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 Description

A non-root-bridge device regards the port on which it received the optimum

1configuration BPDU as the root port. Table 54 describes how the optimum configuration BPDU is selected.

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