Principle Of Mstp - 3Com 4200G 12-Port Configuration Manual

Table 1-6 Combinations of port states and port roles
Port role
Port state
Forwarding
Learning
Discarding

Principle of MSTP

MSTP divides a Layer 2 network into multiple MST regions. The CSTs are generated between these
MST regions, and multiple spanning trees (also called MSTIs) can be generated in each MST region. As
well as RSTP, MSTP uses configuration BPDUs for spanning tree calculation. The only difference is that
the configuration BPDUs for MSTP carry the MSTP configuration information on the switches.
Calculate the CIST
Through comparing configuration BPDUs, the switch of the highest priority in the network is selected as
the root of the CIST. In each MST region, an IST is calculated by MSTP. At the same time, MSTP
regards each MST region as a switch to calculate the CSTs of the network. The CSTs, together with the
ISTs, form the CIST of the network.
Calculate an MSTI
In an MST region, different MSTIs are generated for different VLANs based on the VLAN-to-MSTI
mappings. Each spanning tree is calculated independently, in the same way as how STP/RSTP is
calculated.
Implement STP algorithm
In the beginning, each switch regards itself as the root, and generates a configuration BPDU for each
port on it as a root, with the root path cost being 0, the ID of the designated bridge being that of the
switch, and the designated port being itself.
1) Each switch sends out its configuration BPDUs and operates in the following way when receiving a
configuration BPDU on one of its ports from another switch:
If the priority of the configuration BPDU is lower than that of the configuration BPDU of the port
itself, the switch discards the BPDU and does not change the configuration BPDU of the port.
If the priority of the configuration BPDU is higher than that of the configuration BPDU of the port
itself, the switch replaces the configuration BPDU of the port with the received one and compares it
with those of other ports on the switch to obtain the one with the highest priority.
2) Configuration BPDUs are compared as follows:
For MSTP, CIST configuration information is generally expressed as follows:
(Root bridge ID, External path cost,Master bridge ID, Internal path cost, Designated bridge ID,ID of
sending port,ID of receiving port),so the compared as follows
The smaller the Root bridge ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
For configuration BPDUs with the same Root bridge IDs, the External path costs are compared.
Root/master Designated
port
√ √
√ √
√ √
Region
Boundary
port
port
√
√
√
1-13
Alternate Backup
port port
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