What is a Root Bridge (Root Switch)

The Root Bridge or the Root Switch is a switch that is something special and occurs in the top of the spanning tree, where spanning tree is a tree that is inverted. All the Ethernet connections are started from this root bridge. The Ethernet connections are also called as branches. These branches connect the switches from one to another.

A switch will be denoted as a root switch in Spanning Tree Protocol configured switched LAN or in spanning tree protocol configured broadcast domain. All spanning-tree calculations are done by this root bridge. This calculation is done to decide which link has to be blocked which is redundant. Therefore, it acts as an administrative point for all these calculations.

We can determine a switch as a root bridge among several switches with the help of an election process. All the switches in that particular network have a participation in the election process of a root bridge. Every switches have three properties including,

  • Bridge ID(BID)
  • Extended System ID
  • MAC address.

Where, the first one, called bridge id, have a priority value. MAC address will be the MAC address of the switch. If a switch boots up, every 2 seconds, the switch will send BPDU frames out after the booting up of the switch. This BPDU frame contains two things, the root id and the switch BID. The local BID and the root id of all the switches in a particular network will match by default. Te root bridge can be identified with the help of a root ID. After a switch boots up, it thinks itself as a root bridge initially.

We can explain this with the following example. Consider that A, B, C, and D are 4 switches in the same network. As we stated above, the switches will send out the BPDU frames(Bridge Protocol Data Unit frames) to all the neighboring switches after the switch boots up. The switch which receiving the BPDU will read the information from the BPDU from the neighbors and look at the root ID information.

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After reading this root id information from all neighbors, the root bridge will be identified based on the root id values. The switch which has the minimal(lowest) root id will be denoted as the root bridge for that particular spanning tree. It can be any switch in the network. There is no rule that it should be adjacent.