BPDU hello messages are generated every 2 seconds by switches. BPDUs elect the root, elect one switch per segment to handle forwarding functions, and remove loops by placing ports connected to a redundant path in a blocking state.
The switch with the lowest bridge ID (priority + MAC address) is elected as the root.
Choosing a root port: Choose the path with the lowest accumulated path cost to the root switch. If there is a tie in path cost, choose the neighboring switch bridge ID with the lowest priority. If the bridge ID is the same, choose the port with the lowest priority. If there are still multiple paths and they go through the same neighboring switch, choose the physically lowest numbered port.
Each segment has a designated port that switches traffic to and from the segment. The switch with the best accumulated path cost becomes the designated switch and one port on it is chosen for the segment. If there is a tie, the switch with the lowest bridge ID is chosen.
A blocking port listens only for BPDUs from other switches. In a listening state, a port only listens for frames to detect available paths to the root switch. In a learning state, a port listens for frames and builds the CAM table. After the learning state, a port is placed into a forwarding state, where the bridge performs its normal functioning. A port in the disabled state has either been disabled by the switch because of physical problems or security violations, or has been manually disabled.
STP can take from 30 50 seconds to converge: blocking (20), listening (15), and learning (15). A port will start in a blocking state if a BPDU is received on a nonroot port with a better cost to the root; otherwise, it starts in a listening state.
CST is used on 802.1Q trunks connected to non-Cisco switches. PVST is used between two Cisco switches on ISL and 802.1Q trunks.
