Spanning Tree Bridge Root Priority value & Extended System ID

1. Spanning Tree Bridge Root
Priority value
&
Extended System ID
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2.  Every Bridge (Switch) Participating in a Spanning Tree
Protocol network is assigned with a numerical value
called Bridge Priority (Switch Priority) Value.
 By default, all Cisco Switches has a Bridge Priority
(Switch Priority) value of 32,768. Bridge Priority (Switch
Priority) value decides which Switch can become Root
Bridge (Root Switch).
 You can lower the the Switch Priority value in a
Spanning Tree Protocol switch, so that we can make
that switch elected as the Root Switch.
 To change the Bridge Priority (Switch Priority) Value, to
a particular value, use the following command
from Global Configuration mode.

3.  When you change the Bridge Priority (Switch Priority)
Value, make sure that you are decrementing or
incrementing it by 4096. If you try to decrement or
increment the Bridge Priority (Switch Priority) Value by
any value other than 4096, you will get an error
message similar to below output.
SW1>enable
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#spanning-tree vlan 170 priority 32767
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0 4096 8192 12288 16384 20480 24576
28672
32768 36864 40960 45056 49152 53248 57344
61440
SW1(config)#exit

4.  This is because, by default, Cisco Switches are running
a mode of Spanning Tree Protocol, known as Per-VLAN
Spanning Tree Protocol + (PVST+).
 PVST+ is based on the IEEE 802.1D standard, added
with Cisco proprietary extensions.
 The PVST+ runs on each VLAN on the switch, which
means that there is a separate Spanning Tree Protocol
instance for each VLAN.

5.  The 16-bit Bridge Priority (Switch Priority)
Value included in the BPDU's must hold both the Bridge
Priority (Switch Priority) Value and
the VLAN information, as shown below.
 The VLAN information is added as 12-bit Extended
System ID as shown below.
Switch Priority
Value
Extended System ID (Used to identify VLAN)
Bit
16
Bit
15
Bit
14
Bit
13
Bit
12
Bit
11
Bit
10
Bit
9
Bit
8
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
32768 16384 819
2
4096 2048 1024 512 256 128 64 32 16 8 4 2 1

6.  From above image, we can see that Bridge Priority
(Switch Priority) Value is represented only by using the
left most four bits and the remaining 12 bits are used to
represent Extended System ID. If we want to change
the Bridge Priority (Switch Priority) Value, the least
change is possible only from the 13th bit, which is 2^12.
 So what is Spanning Tree Extended System ID? The
Extended System ID is utilized by spanning-tree to
include the VLAN ID information inside 16-bit STP
Bridge Priority value. Extended System ID is the least
significant 12-bits in 16-bitSTP Bridge Priority value.

7.  Hence the Bridge Priority (Switch Priority) Value 32769
from the output of show command "show spanning-tree" is
the sum of default Bridge Priority (Switch Priority)
Value 32768 and the VLAN number, 1 (above example, I
have only one VLAN).
 Use "spanning-tree vlan <vlan_no> priority <priority_no>"
command to change the Bridge Priority (Switch Priority)
Value, from Global Configuration mode.
SW1>enable
SW1#configure terminal Enter
configuration commands, one per line. End with
CNTL/Z.
SW1(config)#spanning-tree vlan 1 priority 28672
SW1(config)#exit