What is a root switch and how is it chosen?

The root bridge of the spanning tree is the bridge with the smallest bridge ID.

When you type "show spanning-tree root" on the root bridge, you will see "This is the root bridge" in the output.


In any Spanning Tree instance, there is only one Root Bridge, and it must be elected. The Root Bridge is elected in the initial exchange of BPDUs between bridging devices. But how does the Root Bridge get elected? Thats simple. In networks running STP, every bridge has a priority value associated with it. By default, the priority of all bridges is 32,768, unless changed by an administrator. The bridge with the highest priority gets to be the Root Bridge. But wait  youll need to remember that the highest priority is the bridge with the lowest priority value. That is, a bridge priority of 1000 would beat the default priority of 32,768.

The default bridge priority is 32768, all switches start here. When you connect switches together they begin a process called a root war. Every device advertises it's bridge priority in BPDUs, lowest root bid or bridge priority becomes root and backup root bridge. If all things are equal then there is a tie breaking process. The device with the lowest mac address becomes the root bridge and the device with the next lowest mac address becomes backup root. Problem is this process can elect a root bridge is a non-optimal location. Thats why there are ways to influece the process, you can change the bridge priority to force election of a particular device to root and backup root. This allows you to pick the best device for root bridge operation. You can also influence the path traffic takes to get to the root bridge by manipulating the path cost, and path vlan cost of the interfaces
in the switch mesh.

Here is a good doc

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/spantree.html#wp1032530

harbor235 ;}

Did this help get your answer solved?