Principle Of Mstp - H3C S5600 Series Operation Manual

Operation Manual – MSTP
H3C S5600 Series Ethernet Switches

1.2.3 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.
I. 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.
II. 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.
III. 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:
The smaller the root ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
For configuration BPDUs with the same root IDs, the path costs are compared.
Suppose S is the sum of the root path costs and the corresponding path cost of the
port. The less the S value is, the higher the priority of the configuration BPDU is.
For configuration BPDUs with both the same root ID and the same root path cost,
the designated bridge ID, designated port ID, the ID of the receiving port are
compared in turn.
3) A spanning tree is calculated as follows:
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Chapter 1 MSTP Configuration