Switching: How Networks Became Intelligent

The Problem with Early Networks

Let's go back to our earlier network.

You had multiple computers connected using a hub.

      C
      |
A --- HUB --- B
      |
      D

The hub behaved very simply:

Anything received on one port -> sent to ALL ports.

This created serious problems.

Problem 1: Everyone Receives Everything

If A sends data to B:

• C receives it
• D receives it
• Everyone wastes processing power

Like announcing every message on a loudspeaker.

Problem 2: Collision Everywhere

Only one device could talk at a time.

If two tansmitted:

• collision
• corrupted data
• retransmission

As networks grew, performance collapsed.

Problem 3: Shared Bandwidth

If hub speed = 100 Mbps and 10 devices exist:

Each device effectively shares that bandwidth.

Network becomes slow.

Engineers needed something smarter.

Not just signal repetition.

But decision-making.

The Birth of the Network Switch

The solution was the switch.

A switch works at:

Data Link Layer (Layer 2)

Unlike a hub, a switch understands:

• frames
• MAC addresses
• destinations

The Core Idea

A switch asks one question:

“Who exactly should receive this frame?”

Instead of broadcasting blindly.

Modern Network

      PC A
        |
PC B — SWITCH — PC C
        |
      Server

Each device has its own dedicated connection.

How a Switch Learns (The Magic Part)

Switches are intelligent because they learn automatically.

They maintain something called: MAC Address Tabe

Step-by-Step Story

Step 1: First Transmission

Computer A sends a frame to B.

Frame contains.

Source MAC: A
Destination MAC: B

Switch receives it.

Step 2: Learning the Source

Switch records:

MAC A → Port 1

This is called MAC learning.

Step 3: Unknown Destination

Switch doesn't know where B is yet.

So it temporarily broadcasts.

Step 4: B Replies

Now switch sees:

MAC B → Port 3

Tables becomes:

Step 5: Future Communication

Now when A sends to B:

Switch sends frame ONLY to Port 3.

No broadcasting.

Network becomes efficient automatically.

Collisions Domains – The Big Improvement

With hubs:

• Entire network = one collision domain

With switches:

Each port becomes its own collision domain

Meaning:

Devices can transmit simultaneously.

Example:

• A <-> B communication
• C <-> Server communication

Both happen at same time.

No collisions.

Full Duplex Communication

Switches allow:

Full Duplex Mode

Devices can:

• send AND receive simultaneously.

Benefits:

• collisions eliminated
• double effective bandwidth
• smoother communication

CSMA/CD becomes unnecessary.

Switching Methods

Switches can forward frames using different strategies.

1 Store-and-Forward (Most Common)

Process:

1. Receive entire frame
2. Check for errors (CRC)
3. Forward if vaild

Advantages:

• reliable
• error-free forwarding

2 Cut-Through Switching

Switch forward frame immediately after reading destination MAC.

Advantages:

• very low latency

Disadvantages:

• may forward corrupted frames

Used in high-speed environments.

Broadcast, Unicast, and Multicast

Switches handle different traffic types.

Unicast

One sender -> one receiver.

Most traffic.

Broadcast

One sender -> everyone.

Example:

• ARP requests.

Switch forwards to all ports.

Multicast

One sender -> selected group

Used in streaming or conferencing.

VLANs – Virtual Networks Inside One Switch

Large networks need separation.

Example:

• HR department
• Engineering
• Guests

Event on same switch.

Solution:

VLAN (Virtual LAN)

Without VLAN

Everyone in same network

Security risk

With VLAN

Switch logically separates networks:

VLAN 10 → HR
VLAN 20 → Engineering
VLAN 30 → Guests

Devices behave as if on separate physical networks.

Benefits:

• security
• traffic isolation
• easier management

Spanning Tree Protocol (STP)

Large networks often have redundant links.

Example:

Switch A ---- Switch B
     \        /
       Switch C

Problem:

Frames may loop forever.

This causes:

• broadcast storms
• network collapse

Solution: STP

Switches cooperate to:

• detect loops
• disable redundant paths temporarily
• maintain backup links

Network remains stable.

Switch vs Hub

Switches completely replaced hubs.

Big Picture So Far

Your networking stack now looks like:

Layer 2: Switching, Frames, MAC addressing
Layer 1: Signals and transmission

Local communication is solved.

But a bigger problem remains:

How does data travel between different networks?

Your home network -> ISP -> another country.

Switches cannot solve this.

That requires:

Layer 3 – Routing and IP addressing.