3Com WX3000 Principle of Mstp, Calculate the Cist, Calculate an Msti, Implement STP algorithm

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Forwarding state. Ports in this state can forward user packets and receive/send BPDU packets. Learning state. Ports in this state can receive/send BPDU packets.

Discarding state. Ports in this state can only receive BPDU packets.

Port roles and port states are not mutually dependent. Table 1-6 lists possible combinations of port states and port roles.

Table 1-6Combinations of port states and port roles

Port role	Root/
(right)		Designated	Region edge		Alternate
	port/Master					Backup port
Port state		port	port		port
	port
(below)
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 of the devices.

Calculate the CIST

Through comparing configuration BPDUs, the device with 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 device 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 device 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 device, and the designated port being itself.

1)Each device sends out its configuration BPDUs and operates in the following way when receiving a

configuration BPDU on one of its ports from another device:

zIf the priority of the configuration BPDU is lower than that of the configuration BPDU of the port itself, the device discards the BPDU and does not change the configuration BPDU of the port.

zIf the priority of the configuration BPDU is higher than that of the configuration BPDU of the port itself, the device replaces the configuration BPDU of the port with the received one and compares it

with those of other ports on the device to obtain the one with the highest priority.

2)Configuration BPDUs are compared as follows:

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