Three Elements
There are generally three elements used when a ring topology is to be trimmed into a tree topology: root bridge, root port, and designated port. Figure 1 shows the three elements.
Figure 1 STP network architecture
Root bridge
The root bridge is the bridge with the smallest BID. The smallest BID is discovered by exchanging configuration BPDUs.
Root port
The root port is the port with the smallest root path to the root bridge. The root port is determined based on the path cost. Among all STP-capable ports on a network bridge, the port with the smallest root path cost is the root port. There is only one root port on an STP-capable device, but there is no root port on the root bridge.
Designated port
For description of the designated bridge and designated port, see Table 1.
| Table 1 Description of the designated bridge and designated port | ||
| Object | Designated Bridge | Designated Port |
| Device | Device that forwards configuration BPDUs to a directly connected device | Designated bridge port that forwards configuration BPDUs to a device |
| LAN | Device that forwards configuration BPDUs to a network segment | Designated bridge port that forwards configuration BPDUs to a network segment. |
As shown in Figure 2, AP1 and AP2 reside on S1; BP1 and BP2 reside on S2; CP1 and CP2 reside on S3.
S1 sends configuration BPDUs to S2 through AP1. S1 is the designated bridge of S2, and AP1 on S1 is the designated port.
Two devices, S2 and S3, are connected to the LAN. If S2 is responsible for forwarding configuration BPDUs to the LAN, S2 is the designated bridge of the LAN and BP2 on S2 is the designated port.
Figure 2 Networking diagram of the designated bridge and designated port
After the root bridge, root port, and designated port are selected successfully, the entire tree topology is set up. When the topology is stable, only the root port and the designated port forward traffic. All the other ports are in the Blocking state and receive only STP protocol packets instead of forwarding user traffic.
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