Multiple Spanning-Tree (MST)

The most fundamental difference between Multiple Spanning Tree (MST) and Common Spanning Tree Protocols (CSTP) like 802.1D [[Spanning Tree Protocol (STP)]] or 802.1w [[Rapid Spanning-Tree Protocol (RSTP)]] is that MST allows multiple VLANs to share a single spanning-tree instance. This reduces resource usage and allows support beyond the traditional 128-instance limit of PVST+.

Introduction to MST

An MST Instance (MSTI) represents one logical spanning-tree topology — including root election, port roles, path costs, etc.

You can map VLANs to MSTIs, so instead of running one STP instance per VLAN (as with PVST+), MST allows you to group VLANs and run fewer STP processes overall.

MST supports up to 66 MSTIs per region.

When MST is enabled (spanning-tree mode mst), all VLANs default to MSTI 0 (the IST) until explicitly assigned to another instance.

MST uses long path cost values by default.

BPDU Flow in MST

Although MST uses 802.1w (RSTP) [[Rapid Spanning-Tree Protocol (RSTP)]] internally, the BPDU format differs:

In traditional STP/PVST+, a BPDU is sent per VLAN out a trunk.
In MST, a single untagged BPDU is sent per interface, and it includes data for all MSTIs on that trunk.

Basic MST Configuration

spanning-tree mode mst

spanning-tree mst configuration
  name CCIE
  revision 1
  instance 10 vlan 1-5
  instance 20 vlan 6-10
exit

spanning-tree mst 10 priority 4094

spanning-tree mst hello-time 2
spanning-tree mst forward-time 15
spanning-tree mst max-age 20

interface Ethernet1/1
  spanning-tree mst 10 cost 100000

⚠️ MST configuration changes take effect only after exiting spanning-tree mst configuration mode.

Timers apply globally to all MST instances.

MST Regions and Attributes

Each MST region is defined by three region attributes:

Name
Revision number
VLAN-to-Instance mappings

These are hashed into a digest, which is carried in BPDUs. Devices with matching digests are considered in the same MST region.

MST regions enable hierarchical STP domains. Different MST or CST domains are treated as external to one another.

When a BPDU is received from a device in a different region, the port becomes a boundary port.

MST and STP Compatibility

MST + CST (802.1D or 802.1w)

CST (Common Spanning Tree) is the term for legacy STP modes with a single spanning-tree instance.

MST is backward-compatible with CST via MST Instance 0 (MST0), also called the IST (Internal Spanning Tree).

MST0 represents the whole region to external CST domains.
MST sends MST0 BPDUs out boundary ports to communicate as if it were a single CST switch.

This design lets MST plug into an RSTP/PVST+ domain without breaking topology.

MST0 = IST (Industry term), MST0 (Cisco term)

Master Ports in MST

If a superior BPDU is received on a boundary root port, that port becomes a Master Port. All other MSTIs are forced to forward on that port to maintain loop-free convergence.

MST vs PVST+/Rapid-PVST+

Since PVST+ runs one STP per VLAN, and MST uses shared topologies per instance, the topologies don’t align directly.

To ensure compatibility, MST uses a feature called PVST Simulation, which:

Sends one BPDU per VLAN with IST (MST0) information.
Can be enabled globally or per-interface.

spanning-tree mst simulate pvst global
interface Ethernet1/1
  spanning-tree mst simulate pvst

If PVST simulation is disabled and a BPDU is received on that interface, it will enter the STP-inconsistent (blocking) state.

PVST+ Compatibility Caveat

An MST boundary port will enter STP_Inconsistent if:

The boundary port is designated and receives a superior BPDU from PVST+.
The boundary port is root and receives an inferior BPDU compared to VLAN 1.

This commonly happens when VLAN root priorities differ slightly.

Best practice: ensure VLAN 1 has the lowest priority in PVST+ to avoid STP inconsistency on MST boundaries.

Correct PVST+ Interop Configuration

Ensure MST0 connects to a PVST+ domain where:

All PVST VLAN roots have higher priority values than VLAN 1.
MST0 ports receive superior BPDUs only from VLAN 1.

Example:

MST-IST# show running-config interface g0/0
interface GigabitEthernet0/0
 switchport trunk allowed vlan 1-5
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 100
 switchport mode trunk
 switchport nonegotiate
 negotiation auto

RSTP-1-5# show running-config interface g0/0
interface GigabitEthernet0/0
 switchport trunk allowed vlan 1-5
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 100
 switchport mode trunk
 switchport nonegotiate
 negotiation auto

MST-IST# show spanning-tree bridge
                                                   Hello  Max  Fwd
MST Instance                 Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
MST0             32768 (32768,   0) 5254.008d.40a5    2    20   15  mstp

RSTP-1-5# show spanning-tree bridge
                                                   Hello  Max  Fwd
VLAN                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN0001          4097 ( 4096,   1) 5254.00a8.57cd    2    20   15  rstp
VLAN0002             2 (    0,   2) 5254.00a8.57cd    2    20   15  rstp
VLAN0003             3 (    0,   3) 5254.00a8.57cd    2    20   15  rstp
VLAN0004             4 (    0,   4) 5254.00a8.57cd    2    20   15  rstp
VLAN0005             5 (    0,   5) 5254.00a8.57cd    2    20   15  rstp

CBT Nuggets - STP Compatibility Tutorial