Un Tagged & Tagged VLANs: A Practical Guide

2025-02-05

In networking, VLANs (Virtual Local Area Networks) are essential for segmenting network traffic, improving security, and optimizing performance. VLANs can be either tagged or untagged, and understanding the difference is crucial for network administrators. This article will explore the concepts of tagged and untagged VLANs, along with practical commands and configurations.

What Are Tagged and Untagged VLANs?

Untagged VLANs: These are also known as “native VLANs.” Frames on an untagged VLAN do not carry VLAN identification information. When a switch receives an untagged frame, it assigns it to the native VLAN configured on the port.
Tagged VLANs: Frames on a tagged VLAN include a VLAN tag (IEEE 802.1Q) that identifies the VLAN to which the frame belongs. This allows multiple VLANs to traverse a single trunk link between switches.

Practical Configuration on Cisco Devices

1. Configuring Untagged VLANs

To configure an untagged VLAN on a Cisco switch, use the following commands:

Switch> enable
Switch# configure terminal
Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# exit

In this example, port `GigabitEthernet0/1` is assigned to VLAN 10 as an untagged port.

2. Configuring Tagged VLANs

To configure a tagged VLAN (trunk port) on a Cisco switch, use these commands:

Switch> enable
Switch# configure terminal
Switch(config)# interface GigabitEthernet0/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30
Switch(config-if)# exit

Here, port `GigabitEthernet0/2` is configured as a trunk port, allowing VLANs 10, 20, and 30 to pass through.

Verifying VLAN Configuration

To verify your VLAN configuration, use the following command:

Switch# show vlan brief

This command displays a summary of all VLANs and their associated ports.

Troubleshooting VLAN Issues

If you encounter issues with VLAN traffic, use these commands to diagnose the problem:

Switch# show interfaces trunk
Switch# show interfaces switchport

These commands provide detailed information about trunk ports and their VLAN configurations.

What Undercode Say

Understanding and configuring VLANs is a fundamental skill for network administrators. Tagged and untagged VLANs serve different purposes, and their proper implementation ensures efficient network segmentation and traffic management. Below are additional Linux and networking commands to enhance your VLAN knowledge:

1. Linux VLAN Configuration:

To create a VLAN interface on Linux:

sudo ip link add link eth0 name eth0.10 type vlan id 10
sudo ip addr add 192.168.10.1/24 dev eth0.10
sudo ip link set dev eth0.10 up

To verify VLAN interfaces:
```
ip -d link show
```

2. Packet Capturing with VLAN Tags:

Use `tcpdump` to capture VLAN-tagged traffic:
```
sudo tcpdump -i eth0 -nn -e vlan
```

3. Network Troubleshooting:

Use `ping` to test connectivity:
```
ping 192.168.10.1
```
Use `traceroute` to trace the path of packets:
```
traceroute 192.168.10.1
```

4. Advanced VLAN Management:

Use `bridge` command for VLAN filtering:
```
sudo bridge vlan show
```

For further reading, refer to the following resources:

Mastering VLANs requires hands-on practice and a deep understanding of network protocols. By leveraging the commands and configurations provided, you can build a robust and secure network infrastructure.

References:

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