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
The spanning tree calculation process described in the following sections is a simplified process for
STP has the following workflow:
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.
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.
Selection of the root port and designated ports
Table 8 Selection of the root port and designated ports
When the network topology is stable, only the root port and designated ports forward traffic, and other
ports are all in the blocked state in which the port receive BPDUs but do not forward BPDUs or user traffic.
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
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.
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 not send BPDUs
or forward data traffic.