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STP Configuration
144
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 device 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 21-2 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.
Note:
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 uccessfully 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 21-2. 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.