MST does is allow us to map VLANs to a specific instance. So, if we only need 2 STP topologies, we can map half of our VLANs to one instance and the other half to the other instance.
This means that before we configure anything in MST, we need to first design the number of instances and which VLANs to assign to each instance.
MST Regions
An MST region is a group of switches with the same configuration. In most network deployments you would just need a single region, however it’s possible to configure a network to use more than one.
All switches in a single MST region share the following:
MSTP-enabled
Same region name
Same VLAN-MSTI mappings
Same MSTP revision level
Multiple switches can be grouped into an MST region by using MSTP configuration commands. Each MST region has a VLAN mapping table. The VLAN mapping table maps VLANs to MSTIs.
CST
A Common Spanning Tree (CST) connects all MST regions on a switching network. The CST is calculated using STP or RSTP, with each MST region being considered as a single node.
IST
An Internal Spanning Tree (IST) resides within an MST region.
An IST is a special MSTI with an MSTI ID of 0.
An IST is a segment of the CIST in an MST region.
SST
A Single Spanning Tree (SST) is formed in either of the following situations:
A switch running STP or RSTP belongs to only one spanning tree.
An MST region has only one switch.
CIST
A Common and Internal Spanning Tree (CIST) connects all the switches on a switching network and is calculated using STP or RSTP.
Regional Root
Regional roots are classified into Internal Spanning Tree (IST) and MSTI regional roots.
Master Bridge
The master bridge is the switch closest to the CIST root in a region.
Port Roles
MSTP adds two extra port roles to those defined in RSTP. As shown in the below table:
| Port Role | Description |
| Root port | A root port sends data to a root bridge and is the port closest to the root bridge. Root bridges do not have root ports . Root ports are responsible for sending data to root bridges. |
| Designated port | The designated port on a switch forwards BPDUs to a downstream switch. |
| Alternate port | Alternate ports provide an alternate path to the root bridge. This path is different from the path through the root port. |
| Backup port | Backup ports provide a backup path to a segment already connected by a designated port . Backup ports are blocked from sending BPDUs after a BPDU sent by itself is received. |
| Master port | A master port is on the shortest path connecting MST regions to the CIST root . BPDUs of an MST region are sent to the CIST root through the master port. |
| Regional edge port | A regional edge port is located at the edge of an MST region and connects to another MST region or an SST. |
| Edge port | An edge port is located at the edge of an MST region and does not connect to any switching device . Generally, edge ports are directly connected to terminals. |
MSTP Port States
MSTP port states, which are the same as those used in RSTP.
Description | |
Forwarding | A port in this state can send and receive BPDUs. It can also forward user traffic. |
Learning | A port in this state learns MAC addresses from user traffic to construct a MAC address table. In Learning state, the port can send and receive BPDUs, but cannot forward user traffic. |
Discarding | A port in this state can only receive BPDUs. |
NOTE:
Root, master, designated, and regional edge ports support all three port states. Alternate and backup ports support only the Discarding state.
