What Is a VLAN Frame?

Updated on May 5, 2025

A VLAN frame is essentially a standard Ethernet frame enhanced with an IEEE 802.1Q VLAN tag to enable logical network segmentation, traffic isolation, and improved quality of service (QoS). This blog dives into the technical details of VLAN frames, their components, and how they operate within a networking environment.

Definition and Core Concepts

VLAN frames, or Virtual Local Area Network frames, play a pivotal role in segmenting networks logically while sharing the same physical infrastructure. Below, we’ll break down the core components that make up a VLAN frame:

Ethernet Frame Structure

An Ethernet frame serves as the foundation of network data exchange. It consists of:

• Preamble and Start Frame Delimiter (SFD): Indicate the start of a frame.
• Destination and Source MAC Addresses: Define where the frame is going and where it’s coming from.
• EtherType or Length Field: In a standard Ethernet frame, this identifies the protocol used in the payload or the length of the payload. However, in a VLAN-tagged (802.1Q) frame, this field is replaced by the Tag Protocol Identifier (TPID, set to 0x8100), and the original EtherType/Length information is located within the payload after the VLAN tag.
• Payload/Data: The actual data being transmitted.
• Frame Check Sequence (FCS): Verifies data integrity.

The 802.1Q VLAN tag extends this standard Ethernet frame.

What Is a VLAN (Virtual Local Area Network)?

A VLAN is a logical network segment that allows devices to communicate as though they were on the same physical LAN, regardless of their physical locations. VLANs help reduce broadcast domains, enhance data security, and simplify network administration.

The VLAN Tag (IEEE 802.1Q Tag)

The VLAN tag is a 4-byte field inserted into the Ethernet frame between the Source MAC Address and EtherType fields. It contains the following components:

• Tag Protocol Identifier (TPID): A 2-byte field set to 0x8100 that identifies the frame as VLAN-tagged.
• Tag Control Information (TCI): A 2-byte field holding critical information:

1. Priority Code Point (PCP): Uses 3 bits to specify the frame’s priority level (0–7) for traffic scheduling and QoS.
2. Drop Eligible Indicator (DEI): A 1-bit flag that indicates if the frame can be dropped when there’s network congestion.
3. VLAN Identifier (VID): Consists of 12 bits, allowing up to 4096 VLANs (IDs 0 and 4095 are reserved).

This tagging enables the frame to be associated with a specific VLAN while supporting network performance and management.

How It Works

VLAN frames undergo specific processes as they traverse a network. Below is an outline of these mechanisms:

Tagging at Ingress

When a switch receives an untagged Ethernet frame via a VLAN-aware port, it adds a VLAN tag based on the port’s configured membership. This tagging process appends the 802.1Q VLAN tag, thus designating the frame’s VLAN ID.

Forwarding Based on VID

Once tagged, the VLAN ID directs the frame’s path through the network. The switch uses the VID to determine which ports are part of the same VLAN and forwards the frame accordingly.

Untagging at Egress

When the frame exits via an access port, the VLAN tag is removed, reverting to the standard Ethernet frame structure. This ensures the receiving device is unaware of VLAN configurations.

Trunk Ports

Trunk ports are configured to carry traffic for multiple VLANs. Frames passing through trunk ports retain their VLAN tags, allowing VLAN segregation across multiple switches.

Access Ports

Access ports are configured to handle traffic for only a single VLAN. Frames received are transmitted without VLAN tags, enabling seamless device communication within the VLAN.

Key Features and Components

VLAN frames offer numerous advantages for modern networks. Below are the key features and components that make VLAN tagging a vital technology:

1. VLAN Identification (VID): The VLAN ID within the tag clearly indicates the VLAN membership of each frame.
2. Logical Segmentation: VLANs enable multiple logical networks to operate on the same physical network, reducing the complexity and cost of physical segmentation.
3. Traffic Isolation: VLANs ensure that communication within different VLANs remains logically isolated, enhancing security and reducing broadcast traffic.
4. Priority Tagging (PCP): VLAN tagging supports QoS by allowing frames to receive priority treatment on the network.

Use Cases and Applications

VLAN frames are used in a variety of scenarios that require logical segmentation and isolation. Here are some common applications:

Separating Voice and Data Traffic

Many organizations create separate VLANs for voice and data traffic to ensure high-quality voice communication while preventing data congestion.

Guest Networks

Guest networks often use VLANs to securely isolate guest devices from the internal corporate network while providing internet access.

Departmental Segmentation

Large enterprises use VLANs to segment network traffic by department, such as HR, Finance, and Marketing, improving both network management and security.

Server Isolation

Critical servers can be placed in dedicated VLANs to isolate sensitive data from general network traffic, enhancing both security and performance.

Advantages and Trade-offs

When designing networks with VLANs, it’s essential to understand their benefits and possible challenges.

Advantages

• Cost Savings: VLANs eliminate the need for separate physical networks, reducing hardware expenses.
• Flexibility: VLAN-based segmentation enables easy network reconfiguration without the need to rewire infrastructure.
• Enhanced Security: VLANs provide an additional layer of security by logically isolating network segments.

Trade-offs

• Increased Frame Size: The 4-byte VLAN tag increases the frame size, which could impact certain networking environments with tight size constraints.
• Switching Complexity: VLAN-aware switches must be configured correctly, which may increase management complexity.
• Potential for Misconfiguration: Incorrect VLAN tagging or port settings can cause connectivity issues and compromise network performance.

Key Terms Appendix

• VLAN (Virtual Local Area Network): Logical groupings of devices that act as if on the same LAN, regardless of physical location.
• Ethernet Frame: A data transmission unit that includes MAC addresses, payload, and error-checking info.
• IEEE 802.1Q: A standard defining VLAN tagging for Ethernet frames.
• Tag Protocol Identifier (TPID): A 2-byte field identifying VLAN-tagged frames.
• Tag Control Information (TCI): A field within the VLAN tag that includes PCP, DEI, and VID.
• VLAN Identifier (VID): A 12-bit field uniquely identifying VLAN membership.
• Trunk Port: A port carrying multiple VLANs with tagged traffic.
• Access Port: A port handling untagged traffic for a single VLAN.