As previously discussed, MST allows VLANs to be mapped to separate instances. This has the benefit of incorporating different logical topologies without the added resource usage of having a separate STP instance for each VLAN. I'll use a simple topology (below) to demonstrate basic MST configuration.
The MST region name is CCIE and the revision number is 1. VLANs 1-100 are mapped to instance 1 and VLANs 101-200 are mapped to instance 2. The remaining VLANs 201-4094 are assigned to instance 0 by default. MST 0 has a special purpose in terms of IST and CIST, in case of multiple MST regions but it will not be discussed or examined further in this post.
SW1(config)#spanning-tree mst configuration
SW1(config-mst)#name CCIE
SW1(config-mst)#revision 1
SW1(config-mst)#instance 1 vlan 1-100
SW1(config-mst)#instance 2 vlan 101-200
SW1#show spanning-tree mst configuration
% Switch is not in mst mode
Name [CCIE]
Revision 1 Instances configured 3
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 201-4094
1 1-100
2 101-200
-------------------------------------------------------------------------------
SW2(config)#spanning-tree mst configuration
SW2(config-mst)#name CCIE
SW2(config-mst)#revision 1
SW2(config-mst)#instance 1 vlan 1-100
SW2(config-mst)#instance 2 vlan 101-200
SW2#
SW2#show spanning-tree mst configuration
% Switch is not in mst mode
Name [CCIE]
Revision 1 Instances configured 3
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 201-4094
1 1-100
2 101-200
-------------------------------------------------------------------------------
The first part applies the MST configuration but doesn't change the STP mode yet. The switches are still running RSTP, and SW2 is the root.
SW1#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address fa16.3e02.d237
Cost 4
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 4 128.6 P2p
Gi1/2 Altn BLK 4 128.7 P2p
SW2#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.6 P2p
Gi1/2 Desg FWD 4 128.7 P2p
The spanning-tree mode mst command changes the STP mode to MST.
SW1(config)#spanning-tree mode mst
SW2(config)#spanning-tree mode mst
SW1#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address fa16.3e02.d237
Cost 0
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address fa16.3e02.d237
Cost 20000
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
SW2#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
This is because the switches have only default VLANs configured, and none of them are in the MST2 range. As soon as VLAN 111 is created on both switches, MST2 becomes active.
SW1#show spanning-tree mst 2
##### MST2 vlans mapped: 101-200
Bridge address fa16.3e06.1e17 priority 32770 (32768 sysid 2)
Root this switch for MST2
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg BLK 20000 128.6 P2p
Gi1/2 Desg BLK 20000 128.7 P2p
SW1(config)#spanning-tree mst configuration
SW1(config-mst)#name CCIE
SW1(config-mst)#revision 1
SW1(config-mst)#instance 1 vlan 1-100
SW1(config-mst)#instance 2 vlan 101-200
SW1#show spanning-tree mst configuration
% Switch is not in mst mode
Name [CCIE]
Revision 1 Instances configured 3
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 201-4094
1 1-100
2 101-200
-------------------------------------------------------------------------------
SW2(config)#spanning-tree mst configuration
SW2(config-mst)#name CCIE
SW2(config-mst)#revision 1
SW2(config-mst)#instance 1 vlan 1-100
SW2(config-mst)#instance 2 vlan 101-200
SW2#
SW2#show spanning-tree mst configuration
% Switch is not in mst mode
Name [CCIE]
Revision 1 Instances configured 3
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 201-4094
1 1-100
2 101-200
-------------------------------------------------------------------------------
The first part applies the MST configuration but doesn't change the STP mode yet. The switches are still running RSTP, and SW2 is the root.
SW1#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address fa16.3e02.d237
Cost 4
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 4 128.6 P2p
Gi1/2 Altn BLK 4 128.7 P2p
SW2#show spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.6 P2p
Gi1/2 Desg FWD 4 128.7 P2p
The spanning-tree mode mst command changes the STP mode to MST.
SW1(config)#spanning-tree mode mst
SW2(config)#spanning-tree mode mst
SW1#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address fa16.3e02.d237
Cost 0
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address fa16.3e02.d237
Cost 20000
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
SW2#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
Note that the MST2 instance is missing. This is more evident in the following output.
SW1#show spanning-tree mst 1
##### MST1 vlans mapped: 1-100
Bridge address fa16.3e06.1e17 priority 32769 (32768 sysid 1)
Root address fa16.3e02.d237 priority 32769 (32768 sysid 1)
port Gi1/1 cost 20000 rem hops 19
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
SW1#show spanning-tree mst 2
No mst information available for instance(s) 2.
SW2#show spanning-tree mst 1
##### MST1 vlans mapped: 1-100
Bridge address fa16.3e02.d237 priority 32769 (32768 sysid 1)
Root this switch for MST1
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
SW2#show spanning-tree mst 2
No mst information available for instance(s) 2.
This is because the switches have only default VLANs configured, and none of them are in the MST2 range. As soon as VLAN 111 is created on both switches, MST2 becomes active.
SW1#show spanning-tree mst 2
##### MST2 vlans mapped: 101-200
Bridge address fa16.3e06.1e17 priority 32770 (32768 sysid 2)
Root this switch for MST2
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg BLK 20000 128.6 P2p
Gi1/2 Desg BLK 20000 128.7 P2p
SW2#show spanning-tree mst 2
##### MST2 vlans mapped: 101-200
Bridge address fa16.3e02.d237 priority 32770 (32768 sysid 2)
Root this switch for MST2
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
After this, the MST configuration is similar to other STP modes. For example, let's configure SW1 as the root switch for MST1 and SW2 as the root switch for MST2.
SW1(config)#spanning-tree mst 1 root primary
SW1(config)#spanning-tree mst 2 root secondary
SW2(config)#spanning-tree mst 1 root secondary
SW2(config)#spanning-tree mst 2 root primary
SW1# show spanning-tree
MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address fa16.3e06.1e17
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address fa16.3e02.d237
Cost 20000
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28674 (priority 28672 sys-id-ext 2)
Address fa16.3e06.1e17
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
SW2# show spanning-tree
MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address fa16.3e06.1e17
Cost 20000
Port 6 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28673 (priority 28672 sys-id-ext 1)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Root FWD 20000 128.6 P2p
Gi1/2 Altn BLK 20000 128.7 P2p
MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address fa16.3e02.d237
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24578 (priority 24576 sys-id-ext 2)
Address fa16.3e02.d237
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 20000 128.6 P2p
Gi1/2 Desg FWD 20000 128.7 P2p
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