STP notes

1. Root bridge selection: Lowest bridge id [unique bridge id + priority.
2. Priority is considered first if tie happens then check MAC address]
3. Select a least cost path from each bridge to root bridge.
4. Any active port that is not a root port or a designated port is a blocked port.
5. Tie breaker for root port: If a bridge has 2 candidate ports for root position, then the port connecting to the lowest bridge id (neighbor) will be selected as root port
6. Tie breaker for designated port: If a bridge has 2 candidate ports for designated position, then the port connecting to the lowest bridge id (neighbor) will be selected as designated port; in case of tie, port connecting the bridge with lowest MAC address will be designated port.
7. Tie breaker if two bridges are connected with more than one cable: In this case port with lowest port priority will be opted.
8. STP uses multicast address 01:80:C2:00:00:00 for BPDU transmission.
9. BPDU – 8 bytes length [2 bytes priority, 6 bytes MAC]
10. BPDU exchange in every 2 seconds
    
    
    TCNs are injected into the network by a non-root switch and propagated to the root.
    Upon receipt of the TCN, the root switch will set a Topology Change flag in its normal BPDUs
11. Three types of BPDU

• Configuration BPDU [CBPDU]
• Topology change notification [TCN]
• Topology change notification acknowledgement [TCA]

.