3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

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Determining the root port

For each switch in a network, the port on which the configuration BPDU with the 
highest priority is received is chosen as the root port of the switch.

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Determining the designated port

First, the switch calculates a designated port configuration BPDU for each of its 
ports using the root port configuration BPDU and the root port path cost, with the 
root ID being replaced with that of the root port configuration BPDU, root path 
cost being replaced with the sum of the root path cost of the root port 
configuration BPDU and the path cost of the root port, the ID of the designated 
bridge being replaced with that of the switch, and the ID of the designated port 
being replaced with that of the port.

The switch then compares the calculated configuration BPDU with the original 
configuration BPDU received from the corresponding port on another switch. If 
the latter takes precedence over the former, the switch blocks the local port and 
keeps the port’s configuration BPDU unchanged, so that the port can only receive 
configuration messages and cannot forward packets. Otherwise, the switch sets 
the local port to the designated port, replaces the original configuration BPDU of 
the port with the calculated one and advertises it regularly.

MSTP is compatible with both STP and RSTP. That is, MSTP-enabled switches can 
recognize the protocol packets of STP and RSTP and use them for spanning tree 
calculation. In addition to the basic MSTP functions, 3Com series switches also 
provide the following functions for users to manage their switches.

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Root bridge hold

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Root bridge backup

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Root guard

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BPDU guard

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Loop guard

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TC-BPDU attack guard

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BPDU packet drop

STP-related standards include the following.

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IEEE 802.1D: spanning tree protocol

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IEEE 802.1w: rapid spanning tree protocol

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IEEE 802.1s: multiple spanning tree protocol

Table 101 lists the tasks to configure a root bridge.