Simplified calculation overview

A tree-shape topology forms once the root bridge, root ports, and designated ports are selected.

1. Upon initialization of a device, each port generates a BPDU with the following contents:

   • The port as the designated port.

   • The device as the root bridge.

   • 0 as the root path cost.

   • The device ID as the designated bridge ID.

2. 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.
   • A non-root–bridge device regards the port on which it received the optimum configuration BPDU as the root port.

     • Upon receiving a configuration BPDU on a port, the device compares the priority of the received configuration BPDU with that of the configuration BPDU generated by the port. If the former priority is lower, the device discards the received configuration BPDU and keeps the configuration BPDU the port generated. If the former priority is higher, the device replaces the content of the configuration BPDU generated by the port with the content of the received configuration BPDU.

     • The device compares the configuration BPDUs of all the ports and chooses the optimum configuration BPDU.

   • 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 determined, and acts depending on the result of the comparison:

     • If the calculated configuration BPDU is superior, the device performs the following tasks:

       • Considers this port as the designated port.

       • Replaces the configuration BPDU on the port with the calculated configuration BPDU.

       • 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 cannot send BPDUs or forward data traffic.

   When the network topology is stable, only the root port and designated ports forward user traffic. Other ports are all in the blocked state to receive BPDUs but not to forward BPDUs or user traffic.

4. The principles of configuration BPDU comparison:

   • The configuration BPDU with the lowest root bridge ID has the highest priority.

   • If configuration BPDUs have the same root bridge ID, their root path costs are compared. For example, the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The configuration BPDU with the smallest S value has the highest priority.

   • If all configuration BPDUs have the same root bridge ID and S value, the following attributes are compared in sequence: Designated bridge IDs, Designated port IDs, and IDs of the receiving ports.

   The configuration BPDU that contains a smaller designated bridge ID, designated port ID, or receiving port ID is selected.