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:
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.