Mstp Principles - Huawei Quidway S3000 Series Operation Manual

Operation Manual - STP
Quidway S3000 Series Ethernet Switches
Master port is the port connecting the entire region to the Common Root Bridge
and located on the shortest path between them.
Alternate port is the backup of the master port. When the master port is blocked,
the alternate port will take its place.
If two ports of a switch are connected, there must be a loop. In this case, the switch
will block one of them. The blocked one is called BACKUP port.
A port can play different roles in different spanning tree instances.
The following figure illustrates the above mentioned concepts for your better
understanding.
MST region
Designated
port
Figure2-2 Port roles

2.1.2 MSTP Principles

MSTP divides the entire Layer 2 network into several MST regions and calculates and
generates CST for them. Multiple spanning trees are generated in a region and each of
them is called an MSTI. The instance 0 is called IST, and others are called MSTI.
I. CIST calculation
The CIST root is the highest-priority switch elected from the switches on the entire
network through comparing their configuration BPDUs. MSTP calculates and
generates IST in an MST region and also the CST connecting the regions. CIST is the
unique single spanning tree of the entire switching network.
Connected to the
common root
EdgePort
Port 2
Port 1
Alternate port
port
Master
A
B
Port 5
D
Port 3
Port 4
C
Port 6
port
Backup
2-4
Chapter 1 RSTP Configuration