Networking - VLANs and Trunking

1. VLAN (Virtual Local Area Network)

Definition

A VLAN is a logical subdivision of a physical network.
It allows you to group devices on different physical switches into separate broadcast domains.

Works at Layer 2 of the OSI Model (Data Link Layer).
Provides network segmentation, security, and better performance.
Each VLAN has its own VLAN ID (1–4094).

2. Why VLANs Are Needed

Without VLANs:

All devices in a LAN are in the same broadcast domain.
Unnecessary broadcast traffic → Network congestion.
Poor security since all devices can communicate freely.

With VLANs:

You can separate traffic logically.
Devices in different VLANs cannot communicate without a router or Layer 3 switch.
Improves security and performance.

3. Example Scenario

Without VLANs:

[PC1] ----\
[PC2] ----- Switch ---- [PC3]
[PC4] ----/

All PCs are in one broadcast domain.
Broadcast from PC1 → received by PC2, PC3, PC4.

With VLANs:

VLAN 10 (HR) → PC1, PC2  
VLAN 20 (Sales) → PC3, PC4

PC1 and PC2 can communicate.
PC3 and PC4 can communicate.
But PC1 cannot talk to PC3 without routing.

4. VLAN Configuration Example (Cisco)

Creating VLANs on a switch:

Switch> enable
Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name HR
Switch(config-vlan)# exit

Switch(config)# vlan 20
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit

Assigning VLANs to ports:

Switch(config)# interface fastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

Switch(config)# interface fastEthernet 0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20

5. Advantages of VLANs

Feature	Benefit
Segmentation	Divides one big network into smaller networks
Security	Isolates sensitive traffic
Performance	Reduces unnecessary broadcast traffic
Flexibility	Devices can be grouped logically, not physically

6. Trunking

Definition

Trunking is used to carry multiple VLANs over a single physical link between two network devices (e.g., between two switches or between a switch and a router).

Without trunking → You’d need one cable per VLAN.
With trunking → One cable carries traffic from all VLANs.
Uses 802.1Q tagging to identify VLANs.

7. How Trunking Works

When a frame leaves a switch on a trunk port, the switch adds a VLAN tag (802.1Q).
The receiving switch reads the tag and forwards the frame to the correct VLAN.
Access ports don’t tag frames, but trunk ports do.

8. Trunking Configuration Example (Cisco)

Setting a port as a trunk port:

Switch(config)# interface fastEthernet 0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# switchport trunk encapsulation dot1q

This configures FastEthernet 0/24 as a trunk port carrying VLANs 10 and 20.

9. Example Network with VLANs and Trunking

           ┌──────────────┐
           │   Switch A   │
           │ VLAN 10, 20  │
           └──────┬───────┘
                 │ Trunk Link (802.1Q)
                 │ Carries VLAN 10 + VLAN 20
           ┌─────┴─────┐
           │ Switch B  │
           │ VLAN 10, 20 │
           └─────┬─────┘
    ┌───────────┐     ┌───────────┐
   [PC1 VLAN10]     [PC2 VLAN20]

Trunk link carries both VLANs between Switch A and Switch B.
VLAN 10 devices communicate directly.
VLAN 20 devices communicate directly.
VLAN 10 and VLAN 20 devices cannot communicate without routing.

10. VLANs vs Trunking

Feature	VLAN	Trunking
Purpose	Divides a network into smaller segments	Carries multiple VLANs over one link
Works On	Switch ports	Switch-to-switch / Switch-to-router links
Tagging	No tagging on access ports	Uses 802.1Q tagging
Traffic Type	One VLAN per port	Multiple VLANs per link
Example Use	Separate HR and Sales networks	Connect VLANs between switches

11. Key Takeaways

VLANs = Logical segmentation of a network.
Trunking = Carries multiple VLANs over one cable.
802.1Q = Protocol used for VLAN tagging.
Devices in different VLANs cannot communicate without routing (Inter-VLAN Routing).