Hello Aamir
When selecting which port to block on a switch, the following four parameters that are found within received BPDUs. Specifically, one BPDU is considered superior to another if it has:
- The lower root bridge ID
- The lower path cost to the Root
- The lower sending Bridge ID
- The lower sending port ID
These parameters are checked in order, and the next is only checked if the previous is a tie. So in this case, SW2 will receive BPDUs from the ROOT on both links. Both root bridge IDs are the same, both path costs are the same, and both sending Bridge IDs are the same. The last thing to check is the sending port ID.
Fa0/1 on SW2 receives a BPDU with the port ID of Fa0/1 from SW1. Similarly, Fa0/2 on SW2 receives a BPDU with the port ID of Fa0/2 from SW1. The port that receives a BPDU with the lowest port ID is the one chosen to forward traffic. Therefore Fa0/2 on SW2 becomes blocked. Does that make sense?
I hope this has been helpful!
Laz