Understanding Spanning Tree Port States

When a switch is first turned on, all the connection points (interfaces) on it go through 4 states. These situations are; Blocking, Listening, Learning, and Forwarding port status. In addition, status 5 is disabled.

Understanding Spanning Tree Port States

Understanding Spanning Tree Port States

The ports/interfaces on the switch can be examined via LEDs. We see these LEDs when the Switch port status is in progress. These LEDs on the switch turn towards the green from the blinking orange color.

The switch can do this for up to 50 seconds when first switched on. If you have added a Switch to your network before, you have noticed it. The use of port states by switches is, in fact, preventing the cycles that can occur on the network. Switches communicate by sending BPDU frames between themselves.

BPDU frames tell you which Switch will be the Root Bridge or which Switch interface will enter the Blocking mode. In our previous article, we have provided information about Network Redundancy. We mentioned that the STP protocol is used to prevent loops in a network redundancy.

When you add a new Switch to an existing network, the Spanning Tree Protocol on the Switch will pass the ports from 4 states. The reason for spanning tree port statuses is to identify interfaces without looping through a backup network.

Switch Port States on a Redundancy Network

Spanning Tree port states occur in 4 stages. These steps are:

1. Blocking State

It starts in Switch Blocking mode on a backed up network. Blocking mode prevents connection paths that will loop. Also, they do not send Data-Link Frames. They just listen to what is on the network. It retrieves and analyzes BPDUs.

Switch Blocking mode prevents the Loops in the network. The first 20 seconds when the switch is restarted starts Blocking mode. You can view and change the max age timer value for blocking mode.

In addition, Blocking Mode provides notification with plain orange color. In short, it is shown as a blinking orange color.

2. Listening State

Switch, After the Blocking port status completes the task for 20 seconds, it switches to Listening port status. The switch sends out a message with a blinking orange color in this mode. It retrieves and sends BPDUs, but not frames. Also, MAC addresses are not learned in Listening mode.

Listening mode listens to the network with BPDUs and returns to Blocking mode if there is a loop. If there is no loop in the network, it switches to Learning port status.

Listening mode listens BPDUs and ready to send the data-link frames. Switch completes the listening mode in 15 seconds. You can change this value with the forward delay timer command in Cisco IOS software.

3. Learning State

After completing the Switch Listening mode, it will enter Learning mode. The switch will pass after 20 + 15 = 35 seconds to learn mode. The aim of Learning mode is to allow the Switch to build the table of MAC addresses that it sends and receives BPDUs.

The port that is in learning mode does not send data frames. Learning mode default value is 15 seconds and can be changed by forward delay timer command.

Learning mode is also indicated by a blinking orange led.

4. Forwarding State

The switch will pass in Forwarding mode after 20 + 15 + 15 = 50 seconds. In the forwarding mode, BPDUs are processed and MAC addresses are learned. The forwarding mode is indicated by the flashing green led and the data packet is now sent.

If the port is Designated Port or Root Port, it switches to Forwarding Mode.

Disabled State

The Disabled port state is actually the most basic port state. Generally, the system administrator closes the system manually. The disabled port state has no effect on the network. They do not send or receive any data frames. Disabled mode is only off.

   Final Word

When a new Switch is added to a backed up the network, it is in Blocking mode by default. In a backup network with more than one Switch, the Switch does not operate in the forwarding mode directly and can not. This will cause Loop on the network, which will then enter the network in the listening, learning and forwarding mode.

If a computer or server is connected to the Switch port, you can configure it directly in Port Forwarding mode by configuring it with PortFast.

   Related Articles

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Access Port
Trunk Port
Port Security
Default Route

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