What is a VLAN?

VLAN (Virtual Local Area Network) is a virtual local area network that lets you logically split one physical network into several isolated networks at the data link layer (L2).

Briefly and to the point: a VLAN is a way to split devices on one network so that they behave as if connected to different switches, even if physically they are connected to the same one.

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#Why VLAN is needed

VLAN is used to solve several key tasks:

#1. Traffic isolation

Devices in different VLANs:

• do not see each other
• do not receive each other's broadcast frames
• are treated as different logical networks

This improves:

• security
• network manageability

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#2. Limiting the broadcast domain

Each VLAN is:

• a separate broadcast domain

Broadcast traffic:

• does not leave its own VLAN
• does not load the whole network

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#3. Flexibility in building a network

With VLAN:

• network logic does not depend on the physical connection
• devices can be grouped:
  • by department
  • by role
  • by access level

Without re-cabling anything.

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#How VLAN works technically

#VLAN ID

• every VLAN is assigned a number (VLAN ID)
• range: 1-4094

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#VLAN and the Ethernet frame

VLAN transmission uses the IEEE 802.1Q standard:

• a VLAN tag (4 bytes) is added to the Ethernet frame
• the tag contains:
  • the VLAN ID
  • priority
  • service bits

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#Types of switch ports

#1. Access port

• belongs to one VLAN
• traffic goes without a VLAN tag
• used to connect end devices

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#2. Trunk port

• carries traffic for several VLANs
• frames go with VLAN tags
• used between:
  • switches
  • a switch and a router

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#What's important to remember for an interview

• VLAN works at the data link layer (L2)
• different VLANs = different broadcast domains
• to exchange data between VLANs:
  • a router or an L3 switch is needed
• a VLAN is a logical, not a physical, division