning-tree mst 3 root secondary Core-SW-1(config)# end Core-SW-1# wr mem ! On Core-SW-2 (backup for MSTI 1 & 3): Core-SW-2# configure terminal Core-SW-2(config)# spanning-tree mst 1 root secondary Core-SW-2(config)# spanning-tree mst 3 root secondary Core-SW-2(config)# end Core-SW-2# wr mem ! On Core-SW-3 (backup for MSTI 2 & 1): Core-SW-3# configure terminal Core-SW-3(config)# spanning-tree mst 2 root secondary Core-SW-3(config)# spanning-tree mst 1 root secondary Core-SW-3(config)# end Core-SW-3# wr mem
āœ… Redundancy Active: Each MSTI has primary and backup root bridges for failover protection.
Step 9: Set Access Switches to Default Priority Execute on Dis-SW-1 (and any access switches):
! Configure access/non-root switches Dis-SW-1# configure terminal ! Set default priority on all instances Dis-SW-1(config)# spanning-tree mst 0 priority 32768 Dis-SW-1(config)# spanning-tree mst 1 priority 32768 Dis-SW-1(config)# spanning-tree mst 2 priority 32768 Dis-SW-1(config)# spanning-tree mst 3 priority 32768 Dis-SW-1(config)# end Dis-SW-1# wr mem
āœ… Expected Result:
Dis-SW-1 now has default priority (32768) on all instances, ensuring it never becomes root.
āœ… Verification & Validation
Verification Checklist 1: MST Region Configuration
  • ā˜ All 4 switches have identical region name "ENTERPRISE-MST"
  • ā˜ All 4 switches have revision number "2"
  • ā˜ Instance mapping verified on all switches
  • ā˜ IST (Instance 0) vlan 1, 100, 200 confirmed
  • ā˜ MSTI 1 vlan 10, 20, 30 confirmed
  • ā˜ MSTI 2 vlan 40, 50, 60 confirmed
  • ā˜ MSTI 3 vlan 70, 80, 90 confirmed
Verification Checklist 2: Asymmetric Root Bridges
  • ā˜ Core-SW-1: IST root (priority 4096)
  • ā˜ Core-SW-1: MSTI 1 root (priority 8192)
  • ā˜ Core-SW-2: MSTI 2 root (priority 8192)
  • ā˜ Core-SW-3: MSTI 3 root (priority 8192)
  • ā˜ All secondary roots configured
  • ā˜ Load balancing verified with different root bridges per instance
Verification Checklist 3: Trunk Links & Convergence
  • ā˜ All trunk links operational: show interface trunk
  • ā˜ All trunks allow required VLANs
  • ā˜ No errors on trunk ports
  • ā˜ Convergence time < 10 seconds per instance
  • ā˜ IST and all MSTIs converged
  • ā˜ No port role conflicts across instances
šŸ“‹ Critical Verification Commands
1. MST Region Status Verification
! Execute on any switch: show spanning-tree mst configuration ! Expected Output: Name [ENTERPRISE-MST] Revision [2] Instance VlanMapping 0 1,100,200 1 10,20,30 2 40,50,60 3 70,80,90
2. IST Root Bridge Verification
! Execute on any switch: show spanning-tree mst 0 | include "Root\|Priority" ! Expected on non-root switches: Root ID Priority 4096 Address aabb.ccdd.eeff (Core-SW-1 MAC) Cost 0 ! Expected on Core-SW-1: This bridge is the root
3. MSTI Root Bridge Verification
! Verify MSTI 1 Root (should be Core-SW-1): show spanning-tree mst 1 | include "Root\|Priority" ! Verify MSTI 2 Root (should be Core-SW-2): show spanning-tree mst 2 | include "Root\|Priority" ! Verify MSTI 3 Root (should be Core-SW-3): show spanning-tree mst 3 | include "Root\|Priority" ! Expected Output for MSTI 1 on Core-SW-2: Regional Root ID Priority 8192 Address aabb.1111.1111 (Core-SW-1 MAC)
4. Port Roles Across Instances
! Display detailed topology per instance: show spanning-tree mst 0 detail show spanning-tree mst 1 detail show spanning-tree mst 2 detail show spanning-tree mst 3 detail ! Verify roles differ per instance: Interface Role State Cost Gi0/1 Desg Fwd 200000 Gi0/2 Root Fwd 200000 Gi0/3 Altn Blk 200000 ! Different port roles = Load balancing active
5. Load Balancing Verification
! Display all root bridges in single view: show spanning-tree mst root ! Expected Output: Instance Pri RootID RootCost RootPort Cost 0 4096 aabb.0000.0001 0 Gi0/1 200000 1 8192 aabb.0000.0001 200000 Gi0/1 200000 2 8192 aabb.0000.0002 200000 Gi0/2 200000 3 8192 aabb.0000.0003 200000 Gi0/3 200000 ! Shows different roots and different paths for each instance!
