Multiple Spanning Tree (MST) - IEEE 802.1s standard
ccnp-switching
Why Multiple Spanning Tree (802.1s) maps VLANs to a few instances, how MST regions form, and how to configure instances and root bridges.
By default Cisco Catalyst Switches run PVST+ or Rapid PVST+ (Per VLAN Spanning Tree). This means that each VLAN is mapped to a single spanning tree instance. When you have 200 VLANs, it means there are 200 instances of spanning tree. This requires a lot of CPU power and memory.
PVST+ vs MST
- PVST+:
- One instance per VLAN
- 1 VLAN = 1 Instance
- 200 VLAN = 200 Instances
- Cannot change VLAN-to-instance relationship
- A default instance for VLAN 1 always exists
- One instance per VLAN
- MST:
- VLANs can be arbitrarily mapped to instances
- VLANs 1, 5, 7 = 1 instance
- VLANs 2, 20, 200 = 1 instance
- You define VLAN-to-instance mappings
- MST0 is the default instance for all VLANs (1-4094)
- MST0 is also called the Internal Spanning Tree (IST)
- VLANs can be arbitrarily mapped to instances
MST works with the concept of regions. Switches that are configured to use MST need to find out if their neighbors are running MST.
When switches have the same attributes they will be in the same region. It is possible to have one or more regions and here are the attributes that need to match. (YOU explicitly configure these attributes):
- MST configuration name.
- MST configuration revision number.
- MST instance-to-VLAN mapping table.
Example - These are in different regions
Example - These are in the same region
So, here both switches can run MST with each other.
Configuration
Creating VLANs on Distribution_1, Distribution_2, and Access:(config)#vlan 10 (config-vlan)#vlan 20 (config-vlan)#vlan 30 (config-vlan)#vlan 40 (config-vlan)#exitTrunking ports in between the switches:
(config-if)#switchport trunk encapsulation dot1q (config-if)#switchport mode trunkThen we configure spanning-tree MST on all switches as below:
(config)#spanning-tree mode mst (config)#spanning-tree mst configuration (config-mst)#name test (config-mst)#revision 1 (config-mst)#instance 1 vlan 1,10 (config-mst)#instance 2 vlan 20,30,40 (config-mst)#exit (config)#spanning-tree mst configuration (config-mst)#show Pending MST configuration Name [test] Revision 1 Instances configured 3 Instance Vlans mapped -------- --------------------------------------------------------------------- 0 1-9,11-19,21-29,31-39,41-4094 1 10,20 2 30,40 -------------------------------------------------------------------------------
Verification
Access#show spanning-tree mst configuration Name [test] Revision 1 Instances configured 3 Instance Vlans mapped -------- --------------------------------------------------------------------- 0 1-9,11-19,21-29,31-39,41-4094 1 10,20 2 30,40 -------------------------------------------------------------------------------
Distribution_1#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address 001d.707b.4200
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 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Desg FWD 200000 128.26 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address 001d.707b.4200
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 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Desg FWD 200000 128.26 P2p
MST2
Spanning tree enabled protocol mstp
Root ID Priority 32770
Address 001d.707b.4200
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Desg FWD 200000 128.26 P2p
As you can see above, Distribution_1 is root for all MST instances which is not very efficient. Now, we want Distribution_1 to be the root bridge for MST0, and Distribution_2 to be root bridge for MST1 and MST2.
Distribution_1(config)#spanning-tree mst 0 root primary Distribution_1(config)#spanning-tree mst 1,2 root secondary
Distribution_2(config)#spanning-tree mst 0 root secondary Distribution_2(config)#spanning-tree mst 1,2 root primaryTo verify:
Distribution_1#show spanning-tree
MST0
Spanning tree enabled protocol mstp
Root ID Priority 24576
Address 001d.707b.4200
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24576 (priority 24576 sys-id-ext 0)
Address 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Desg FWD 200000 128.26 P2p
MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address 0022.0dec.1200
Cost 200000
Port 26 (FastEthernet1/0/24)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28673 (priority 28672 sys-id-ext 1)
Address 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Root FWD 200000 128.26 P2p
MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address 0022.0dec.1200
Cost 200000
Port 26 (FastEthernet1/0/24)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28674 (priority 28672 sys-id-ext 2)
Address 001d.707b.4200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa1/0/23 Desg FWD 200000 128.25 P2p
Fa1/0/24 Root FWD 200000 128.26 P2p
Tuning MST timers
(config)#spanning-tree mst ? WORD MST instance range, example: 0-3,5,7-9 configuration Enter MST configuration submode forward-time Set the forward delay for the spanning tree hello-time Set the hello interval for the spanning tree max-age Set the max age interval for the spanning tree max-hops Set the max hops value for the spanning tree