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 to 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 28, 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:
The designated bridge for Device B is Device A.
The designated port for Device B is port A1 on Device A.
If Device B forwards BPDUs to the LAN, the designated bridge and designated port are as follows:
The designated bridge for the LAN is Device B.
The designated port for the LAN is port B2 on Device B.
Figure 28: Designated bridges and designated ports
Port states
Table 6 lists the port states in STP.
Table 6: STP port states
State | Receives/sends BPDUs | Learns MAC addresses | Forwards user data |
|---|---|---|---|
Disabled | No | No | No |
Listening | Yes | No | No |
Learning | Yes | Yes | No |
Forwarding | Yes | Yes | Yes |
Blocking | Receive | No | No |
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.