H3C S5600 SERIES Operation Manual page 228

Selection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to be the root bridge,
with the root bridge ID being its own bridge ID. By exchanging configuration BPDUs, the devices
compare one another's root bridge ID. The device with the smallest root bridge ID is elected as the root
bridge.
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 1-3 Selection of the root port and designated ports
Step
1
2
3
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 1-2. 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.
A non-root-bridge device takes the port on which the optimum configuration
BPDU was received as the root port.
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.
The device compares the calculated configuration BPDU with the
configuration BPDU on the port whose role is to be determined, and acts as
follows based on the comparison result:
If the calculated configuration BPDU is superior, this port will serve 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 stops updating
the configuration BPDUs of the port and blocks the port, so that the port
only receives configuration BPDUs, but does not forward data or send
configuration BPDUs.
Description
1-5