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MST - Best Practices for Core and Access Switch Configurations

In this post, we will configure Multiple Spanning Tree (MST), a protocol designed to optimize spanning tree instances by mapping multiple VLANs to fewer instances. This reduces overhead on network devices, enhances scalability, and speeds up convergence. We'll configure MST on both core/root switches and access switches, ensuring that only the required VLANs are active on each switch. The configuration will focus on assigning VLANs to specific MST instances, defining root priorities, and controlling VLAN availability on trunk links between switches.

This setup ensures efficient traffic flow, minimizes network downtime, and improves overall stability. We'll also define MST regions and revision numbers to maintain consistency across the network. By following this guide, you'll optimize spanning tree operations while maintaining flexibility in VLAN creation and deployment across your infrastructure.


Configuration for Core-SW1 (Primary Root for Instance 1)

! Define MST region and name it "MSTRegion1" with revision number 1

core-sw1(config)# spanning-tree mode mst

core-sw1(config)# spanning-tree mst configuration

core-sw1(config-mst)# name MSTRegion1

core-sw1(config-mst)# revision 1

core-sw1(config-mst)# instance 1 vlan 2-50

core-sw1(config-mst)# instance 2 vlan 51-100

core-sw1(config-mst)# exit

 

! Set priorities to define the root for MST instances

core-sw1(config)# spanning-tree mst 1 priority 4096  ! Primary root for instance 1

core-sw1(config)# spanning-tree mst 2 priority 8192  ! Secondary root for instance 2

 

! Save the configuration

core-sw1# write memory


Configuration for Core-SW2 (Primary Root for Instance 2)

! Define MST region and name it "MSTRegion1" with revision number 1

core-sw2(config)# spanning-tree mode mst

core-sw2(config)# spanning-tree mst configuration

core-sw2(config-mst)# name MSTRegion1

core-sw2(config-mst)# revision 1

core-sw2(config-mst)# instance 1 vlan 2-50

core-sw2(config-mst)# instance 2 vlan 51-100

core-sw2(config-mst)# exit

 

! Set priorities to define the root for MST instances

core-sw2(config)# spanning-tree mst 1 priority 8192  ! Secondary root for instance 1

core-sw2(config)# spanning-tree mst 2 priority 4096  ! Primary root for instance 2

 

! Save the configuration

core-sw2# write memory


Configuration for Access Switches

! Define MST region to match the core switches

access-sw(config)# spanning-tree mode mst

access-sw(config)# spanning-tree mst configuration

access-sw(config-mst)# name MSTRegion1

access-sw(config-mst)# revision 1

access-sw(config-mst)# instance 1 vlan 2-50

access-sw(config-mst)# instance 2 vlan 51-100

access-sw(config-mst)# exit

 

! Configure trunk to Core-SW1

access-sw(config)# interface g1/0/1

access-sw(config-if)# switchport mode trunk

access-sw(config-if)# switchport trunk allowed vlan 2-50,51-100

access-sw(config-if)# exit

 

! Configure trunk to Core-SW2

access-sw(config)# interface g1/0/2

access-sw(config-if)# switchport mode trunk

access-sw(config-if)# switchport trunk allowed vlan 2-50,51-100

access-sw(config-if)# exit

 

! Enable PortFast and BPDU Guard on access ports

access-sw(config)# interface range g0/1-24

access-sw(config-if-range)# spanning-tree portfast

access-sw(config-if-range)# spanning-tree bpduguard enable

access-sw(config-if-range)# exit

 

! Example: Allow VLAN 10 on specific access ports

access-sw(config)# interface range g0/5-8

access-sw(config-if-range)# switchport access vlan 10

access-sw(config-if-range)# spanning-tree portfast

access-sw(config-if-range)# exit

 

! Verify MST configuration

access-sw# show spanning-tree mst configuration

! Verify spanning tree status

access-sw# show spanning-tree mst

! Verify trunk status

access-sw# show interface trunk


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