Spanning tree port priority increments of 64, why not 16?

Looking at the port priority field in a BPDU, 4 bits are assigned to the port priority. So 128, 64, 32 and 16, suggesting to me that 16 would be the smallest increment that could be used for port priority. So why on Cisco switches (that I have configured) does it say "% Port Priority in increments of 64 is required"?

Answers

  • Looking at it more, I can see that the only options on the cisco switch (which allows a range of 0 - 192) would be 0, 64, 128, 192. This would suggest that it is only using the first 2 of the 4 bits assigned to the priority. So why? Is it using the last 2 bits for something else?

  • After lots of research, it looks like on different switches you have different increments. But essentially the first 4 bits would make increments of 16. If anyone has any information on why this increment changes on some kit or maybe it is STP type, please reply as I would be very interested to hear?

  • I guess this heavily depends on the platform you are using.

    To my knowledge the port prio is a subfield of the port ID field which is 16 bits. So depending on how many bits you take to identify the port and how many you take to identify the priority of the port, you will get different values here.

    i tested this in a 3850 with 16.12.4 IOS-XE. Accoring to the config guide I should be able to use increments of 4, however my switch only allows increments of 16:

    lab-sw05(config-if)#spanning-tree port-priority ?
      <0-240>  port priority in increments of 16
    
    lab-sw05(config-if)#spanning-tree port-priority 8
    % Port Priority in increments of 16 is required
    
    lab-sw05(config-if)#
    

    So in this example I should have 4 bits to use for port priority, with 16 being the value of the lowest bit (the rest is used for the port identifier elsewhere). This allows me to express a maximum value for 240 with those 4 bits available.

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