Back
Lab 05Hands-on Lab

Implement MST

Multi-Instance STP

Goal

Configure an MST region, map VLANs to MST instances, and control instance roots so different VLANs use different forwarding paths (load sharing).

MSTMSTPRegionLoad SharingCIST
1

Implement MST

Configure an MST region, map VLANs to MST instances, and control instance roots so different VLANs use different forwarding paths (load sharing).

2

Multi-Instance STP

R1#
spanning-tree mst configuration
 name <region-name-from-lab>
 revision <n>            ! typically 1
 instance 1 vlan 10, 20
 instance 2 vlan 30, 40
 exit

Concept

3

MST groups VLANs into a few spanning-tree instances. Configure an identical region (name + revision + instance/VLAN map) on every switch, then choose a root per instance.

R1#
spanning-tree mode mst

Define the MST Region (SAME on all 3 switches)

4

The name, revision, AND instance-to-VLAN map must be identical on S1, S2, S3 or they will not be in the same region (and verification fails).

Enable MST Mode

R1#
! S1
spanning-tree mst 1 root primary
spanning-tree mst 2 root secondary
! S2
spanning-tree mst 2 root primary
spanning-tree mst 1 root secondary
! S3 (default priorities, no change)
5

Verify: show spanning-tree → "Spanning tree enabled protocol mstp" with CIST root info.

Set Instance Roots

R1#
interface g0/2
 spanning-tree mst 1 cost 10        ! lower cost = preferred path for instance 1
 spanning-tree mst 2 cost 20
! or
 spanning-tree mst 1 port-priority 16
6

Example policy: S1 is root for instance 1 (VLANs 10/20), S2 is root for instance 2 (VLANs 30/40), S3 keeps default priorities.

R1#
show spanning-tree mst              ! region info + instance summary
show spanning-tree mst 1
show spanning-tree mst 2
show spanning-tree mst configuration   ! verify region matches on all switches
show running-config | include spanning-tree

Common Pitfalls