In this article I am going to discuss about How to Change Spanning Tree Bridge Root Priority value and what is Extended System ID so keep reading this article till the end.

Before we knowing about the changing of the bridge priority value, w have to know what is bridge priority value. It is also known as the Switch Priority Value. There will be a value that is assigned to all the switches or the bridges that are participating in the spanning tree protocol networks, and this assigned value is the bridge priority value(Switch priority value).

The bridge priority values are a numerical value having a default value of 32768. With the help of this switch priority value, it can decide that which switch becomes the root switch. The selection of the root switch is based on the value contained in the switch priority value. This is done by, selecting a switch as root switch which having the minimal switch priority value. Thus we can select a switch as the root switch by lowering the value of switch priority value.

We use some commands from Global Configuration Mode to change the value of Switch Priority Value from one particular value to another value.

I take the following format:

“spanning-tree vlan <vlan number> priority <priority number>”

The one thing that we should care about when we changing this value is that we are either decrement or increment this value by 4096. If we change the Switch priority value by decrementing or incrementing by another value, it will make some error messages.

The following command  shows an example for such case:

enable

#configure terminal

#spanning-tree vlan 170 priority 32767

#exit

#

The command in the third line will show some error message since the switch priority value is changed not by incrementing or decrementing it by 4096. The reason for happening like this because of a mode of Spanning Tree Protocol ran by all Cisco switches which is Per-VLAN Spanning Tree Protocol + (PVSP+). This protocol has the Cisco proprietary extensions and is based on the standard, IEEE 802.1D. Also, this protocol runs switch’s each VLAN. This indicates that, for each VLAN there is a different Spanning tree Protocol.

As we know, the  BPDU’s Switch Priority Value is 6 bit and it contains 2 types of values, VLAN information, and the Switch Priority value. VLAN information is 12 bit in length and is added as an Extended System ID.

We can represent this using the following format:

Bit 1 – 1  : Switch Priority Value

Bit 2 – 2  :     ’’

Bit 3 – 4  :  ’’

Bit 4 – 8  :  ’’

Bit 5 – 16 :   ’’

Bit 6 – 32 :   ’’

Bit 7 – 64 :   ’’

Bit 8 – 128 :   ’’

Bit 10 – 256 :   ’’

Bit 11 – 512 :   ’’

Bit 12 – 1024 : Extended System ID

Bit 13 – 2048 :  ’’

Bit 14 – 4096 :  ’’

Bit 15 – 8192 :  ’’

Bit 16 – 32768 :    ’’

Also Read:

Here we can see that the first 12 bits(from bit 1 to bit 12) are used to represent the extended system ID. And the remaining 4 bits(from bits 13 to bit 16). We can only change the 13th bit if we wanted to change the value of Bridge Priority Value. This Extended System ID is used to include the information about the VLAN ID.

I hope you have understood the written article on How to Change Spanning Tree Bridge Root Priority value and what is Extended System ID.