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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?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. – Ron Maupin Aug 7 '17 at 16:03
3

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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1

As Ron Trunk pointed out, the switch priority is a 16-bit value made up of two parts: the bridge priority is the high-order four bits, and the VLAN number is the low-order 12 bits.

This Cisco document has a diagram demonstrating this:

Bridge ID, Switch Priority, and Extended System ID

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which controls the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+ and rapid PVST+, the same switch must have a different bridge IDs for each configured VLAN. Each VLAN on the switch has a unique 8-byte bridge ID. The 2 most-significant bytes are used for the switch priority, and the remaining 6 bytes are derived from the switch MAC address.

The switch supports the IEEE 802.1t spanning-tree extensions, and some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 17-1, the 2 bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.

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