Operation Manual - STP
Quidway S3500 Series Ethernet Switches Chapter 1 MSTP Region-configuration
Huawei Technologies Proprietary
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1.1.2 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.
II. MSTI calculation
Inside an MST region, MSTP generates different MSTIs for different VLANs according
to the association between VLAN and the spanning tree. The calculation process of
MSTI is same like RSTP.
In this way, the packets of a VLAN travel along the corresponding MSTI inside the MST
region and the CST between different regions.
Followed introduce the calculation process of one MSTI.
The fundamental of STP is that the switches exchange a special kind of protocol packet
(which is called configuration Bridge Protocol Data Units, or BPDU, in IEEE 802.1D) to
decide the topology of the network. The configuration BPDU contains the information
enough to ensure the switches to compute the spanning tree.
The configuration BPDU mainly contains the following information:
z The root ID consisting of root priority and MAC address
z The cost of the shortest path to the root
z Designated switch ID consisting of designated switch priority and MAC address
z Designated port ID consisting of port priority and port number
z The age of the configuration BPDU: MessageAge
z The maximum age of the configuration BPDU: MaxAge
z Configuration BPDU interval: HelloTime
z Forward delay of the port: ForwardDelay.
What are the designated switch and designated port?
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