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MSTP Elements
MST Region (Multiple Spanning Tree Region): An MST Region comprises switches with the
same region configuration and VLAN-to-Instances mapping relationship.
IST (Internal Spanning Tree): An IST is a spanning tree in an MST.
CST (Common Spanning Tree): A CST is the spanning tree in a switched network that connects
all MST regions in the network.
CIST (Common and Internal Spanning Tree): A CIST, comprising IST and CST, is the spanning
tree in a switched network that connects all switches in the network.
The following figure shows the network diagram in MSTP.
Figure 7-2 Basic MSTP diagram
MSTP
MSTP divides a network into several MST regions. The CST is generated between these MST
regions, and multiple spanning trees can be generated in each MST region. Each spanning tree
is called an instance. As well as STP, MSTP uses BPDUs to generate spanning tree. The only
difference is that the BPDU for MSTP carries the MSTP configuration information on the
switches.
Port States
In an MSTP, ports can be in the following four states:
Forwarding: In this status the port can receive/forward data, receive/send BPDU packets
as well as learn MAC address.
Learning: In this status the port can receive/send BPDU packets and learn MAC address.
Blocking: In this status the port can only receive BPDU packets.
Disconnected: In this status the port is not participating in the STP.
Port Roles
In an MSTP, the following roles exist:
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