Calculation Process Of The Stp Algorithm - HP 1920 Gigabit Ethernet Switch Series User Manual

Figure 173 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.
All the ports on the root bridge are designated ports.

Calculation process of the STP algorithm

The spanning tree calculation process described in the following sections is a simplified process for
example only.
Calculation process
The STP algorithm uses the following calculation process:
1. Network initialization.
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. Root port and designated ports selection on the non-root bridges.
Step
1
Description
A non-root-bridge device regards the port on which it received the optimum
configuration BPDU as the root port.
BPDU is selected.
179
Table 54 describes how the optimum configuration