H3C Technologies H3C S12500 Series Switches - Manual (Page 69)

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Path cost
Path cost is a reference value used for link selection in STP. STP calculates path costs to select the most
robust links and blocks redundant links that are less robust, to prune the network into a loop-free tree.
Calculation process of the STP algorithm
The STP algorithm uses the following calculation process:
1. Initial state
Upon initialization of a device, each port generates a BPDU with itself 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 6 describes the process of selecting the root port and designated ports.
Table 6 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 7 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 its 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 is to be determined:
• 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 out 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.