3Com MSR 50 Series Configuration Manual page 447

Table 12 Selection of the root port and designated ports
Step Description
1 A non-root-bridge device regards the port through which it received the
optimum configuration BPDU as the root port.
2 Based on the configuration BPDU and the path cost of the root port, the device
calculates a designated port configuration BPDU for each of the rest ports.
■
■
■
■
3 The device compares the calculated configuration BPDU with the configuration
BPDU on the port of which the port role is to be determined, and proceeds the
following according to the comparison result:
■
■
n
When the network topology is stable, only the root port and designated ports
forward traffic, while other ports are all in the blocked state - they only receive STP
packets but do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated ports
have been successfully elected, the entire tree-shaped topology has been
constructed.
The following is an example of how the STP algorithm works. The specific network
diagram is shown in
priority of Device B is 1, the priority of Device C is 2, and the path costs of these
links are 5, 10 and 4 respectively.
Figure 128 Network diagram for STP algorithm
AP 1
5
BP 1
BP 2
Device B
With priority 1
The root bridge ID is replaced with that of the configuration BPDU of the
root port.
The root path cost is replaced with that of the configuration BPDU of the
root port plus the path cost corresponding to the root port.
The designated bridge ID is replaced with the ID of this device.
The designated port ID is replaced with the ID of this port.
If the calculated configuration BPDU is superior, the device will consider this
port as the designated port, and the configuration BPDU on the port will be
replaced with the calculated configuration BPDU, which will be sent out
periodically.
If the configuration BPDU on the port is superior, the device will block this
port without updating its configuration BPDU, so that the port will only
receive BPDUs, but not send any, and will not forward data.
Figure 128. In the feature, the priority of Device A is 0, the
Device A
With priority 0
AP 2
10
4
CP 2
MSTP Overview
CP 1
Device C
With priority 2
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