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How BPDU is Generated and How BPDU Works?

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Before starting our topic we have to know about some basic things about computer networks and the role of BPDU in computer networks:

  • What is Computer Networks?
  • Types of Computer Networks.
  • What is BPDU?

Computer networks are a procedure of connecting devices such as laptops, desktops, smartphones, servers, and various IoT devices such as cameras, doorbells, refrigerators, audio systems that communicate with one another.

There are various devices that are used as a foundation in computer networks. these devices have their specific on the whole procedure of computer network works. There are various devices such as switches, routers, access points.

Types of Computer Network:

  • LAN: A LAN is a collection of various connected devices in any physical location. a LAN has both wired and wireless devices. A LAN can be range from small to large networks such as from home networks to big enterprises network.
  • WAN: A WAN can be extended over a large area and used to connect individual users and various LANs. There are various organizations that are using WAN to connect various sites, remote employees.
  • Enterprise Network: Network is crucial for the various modern enterprise to function properly so to overcome these problems enterprise networks have tools that allow engineers to design, deploy and debug them. LAN and WAN are both used in enterprise networks.
  • Service Provider Network: Service networks operate WANs to give connectivity to various organizations or individual users.

What is BPDU?

BPDU are the parts of the spanning tree protocol (STP). they allow the switches to obtain information about each other. The spanning tree protocol enabled the switches in redundant local area network need to exchange the information between each other for spanning tree protocol to work properly so, for the proper exchange of information between spanning tree protocol the bridge protocol data unit (BPDU) is used.

The bridge protocol data unit frame is mainly used for containing all information about switch ID, MAC address, switch port and it also stores information regarding priority and cost of switch ports.

There are mainly two types of bridge protocol data units:

  • Configuration BPDUs.
  • Topology Change Notification (TCN) BPDUs.

How BPDU is Generated?

As we discussed above there are two types of BPDU, Hence generation of both BPDUs are different:

1. Configuration Bridge Protocol Data Unit: These BPDUs are generated from the root bridge(Root switch) and they also move away from the root bridge(root switch). These BPDUs is different from other ones because it generates and moves away from the root bridge.

Working: The working of this configuration BPDU is shown in the below figure:

Configuration BPDUs are generated from Root Bridge(root switch) and they move away from Root Bridge(root switch). The root bridge is the switch having the lowest switch ID. The process starts with comparing the switches. When a network switch receives a configuration BPDU that is having a lower root switch and is compared with what the network has the lowest root switch, Then automatically network switch will choose the switch with the lowest root switch as a Root Bridge.

But in this whole process, we might think that what is the role of non-root switches in this, so the non-root switches do not generate any configuration BPDUs instead of the Non-root switches can only propagate BPDUs generated by the Root bridge.

In figure 1 given below, there is shown a working of configuration BPDU, as we can see there are five switches and switch 1 is described as a Root bridge and then Non-Root switches do not generate the configuration BPDUs but they actually propagate the BPDUs generated by Root Bridge.

 Configuration Bridge Protocol Data Unit

2. Topology change notification BPDUs: These BPDUs are generated from Non-root switches and they flow towards the Root bridge its main work is to inform the root bridge that topology has been changed in the network.

Working: The working of topology change notification BPDUs is shown in the below figure:

Topology change notification BPDUs are generated when a switch encounters that topology is changed in-network and all the information about topology is being used currently and sent to the root part. Now when the root port receives the TCN BPDU it sends back the sender with topology change acknowledgment (TCA)BPDU. After when root port sends back TCN BPDU generates its own TCN BPDU and forwards it to the root port. This whole is continued till TCN BPDU is received by Root bridge. If a process is completed and the root bridge is notified that topology is changed in-network, then it generates a configuration BPDU to the network.

 Topology change notification BPDUs

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Last Updated : 23 Feb, 2022
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