What is a network switch (switch)?

A switch is a data link layer (L2) network device that connects devices within a local network and forwards Ethernet frames based on MAC addresses.

Briefly and precisely: a switch delivers frames specifically to the device they are intended for, not to everyone at once.

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#What a switch does

#1. Receives Ethernet frames

The switch receives a frame on one of its ports and analyzes the MAC addresses in the frame header.

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#2. Learns MAC addresses (learning)

The switch remembers:

• the source MAC address
• the port the frame arrived on

This information is stored in the MAC table (CAM table).

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#3. Decides where to send the frame

Based on the destination MAC address, the switch:

• finds the matching port in the MAC table
• sends the frame only to the right port

If the destination MAC address is unknown:

• the frame is temporarily flooded to all ports
• after a reply, the table is updated

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#4. Reduces the load on the network

Because of targeted delivery:

• the amount of unnecessary traffic decreases
• network performance improves
• the chance of collisions is reduced

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#What layer a switch operates at

• Its main mode of operation: the data link layer (L2)
• It works with:
  • Ethernet frames
  • MAC addresses

Some switches can also operate at L3 (a Layer 3 switch), but a classic switch is an L2 device.

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#What a switch does NOT do

Important for an interview: A switch:

• does not work with IP addresses (in basic mode)
• does not route traffic between networks
• does not analyze TCP/UDP ports
• does not make application-layer decisions

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#Difference from a hub (briefly)

• A hub floods everything to all ports
• A switch sends a frame only where the matching MAC address is located