Cisco Multiple Spanning Tree Protocol MSTP Network

The Cisco Multiple Spanning Tree Protocol (MSTP) network is defined under IEEE 802.1s which was developed to address the limitation of the conventional spanning tree where only a single spanning-tree instance for the whole network can only be used. All VLANs are in the same spanning tree. Multiple Spanning Tree (MST) convergence is fast because it overlays on top of the Rapid spanning tree protocol (RSTP). Scalability has been an issue with the common spanning tree (CST), but MST provides load balancing, allowing an extensive Layer 2 network.


Multiple Spanning Tree Topologies

As you can see on the diagram below, all 4 VLANs maintain STP. Let us say another 5 more VLANs were added to this network. The switches will still support 3 STP topologies if aligned to one of the three existing MSTIs. MST gives a mixed approach by mapping one or multiple VLANs onto different spanning-tree instances. A group of VLANs in one spanning-tree instance is called an MST Instance (MSTI).

MST region is defined as a grouping of MST switches with the same high-level configuration. MST includes mechanisms that make an MST region looks like a single virtual switch to outside switches as part of the mechanism for compatibility.

MSTP Network


The diagram below shows the concept of an actual STP topology compared to what the devices outside the MST region perceived it to be.

MST Topology


MSTP Network Configuration

The first step is to establish MST as the spanning tree protocol using the command ‘spanning-tree mode mst’. Then, optionally, you can define either one of the two methods below for the multiple spanning-tree instances.

  • spanning-tree mst <instance-number> priority <priority>
  • spanning-tree mst <instance-number> root {primary | secondary}[diameter diameter]


The VLANs should be associated with a multiple spanning-tree instance. By default, all VLANs are associated with MST instance 0, called Internal Spanning Tree (IST). The MST configuration submode must be entered in global configuration mode using the command ‘spanning-tree mst configuration’. Once done, the VLANs can now be assigned to a different MST instance with the command ‘instance <instance-number> vlan <vlan-id>’.

The MST version number should also be specified. The MST version number must match all switches in the same MST region. The MST version number is configured with the submode configuration command ‘revision number’. Finally, define the MST region name using the command ‘name <mst-region-name>’.


Here is a sample configuration for MST:

SW2(config)# spanning-tree mode mst
SW2(config)# spanning-tree mst 0 root primary
SW21(config)# spanning-tree mst 1 root primary
SW2(config)# spanning-tree mst 2 root primary
SW2(config)# spanning-tree mst configuration
SW2(config-mst)# name STUDY_CCNP
SW2(config-mst)# revision 2
SW2(config-mst)# instance 1 vlan 15,25
SW2(config-mst)# instance 2 vlan 99


MSTP Network Verification

To quickly verify our MST configuration, we can use the command ‘show spanning-tree mst configuration’, which will yield the following output:

SW2# show spanning-tree mst configuration
Revision 2 Instances configured 3
Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 1-9,11-19,21-98,100-4094
1 15,25
2 99


You can see that MST instance 0 contains all the VLANs except 15, 25, and 99, regardless of the VLANs configured on the switch. MST instance 1 contains VLAN 15 and 25, and MST instance 2 has only VLAN 99.

The command’ spanning-tree mst interface <interface-id>’ can also be used to check the MST configuration on a specific interface.

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