6. VLAN to Instance Mapping Verification
! Verify VLAN belongs to correct instance: show spanning-tree vlan 10 show spanning-tree vlan 40 show spanning-tree vlan 70 ! Expected Output shows which instance each VLAN uses: VLAN 10 is executing the mst1 (MST Instance) version of the STP Spanning Tree enabled protocol mst Root ID Priority 8192 Address aabb.0000.0001 (Core-SW-1) Cost 200000 Port 3 (Gigabit 0/3)
šŸ” Troubleshooting Guide
Issue 1: Region Mismatch - Different Names or Revisions Symptom: Switches show different region names or revision numbers Root Cause: Configuration mismatch prevents unified region formation Solution:
! On mismatched switch: Switch# configure terminal Switch(config)# spanning-tree mst configuration Switch(config-mst)# name ENTERPRISE-MST Switch(config-mst)# revision 2 ! Verify region matches Switch(config-mst)# exit Switch(config)# end Switch# show spanning-tree mst configuration ! Region name and revision must match all other switches
Issue 2: Different Root Bridges Not Applied Correctly Symptom: All MSTIs show same root bridge instead of different ones Root Cause: Root bridge commands not executed on correct switches or priority values still at defaults Solution:
! Verify on each switch: Core-SW-1# show spanning-tree mst 1 | include "Bridge ID\|Priority" Core-SW-1# show spanning-tree mst 2 | include "Bridge ID\|Priority" Core-SW-1# show spanning-tree mst 3 | include "Bridge ID\|Priority" ! Should show different priorities on different switches ! Check bridge ID addresses - should be different per instance ! If all roots are same, verify root primary commands executed: Core-SW-1# show spanning-tree mst 1 root Core-SW-2# show spanning-tree mst 2 root Core-SW-3# show spanning-tree mst 3 root
Issue 3: Load Balancing Not Working Symptom: Traffic for all VLANs follows same path (single root dominates) Root Cause: Different MSTIs don't have different root bridges or port cost calculations favor one path Solution:
! Check if all instances have their intended roots: show spanning-tree mst root ! Output should show different Root IDs per instance: Instance Pri RootID RootCost RootPort Cost 0 4096 aabb.0000.0001 Gi0/1 1 8192 aabb.0000.0001 Gi0/1 2 8192 aabb.0000.0002 Gi0/2 ā† Different! 3 8192 aabb.0000.0003 Gi0/3 ā† Different! ! If not different, reconfigure root priorities: Core-SW-2(config)# spanning-tree mst 2 priority 8192 Core-SW-3(config)# spanning-tree mst 3 priority 8192
Issue 4: Convergence Delays on Instance Change Symptom: One MSTI converges quickly, another takes 30+ seconds Root Cause: Possible spanning tree port issues, mismatched timers, or RSTP not properly enabled Solution:
! Check if RSTP is enabled in MST: show spanning-tree detail | include "Protocol" ! Should show: STP Protocol = rstp (RSTP runs within MST) ! Verify port states after simulated link failure: show spanning-tree mst 0 detail show spanning-tree mst 1 detail show spanning-tree mst 2 detail show spanning-tree mst 3 detail ! All ports should transition within 6-10 seconds ! If taking longer, check for BPDUguard or other port configurations
šŸ“ˆ Performance Comparison: MST Load Balancing vs Single STP Instance
Metric Single STP Instance MST 4 Instances (Lab Result) Improvement
Bandwidth Utilization Single path active Multiple paths active 300-400% throughput gain
CPU Load 35-40% baseline 10-15% baseline 70% reduction
Memory Usage 20 MB (20 PVST+ instances) 4 MB (4 MSTI instances) 80% reduction
Convergence Time 30-50 seconds 6-10 seconds ~80% faster
Load Distribution Unbalanced Optimized Better utilization
BPDU Overhead Multiple per port 1 BPDU/port (4 instances inside) Significant reduction
šŸ“š Key Learning Points
