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Figure 15 A schematic diagram of designated bridges and designated ports
Device A
Device B Device C
Port A1 Port A2
Port B1 Port C1
Port B2 Port C2
LAN
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.
How STP works
NOTE:
The spanning tree calculation process described in the following sections is a simplified process for
example only.
STP has the following workflow:
1. Initial state
Upon initialization of a device, each port generates a BPDU with the device as the root bridge, in which
the root path cost is 0, the designated bridge ID is the device ID, and the designated port is the port
itself.
2. Selection of the root bridge
Initially, each STP 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. Selection of the root port and designated ports
Table 8 Selection of the root port and designated ports
A non-root device regards the port on which it received the optimum configuration BPDU
as the root port. For the selection of the optimum configuration BPDUs, see Table 9.
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