Root Bridge Election

Spanning Tree Communication:

STP exchanges its data messages in the form BPDU’s (Bridge Protocol Data Units). Since the switch is unaware of the other switches around it, so it sends BPDU frames with the source MAC address of its port, and the destination MAC address is the STP Multicast address, i.e., 0180.C200.0000....

Root Bridge Election:

The switch with the lowest Bridge ID (the least Priority or Lowest MAC address) wins the election and becomes the Root Bridge for the STP topology. Initially, each switch begins its STP logic by creating and sending a Hello BPDU message, in which it claims to be the Root Bridge (because priority is by default the same for every switch, and it is unaware of the other switch’s Base MAC). If a switch receives a superior Hello BPDU message on any of its ports, it stops claiming to be the Root Bridge and stops sending Hello BPDUs. Ultimately, the switch with the lowest Bridge ID stops sending Hello BPDUs and wins the election process....

Type of Ports in STP:

Root Port (RP): These are the ports that are closest to the Root Bridge in terms of Cost and are responsible for receiving the STP’s control-plane Traffic. Every Non-Root Bridge has only one Root Port and other ports can be either Designated Ports or Alternate Ports (based on the election). Designated Port (DP): It is responsible for forwarding traffic.  Every port of the Root Bridge is a Designated Port. Alternate Port (Alt)/Blocked Port: It is a backup port that is in a Blocking State. It comes up only if the current Root Port goes down....

Port Role Election:

After the Root Bridge is elected, the Port Role election takes place on every switch (non-root bridge), based on the following criteria in order: –...

Port Role Election Process:

As we have seen that Switch1 has won the Root Bridge election and has become the Root Bridge. Therefore, all its ports become the Designated Ports (DP). Thus, Switch1 starts sending C-BPDUs (Configuration BPDU – generated by Root Bridge) through its DP ports with a cost to reach the root bridge as 0....