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 ```none 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**. ```none 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: ```none 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 ``` ```none 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](https://learn.adept.at/cbtnuggets/layer-2-ccie-training-vlan-etherchannel-stp-tutorial/skill/configure-stp-compatibility#component-9d7c5)