What is MSTP and when should I use it instead of RSTP?

MSTP (Multiple Spanning Tree Protocol), defined in IEEE 802.1s, allows multiple spanning tree instances to run simultaneously, each covering different VLANs.

The Problem with STP/RSTP

Classic STP and RSTP run a single spanning tree for all VLANs, meaning only one topology is active. In multi-VLAN networks, this wastes redundant links.

MSTP Advantages

• Load balances traffic across redundant links per VLAN group
• Multiple MST instances (typically up to 64)
• Backward compatible with STP/RSTP
• Reduces convergence CPU load vs per-VLAN spanning tree

Key Concepts

• MST Region: Group of switches with same MSTP config
• MST Instance (MSTI): Individual spanning tree for VLAN group
• IST (Internal Spanning Tree): Default MSTI 0
• CIST (Common and Internal Spanning Tree): Interconnects regions

Design Example

In a network with 4 core switches and VLANs 10-40: Assign VLANs 10,20 to MSTI 1 (root at Switch A), VLANs 30,40 to MSTI 2 (root at Switch B). Both trunks are actively used.

When to Use MSTP

• Networks with 10+ VLANs and redundant links
• When traffic engineering across paths is needed
• Large enterprise or campus networks
• When migrating from per-VLAN STP

When RSTP Is Enough

• Small networks with few VLANs
• Simple hub-and-spoke topologies
• When ERPS/proprietary ring protocols are used instead