H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 163)

20-5
compare one another’s root bridge ID. The device with the smallest root bridge ID is elected as the root
bridge.
z
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 20-3 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.
z
The root bridge ID is replaced with that of the configuration BPDU of the root port.
z
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.
z The designated bridge ID is replaced with the ID of this device.
z
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:
z 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.
z 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.
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 20-2 . 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.