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STP Tiebreaker
iworms
Member Posts: 53 ■■□□□□□□□□
in CCNA & CCENT
I've just cleared up an STP confusion, which I'm sure some people will have as well. When multiple equal-bandwidth cables connect two switches and all ports have the same priority, which port becomes the root port? Many places I've read simply say the tiebreaker is the port ID, or that the port with a lower/lowest ID becomes the root port. One book even shows this figure:
which led me to think the lowest port on Switch B is always the root port. But actually it is a Switch B port, which connects to Switch A's lowest port, that becomes the root port. Changing the diagram this way makes it more accurate:
The thing to remember is that all decision/tie-breaking information come from the received BPDUs. Switch B does not look at its own ports to decide. The confusing part is that physically this is determined at Switch A (i.e. which cable gets plugged into the lowest port), while it is Switch B that uses this information to decide which ports to block.
which led me to think the lowest port on Switch B is always the root port. But actually it is a Switch B port, which connects to Switch A's lowest port, that becomes the root port. Changing the diagram this way makes it more accurate:
The thing to remember is that all decision/tie-breaking information come from the received BPDUs. Switch B does not look at its own ports to decide. The confusing part is that physically this is determined at Switch A (i.e. which cable gets plugged into the lowest port), while it is Switch B that uses this information to decide which ports to block.