STP Root Port and Designated Port

[Tyler Jackson]

I understands the 3 step STP election process:

1) Elect root bridge,

2) Elect root port on non-root bridge,

3) Elect designated port.

However, my question revolves around the tie-breaker process regarding the Root Port and Designated Port.

Specifically, the order and if the tie-breaker is the same for both.

Please assist in drawing clarity as to the this because various text has made multiple arguments.  For example:

Root Port [Decision/Tie-Breaker]-

1) Cost [Decision]

2) Lowest Sending BID (Bridge priority + MAC address) [TB]

3) Lowest Sending PID (Port priority + a Port number) [TB]

4) Lowest Sending Port Interface Number [TB]

Designated Port [Decision/Tie-Breaker]-

1) Lowest root path cost to root bridge [Decision]

2) Lowest root BID [TB]

3) Lowest sender BID [TB]

4) Lowest sender PID [TB]

[piyushhote · Accepted solution]

Actually, the complete sequence as per 802.1D/Q is:

1. Lowest root bridge ID
2. Lowest total root path cost
3. Lowest sender bridge ID
4. Lowest sender port ID
5. Lowest receiver port ID

The rule #5 is usually not mentioned because it is only seldom invoked - if a switch is connected via several uplinks to a common segment (hub/unmanaged switch) and has to select its root port from among these links. The rules #1-#4 would not break the tie here.

Regards,

Piyush

[Tyler Jackson]

Thanks for the clarity.  This helps.