Cisco – Set priority of bridge to 0 but root bridge shows now priority 1 in STP

ciscopacket-tracerspanning treeswitch

I have configured a network in Packet Tracer with 5 switches and connected them all together. Then, I configured a priority of 0 and wanted to see how the election of the root bridge takes effect.

Switch(config)#spanning-tree vlan 1 priority 0
Switch#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    1
             Address     0060.47A2.34E8
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    1  (priority 0 sys-id-ext 1)
             Address     0060.47A2.34E8
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg LRN 19        128.1    P2p
Fa0/3            Desg FWD 19        128.3    P2p
Fa0/4            Desg FWD 19        128.4    P2p
Fa0/2            Desg FWD 19        128.2    P2p

Why do I have a priority 1 and not 0?

Best Answer

Your clue is in the listing you posted:

 Bridge ID  Priority    1  (priority 0 sys-id-ext 1)

The priority is made up of two parameters: the priority value you set (0) and the extended ID (sys-ext-id) The sys-ext-id identifies the specific instance of STP on the switch (Cisco by default runs a separate instance for each VLAN). In this case you're using VLAN 1, so that is the sys-ext-id. Combining the two (0 and 1) gives you 1 as the bridge priority.

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