Switch – Spanning Tree Default Priority Values

spanning treeswitch

This question is a repost of the same question in The Cisco Learning Network; however, the answers are unique to Stack Exchange.

I have always accepted that the defualt priority value of STP (802.1d) is 32768 (32768 + sys-id-ext x) as show in "show spanning-tree" below –

SW3#show spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     00D0.D3E6.9838
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     00D0.BCB5.B556
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Desg FWD 19        128.8    P2p
Fa0/1            Root FWD 19        128.1    P2p
Fa0/24           Altn BLK 19        128.24   P2p

SW3#

Now, when I want to configure priority I don't undertand WHY the priority must be in increments of 4096 ?
Is there a specific reason for the default value of both of these numbers?

SW3(config)#spanning-tree vlan 1 priority ?
  <0-61440>  bridge priority in increments of 4096
SW3(config)#spanning-tree vlan 1 priority

Thanks in advance.

Best Answer

In the spanning tree algorithm the process of determining the root bridge is based on the bridge priority (BID).

When there were no VLANs (meaning that switches had to deal with only one broadcast domain) the BID was equal to:

Bridge priority - MAC Address
   <2 bytes>       <6 bytes>

As Network administrators we can modify the bridge priority value.

When VLANs were introduced the needing of differentiating the STP protocol raised (Per VLAN Spanning Tree - PVST).

To accomplish this the bridge priority value was modified as follows:

Bridge priority - Exstended system ID (VLAN ID)
     4 bit      -           12 bit

We are still able to modify the bridge priority but this means to operate to the 4th most significative bit over 16 bits.

To put it simple:

Legend : C= changeable ; U= unchangeable

CCCCUUUUUUUUUUUU 16 bits

0001UUUUUUUUUUUU 1*2^12 = 4096

0111UUUUUUUUUUUU Default: 32768

0111000000000001 Default for VLAN 1: 32769

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