MSTP divides a bridged network into multiple regions called MST regions. Within each region, multiple RSTP instances called MSTIs are enabled to map VLANs and build separate spanning trees. A representative bridge in each region communicates with representatives of other regions to form a common spanning tree (CST) that connects all the MST regions as a single spanning tree across the entire network.

1. MSTP High Level Overview
Multiple Spanning Tree Protocol
Gary Jan

2. MSTP Introduction
• Originally defined in 802.1s and later merged into 802.1q.
• Divides a bridged network into multiple bridged networks.
• Each divided bridged networks is autonomous to one another.
• Inside a divided bridged-network
• More than one RSTP instances are enabled
• Each RSTP instance is mapped for one or more VLAN(s).
• Each MSTP divided bridged-network is perceived as a single Bridge to other
divided bridged-network.
• A representative is elected inside a MSTP divided network
• Representatives from divided networks exchange information
• Non-representative devices are unaware of the outside world.

3. MSTP Regions
•MSTP divides a bridged network into multiple regions.
•In MSTP jargon, a divided region is called a MST Region
MST 1
MST 2
MST 3
MST 4

4. IST VLAN 1
MSTP Region 1/2
MST 2
X
X
MST 2
• MSTP enables multiple RSTs inside a region
• Each RST in a region is called a “instance”
• The RST that connects all devices is called “IST” – Internal Spanning Tree
• IST is a ST running RSTP across all devices in the region
• VLAN 1 is mapped to IST by default

5. X
X
MST 2
MST 2
IST VLAN 1
MSTI 3 VLAN 2, 3, 4
MSTI 4 VLAN 5, 6
MSTI 5 VLAN 10, 11, 12
MSTI 6 VLAN 13
MSTP Region 2/2
• Multiple RST can be built beside IST
• These RSTs are called MSTIs (Multiple Spanning Tree Instance)
• Runs STP/RSTP
• One or multiple VLANs can be associated with each MSTI
• A VLAN # can only be associated with one MSTI in a region

6. Regional Interactions
MST 1
MST 2
MST 3
MST 4
X X
X X
X
X
X
• In MSTP jargon, regional representatives are called “CIST Regional Root”
• Regional representatives form a network wide ST called Common Spanning Tree (CST)
• CST is a RSTP instance connecting regions
• Each region is perceived as a switch to other regions
• In MSTP jargon, the root of CST is called “CIST Root”
• CST and ISTs form a single spanning tree across the network, called CIST- “Common and Internal
Spanning Tree”
CST
CIST Regional Root
CIST Root
CIST
X