14.8.5 What Is an Ethernet Switch and How Does It Forward Data?
- Why Do Networks Need Switches?
- What Is an Ethernet Switch?
- How Does a Switch Know Where to Send a Frame?
- What Happens If the Destination Is Unknown?
- Why Are Switches Faster Than Hubs?
- What Is a Collision Domain?
- What Is a Broadcast Domain?
- What Is a VLAN?
- Why Does Modern Ethernet Use Full-Duplex Links?
- How Large Can Switched Networks Become?
- Are Switches Used Outside Local Area Networks?
- Why Are Ethernet Switches So Important?
- What Should You Remember?
Short Answer
An Ethernet switch is the central device in most modern local area networks. It receives Ethernet frames from connected devices, examines the destination Media Access Control (MAC) address, and forwards each frame only to the port leading toward its intended destination. By creating dedicated communication paths between devices, switches greatly improve network performance, reduce unnecessary traffic, and eliminate collisions that were common in early shared Ethernet networks.
Why Do Networks Need Switches?
In the earliest Ethernet networks, every computer shared the same communication cable.
Whenever one computer transmitted, every other device received the transmission, even though it was intended for only one destination. As networks became larger, this approach created several problems:
- collisions became more frequent;
- available bandwidth was shared by all users;
- unnecessary traffic increased; and
- overall network performance declined.
Ethernet switches were developed to overcome these limitations.
What Is an Ethernet Switch?
An Ethernet switch is a specialized networking device that interconnects multiple Ethernet devices within a local area network.
Each connected device has its own dedicated communication link to the switch. The switch acts as an intelligent traffic director, ensuring that information is delivered only to the intended recipient whenever possible.
Unlike the shared Ethernet systems of the past, many conversations can occur simultaneously through the switch without interfering with one another.
How Does a Switch Know Where to Send a Frame?
Every Ethernet frame contains:
- a destination MAC address; and
- a source MAC address.
Whenever a frame arrives, the switch records the source MAC address together with the port on which the frame was received. Over time, the switch builds a MAC address table (sometimes called a forwarding table) that associates each device with a particular switch port.
When another frame arrives, the switch simply looks up the destination address and forwards the frame through the correct port.
What Happens If the Destination Is Unknown?
Occasionally, the switch receives a frame addressed to a device that it has not yet learned. In this situation, the switch temporarily floods the frame to all ports except the one on which it arrived. When the destination device responds, the switch learns its location and records it in the forwarding table. Future frames destined for that device can then be forwarded directly.
This learning process occurs automatically without requiring manual configuration.
Why Are Switches Faster Than Hubs?
Early Ethernet networks often used hubs rather than switches.
A hub simply repeats every incoming signal to every connected device. It has no knowledge of addresses or destinations. Consequently:
- every transmission is received by every device;
- collisions occur whenever two stations transmit simultaneously; and
- all users share the same communication bandwidth.
An Ethernet switch behaves very differently. It forwards frames intelligently, allowing several independent conversations to occur at the same time.
This dramatically improves network efficiency.
What Is a Collision Domain?
A collision domain is a portion of a network in which simultaneous transmissions can interfere with one another.
In hub-based Ethernet, all connected devices formed a single collision domain. Modern switches isolate every port into its own collision domain. Because each device has a dedicated connection to the switch, collisions are effectively eliminated.
This is one of the principal reasons why switched Ethernet provides much higher performance than shared Ethernet.
What Is a Broadcast Domain?
Some Ethernet frames are intended for every device on the network.
These are called broadcast frames. Unlike ordinary frames, broadcasts must be delivered to all connected devices. The collection of devices that receive these broadcasts forms a broadcast domain.
Large broadcast domains generate additional traffic, which can reduce network performance if they become excessively large.
What Is a VLAN?
A Virtual Local Area Network (VLAN) allows one physical Ethernet switch to be divided into several independent logical networks.
Devices assigned to different VLANs behave as though they are connected to separate physical switches. VLANs provide several important advantages:
- improved security;
- reduced broadcast traffic;
- simpler network administration;
- better traffic isolation; and
- greater flexibility when relocating users.
Although the devices share the same switch hardware, communication between VLANs normally requires routing through a higher-layer device.
Why Does Modern Ethernet Use Full-Duplex Links?
Each switch port normally connects to only one device.
This dedicated connection allows communication in both directions simultaneously. Full-duplex operation provides:
- higher throughput;
- lower latency;
- elimination of collisions;
- more efficient use of bandwidth; and
- simpler communication protocols.
As a result, modern Ethernet no longer requires the collision-detection procedures used by earlier shared networks.
How Large Can Switched Networks Become?
Modern Ethernet switches range from small desktop units with only a few ports to large modular switches supporting hundreds of connections.
Multiple switches can also be interconnected to form much larger networks. Large enterprise networks commonly consist of:
- access switches connecting users;
- distribution switches aggregating traffic; and
- high-speed core switches interconnecting different parts of the organization.
This hierarchical architecture provides excellent scalability while maintaining high performance.
Are Switches Used Outside Local Area Networks?
Yes.
Although Ethernet switches are most commonly associated with LANs, switching technology is also widely used in:
- industrial control systems;
- data centres;
- telecommunications networks;
- cloud computing infrastructure; and
- campus networks.
Many modern communication systems rely on high-capacity Ethernet switching as their fundamental building block.
Why Are Ethernet Switches So Important?
The introduction of Ethernet switching transformed computer networking.
Instead of competing for access to a shared communication medium, devices now enjoy dedicated high-speed connections with intelligent forwarding performed automatically by the network.
This simple idea enabled Ethernet to evolve from a relatively modest office network into the dominant technology for local area networking throughout the world.
What Should You Remember?
- Ethernet switches intelligently forward frames according to destination MAC addresses.
- Switches automatically learn the location of connected devices by building MAC address tables.
- Unknown destination frames are temporarily flooded until the destination location is learned.
- Modern switches eliminate collisions by providing dedicated full-duplex links to every connected device.
- Broadcast frames are delivered throughout a broadcast domain, while VLANs divide large networks into smaller logical segments.
- Switched Ethernet supports multiple simultaneous conversations, providing much higher performance than early hub-based networks.
- Ethernet switches are fundamental components of modern LANs, enterprise networks, and data centres.