MSTP Overview 201
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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 Figure 54. In the feature, the priority of Device A is 0, the
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 54 Network diagram for the STP algorithm
■ Initial state of each device
The following table shows the initial state of each device.
Table 39 Selection of the root port and designated ports
Step Description
1 A non-root-ridge device regards the port on 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.
■ 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.
3 The device compares the calculated configuration BPDU with the configuration BPDU
on the port of which the port role is to be defined, and does different things according
to the comparison result:
■ 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.
Device A
With priority 0
Device B
With priority 1
Device C
With priority 2
AP 1
AP 2
BP 1
BP 2
CP 1
CP 2
5
10
4
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