H3C Technologies H3C S10500 Series Switches - Manual (Page 73)

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Calculation process of the STP algorithm
NOTE:
The spanning tree calculation process described in the following sections is a simplified process for
example only.
The STP algorithm uses the following calculation process:
1. Initial state
Upon initialization of a device, each port generates a BPDU with the port as the designated port, the
device as the root bridge, 0 as the root path cost, and the device ID as the designated bridge ID.
2. Root bridge selection
Initially, each STP-enabled device on the network assumes itself to be the root bridge, with its own device
ID as the root bridge ID. By exchanging configuration BPDUs, the devices compare their root bridge IDs
to elect the device with the smallest root bridge ID as the root bridge.
3. Non-root bridge: Selection of root port and designated ports
Table 9 Selection of the root port and designated ports
Step Description
1
A non-root-bridge device regards the port on which it received the optimum configuration BPDU
as the root port.
Table 10 describes how the optimum configuration BPDU is selected.
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 other 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 of 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
whose port role will be defined, and acts depending on the result of the comparison.
• If the calculated configuration BPDU is superior, the device considers this port as the
designated port, replaces the configuration BPDU on the port with the calculated configuration
BPDU, and periodically sends the calculated configuration BPDU.
• If the configuration BPDU on the port is superior, the device blocks this port without updating its
configuration BPDU. The blocked port can receive BPDUs, but cannot send BPDUs or forward
data traffic.
NOTE:
When the network topology is stable, only the root port and designated ports forward user traffic, while
other ports are all in the blocked state to receive BPDUs but not forward BPDUs or user traffic.