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When you want to create a logically loop-free topology, the spanning tree always uses the following four comparison principles:
- Minimum root bridge bid
- Minimum root path overhead
- Minimum sender bid
- Minimum Port ID
Each bridge transmits spanning tree information through a Network Bridge Protocol data Unit (BPDU), each of which computes an optimal BPDU on each port by using the above four comparison principles, and all BPDUs received and sent on the port are included in the calculation. When a port receives BPDUs, the BPDU calculates the above four values to see if the computed value is better than the already saved BPDUs (that is, if the value is smaller), replace the previous value. Note: Configuration BPDUs are sent when the bridge receives better BPDUs.
In addition, it also controls the transmission of BPDUs while "saving optimal BPDUs". When the bridge is first activated, all its ports start to send BPDUs every 2 seconds (if the default timer configuration is used), but if a port receives a BPDU from another bridge and the BPDU is better than the sent BPDU, the local port stops sending BPDUs. If a better BPDU is not sent from an adjacent bridge within a period of time (the default of 20 seconds), the local port resumes sending BPDUs. Note: There are two types of BPDUs here: Configuring BPDUs and Topology Change notification (TCN) BPDUs, half of which is only about configuring BPDUs, and the other half discusses the differences between TCN BPDUs and the two.
The classic "Cisco Lan Switching" chapter sixth (iv): Four-step STP decision Sequence
