Basic concepts in STP

Root bridge

A tree network must have a root bridge. The entire network contains only one root bridge, and all the other bridges in the network are called leaf nodes. The root bridge is not permanent, but can change with changes of the network topology.

Upon initialization of a network, each device generates and periodically sends configuration BPDUs, with itself as the root bridge. After network convergence, only the root bridge generates and periodically sends configuration BPDUs. The other devices only forward the BPDUs.

Root port

On a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates with the root bridge. Each non-root bridge has only one root port. The root bridge has no root port.

Designated bridge and designated port

Classification

Designated bridge

Designated port

For a device

Device directly connected with the local device and responsible for forwarding BPDUs to the local device

Port through which the designated bridge forwards BPDUs to this device

For a LAN

Device responsible for forwarding BPDUs to this LAN segment

Port through which the designated bridge forwards BPDUs to this LAN segment

As shown in Figure 20, Device B and Device C are directly connected to a LAN.

If Device A forwards BPDUs to Device B through port A1, the designated bridge and designated port are as follows:

If Device B forwards BPDUs to the LAN, the designated bridge and designated port are as follows:

Figure 20: Designated bridges and designated ports

Path cost

Path cost is a reference value used for link selection in STP. To prune the network into a loop-free tree, STP calculates path costs to select the most robust links and block redundant links that are less robust.