Before we learning about the Spanning Tree Bridge Priority and Bridge ID Values, we all should have a deep knowledge about what is spanning tree and why it is used?. The Broadcast storms and the switching loops in layer 2 are the major problems occurring in the network. These two problems in a Local Area Network can be avoided by using the protocol called Spanning Tree Protocol.

We are going to learn what is Spanning Tree Bridge Priority Value(this is also referred by using Switch Priority Value). A value that is numerical is assigned to all the switches or the bridges which are participating in the network using the Spanning tree Protocol. This assigned numerical value is Switch Priority Value/ Bridge Priority Value which is a binary number having a length of 16 bit.

There is a default value for switch priority for all the Cisco Switches, which is 32768. This value, Switch Priority, is used for identifying which switch has to become the Root Switch/ Root Bridge. This is done by finding the switch having the smallest switch Priority Value. This switch having the lowest Switch Priority Value is then identified ad as the Root Bridge

Next, we are going to learn about the Bridge ID. If a Spanning Tree Protocol wants to find out the best path to the root switch, it then first need to find that which switch has to be selected as a root switch. The Bridge ID has an important role here in this process. For all the switches which become a part of a network will have an ID, called the Bridge ID. Which is also referred to as BID.

This ID having a length of 8-byte memory and this 8 byte is divided into two unequal parts. The first part contains the bridge priority taking a memory of 2 bytes and the later part contains the MAC address of that particular switch having a length of 6-byte memory. The Bridge ID can find out by using the above Switch Priority Value/ Bridge Priority Value.

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The Bridge ID is also referred to by using the term Switch ID. This ID is generated from the following two values:

  • From the Bridge Priority/ Switch Priority.
  • From the MAC Address of the Switch.

As we know, the Spanning Tree Bridge Priority values are a numerical value having a default value of 32768. IEEE 802.1D defines the above value.