1

I'm very confused by this spanning-tree output. Each subsequent vlan id has a Priority that is higher than the configured 16384. I think this has something to do with the extended system id, but I'm not sure how.

VLAN0001
  Spanning tree enabled protocol rstp
  ***Root ID    Priority    16385***
             Address     xxxx.xxxx.xxxx
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16385  (priority 16384 sys-id-ext 1)
             Address     xxxx.xxxx.xxxx
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

...

VLAN0005
  Spanning tree enabled protocol rstp
  ***Root ID    Priority    16389***
             Address     xxxx.xxxx.xxxx
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16389  (priority 16384 sys-id-ext 5)
             Address     xxxx.xxxx.xxxx
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec
...

VLAN0020
  Spanning tree enabled protocol rstp
  ***Root ID    Priority    16404***
             Address     xxxx.xxxx.xxxx
             This bridge is the root
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    16404  (priority 16384 sys-id-ext 20)
             Address     xxxx.xxxx.xxxx
             Hello Time  2  sec  Max Age 20 sec  Forward Delay 15 sec
4

if you set the priority for a VLAN to 0 on the switch, and enable sys-id extension it just adds the system extension number to the set priority. This is most likely why you are seeing the number higher.

F.e., You have the priority for VLAN20 set as 16384 but the sys-id-ext is 20. So its as simple as the set priority plus the sys-id-ext.

Btw, the sys-id-ext is equal to the VLAN identifier.

See also here: Spanning tree Default Priority Values

2
  • So the displayed priority is correct?
    – stets
    Feb 24 '17 at 16:18
  • Yes it is correct just going by the math. Is the Nexus supposed to be the root bridge?If so, you may want to lower the configured Priority. This is a suggestion.
    – HAL
    Feb 24 '17 at 21:33

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