Which switch is the root bridge

This means that possible values are: , , , , , , , etc. The election process uses several STP messages sent between switches which help each switch to decide, who is the Root Bridge. It is important to understand the information these BPDUs carry as it will help understand the election process itself.

Each BPDU carries several fields in it. The following table defines each field:. For the purpose of this exercise, we will only concentrate on the first three fields. Now, the election process itself is very simple. If there is a tie between two switches having the same priority value, then the switch with the lowest MAC address becomes the Root Bridge. This process continues till every switch agrees on which switch has the lower BID , and hence deserves to be the Root Bridge.

Let's look at this process using a three switch combination within a network. For the sake of simplicity, the MAC address of each switch has been changed to a simple value:. Now, the election process commences with the advertisement of the individual Hello BPDU's from each switch, as indicated by the arrows in our diagram. These BPDUs originate from each switch and end up at the other switches.

Let's take up one switch at a time to see how it reacts to the BPDUs it receives from the other switches. Switch and port priority? When switches and bridges are running the STP, each has a bridge or switch port priority associated with it. By default, all STP switches are configured with a bridge priority value of 32, After the exchange of BPDUs, the switch with the lowest priority value becomes the root bridge.

Port cost? Cost is determined according to the speeds that the ports support; the faster the port, the lower the port cost. Switches use port costs in determining the root port for each and every switch. This election is similar to a bunch of switches going to a voting booth and choosing their favorite switch.

The BPDUs are used as a voter information guide, or ballot, to select the correct candidate. The purpose of this election is to determine which switch has the lowest identifier. The shortest distance to the root switch is calculated for each switch. A designated switch is selected.

This designated switch is the closest switch to the root switch through which frames will be forwarded to the root. There is only one designated switch per segment or VLAN. A designated port for each switch is selected, providing the best path to the root switch.

Ports included in the STP are selected. Because all ports might not be part of the spanning tree, the exchange of BPDUs determines which ports have an invitation to the spanning tree forwarding and which ports do not. Those that don't are disabled.

The ports not turned on are not included as part of the spanning tree. Loops in the switched network are removed. Loops are detriments to networks because traffic on a network containing loops goes around in circles? These network loops are prevented by each switch placing redundant switch ports in a backup state as directed by the STP. Toggle navigation. See also. Home Networking Lan switching first-step. Multiple Routes Between the Source and Destination The root switch is elected as part of the STP and is necessary to build a reference point for the spanning-tree algorithm calculations.

The topology of a switched LAN is determined by the following: The unique switch identifier or bridge ID associated with each switch? Remember the name: eTutorials. Chapter 1. Networking Basics. What Is a Network? Logical Networks. Data Transmission Modes. Types of Networks. Network Standards and Models. Chapter Summary. Chapter Review Questions. The way this works is that the switch looks at the priority of the current root bridge and then decreases its own priority so that it becomes the new root bridge.

Above, we see that the priority is now 20 priority 0 and sys-id-ext Ask a question or join the discussion by visiting our Community Forum.

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