šŸŽÆ Critical Concepts to Understand: 1. MST Region Requirement All switches in an MST region must have identical region name, revision, and instance VLAN mapping. Any mismatch creates separate spanning trees instead of a unified region. 2. Asymmetric Load Balancing By configuring different switches as root bridges for different MSTI instances, traffic for different VLAN groups takes different paths, maximizing bandwidth utilization across available redundant links. 3. IST (Internal Spanning Tree) MSTI 0 (Instance 0) acts as the IST and encapsulates all other MSTIs. All switches must participate in IST for inter-region communication. Configuration errors in IST affect all MSTIs. 4. Priority Values ā€¢ Use primary/secondary commands for automatic priority calculation (4096 for primary, 16384 for secondary) ā€¢ Increment by 4096 in manual configurations for easy management ā€¢ Higher priority value = less preferred as root 5. BPDU Encapsulation Efficiency MST encapsulates all MSTI BPDUs into a single BPDU format (M-BPDU). This dramatically reduces bandwidth overhead compared to 20 separate STP instances in PVST+ mode. 6. Convergence Advantages RSTP-like rapid convergence (6-10 seconds) within MST replaces 30-50 second convergence times in traditional spanning tree, improving network resilience. 7. Per-VLAN Load Balancing Each VLAN follows its mapped instance's topology. This allows true asymmetric load balancing where different VLAN groups actively use different physical paths.
āŒ Common Mistakes to Avoid
Mistake 1: Mismatched Region Configuration āœ— Setting different region names or revisions on switches āœ“ Verify ALL switches have identical: name ENTERPRISE-MST, revision 2 Mistake 2: Not Creating Backup Root Bridges āœ— Only configuring primary roots without secondaries āœ“ Always configure secondary roots for failover protection Mistake 3: Same Root for All Instances āœ— Letting all instances use single root bridge by default āœ“ Explicitly configure different roots (primary commands) on different switches per instance Mistake 4: Forgetting to Include Unassigned VLANs in Instance 0 āœ— Creating instances that don't include Instance 0 mapping āœ“ Instance 0 (IST) must include at least one VLAN (usually management VLANs 1, 100, 200) Mistake 5: Not Verifying with Show Commands āœ— Applying configuration without verifying actual state āœ“ After each major config step, run verification commands to confirm expected behavior Mistake 6: Forgetting "write memory" After Configuration āœ— Configuring switches but not saving with wr mem āœ“ Always save configuration after making changes to persist across reboots
āœ… Lab Completion Criteria
Successfully Complete This Lab When:
  • āœ“ All 4 switches in same MST region (ENTERPRISE-MST, Rev 2)
  • āœ“ IST and 3 MSTIs created with correct VLAN mappings
  • āœ“ Asymmetric root bridges configured (Core-SW-1, Core-SW-2, Core-SW-3 roots)
  • āœ“ Load balancing verified with different paths per VLAN group
  • āœ“ All trunk links operational and error-free
  • āœ“ Convergence time < 10 seconds per instance
  • āœ“ All verification commands execute successfully
  • āœ“ Performance metrics show resource reduction (CPU, Memory)
  • āœ“ Secondary root bridges functional (manual failover test)
  • āœ“ No region mismatches or BPDU errors in logs
šŸŽ“ Advanced Challenge (Optional)
Challenge 1: Failover Testing Simulate Core-SW-1 failure while monitoring MSTI 1 convergence. Verify Core-SW-2 takes over as backup root within 10 seconds. Challenge 2: Dynamic Load Rebalancing Add a 5th switch and redistribute VLANs across 5 instances. Measure CPU/memory reduction compared to 4 instances. Challenge 3: MST to PVST+ Interoperability Connect a PVST+ switch to the MST region. Analyze BPDU exchanges and verify proper boundary handling. Challenge 4: Cost Manipulation Adjust link costs on specific instances to force asymmetric traffic patterns. Example: Make VLAN 10-30 take path A, force VLAN 40-60 through path B intentionally.