HOW STP WORKS

How Spanning Tree Protocol Works?

When a switch first power up it assumes that it is the Root Bridge itself and switch begins by sending out BPDUs with a Root Bridge ID and Sender ID equal to its own Bridge ID. As the BPDU goes out through the network, each switch compares its own BPDU with the one that switch receives from the neighbors. The comparison is done on bridge ID. When a switch hears of a better Root Bridge (lower bridge ID), it replaces its own Root Bridge ID with the Root Bridge ID announced in the BPDU although it still identifies itself as the Sender Bridge ID. After sometime the switch with the lowest bridge ID in the network wins this Root Bridge election process. 

• STP Rule 1: All ports of the root bridge will be in forwarding mode. 

o Next, each switch determines the best path to get to the root. The switches determine this path based on Root Path Cost. This value is the cumulative cost of all the links leading to the Root Bridge. The switch uses the port with the least Root Path Cost in the BPDU in order to get to the root switch; the port with the least Root Path Cost in the BPDU is the root port. 

• STP Rule 2: The root port must be set to forwarding mode. 

o In addition, the switches on each LAN segment communicate with each other to determine which switch is best to use in order to move data from that segment to the root bridge and this determination is based on the lowest cumulative Root Path Cost to the Root Bridge. And the port is called the designated port. 

• STP Rule 3: The designated port must be set to forwarding mode. 

• STP Rule 4: All the other ports in all the switches must be placed in blocking mode. 

If two or more links might identical Root Path Costs then that will results in a tie condition. All tiebreaking STP decisions are based on the following sequence of four conditions:

• Lowest Root Bridge ID

• Lowest Root Path Cost to Root Bridge

• Lowest Sender Bridge ID

• Lowest Sender Port ID

STP PORT STATE:

• Blocking:

All the port by default on a switch will be in the blocking state. In blocking state we are only allowed to receive BPDU. We can't build our MAC Table at this state.

• Listening:

If the port on the switch is administratively enabled it move to listening state. At this state it can receive and transmit BPDU. Hence it involved in the election of Root Bridge. This state has a timer enabled which has to be elapsed before getting to the next state. This timer is called as forward delay timer and its default value is 15sec. 

• Learning:

At this state port can receive and transmit BPDU and also can start building MAC table. But it can't receive or transmit data frames at this state. This state also has forward delay timer and the default value of the timer is 15sec.

• Forwarding:

At this state port can receive and transmit BPDU & DATA Frames and also we will continue to build our MAC table.