Calculation Process Of The Stp Algorithm - H3C S5120-SI series Configuration Manual

Figure 12 Schematic diagram of designated bridges and designated ports
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 block redundant links that are less robust, to prune the network into a loop-free tree.

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 device 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.
Root bridge selection
2.
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 8 Selection of the root port and designated ports
Step
1
Description
A non-root–bridge device regards the port on which it received the optimum configuration BPDU
as the root port. Table 9 describes how the optimum configuration BPDU is selected.
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