A VLAN (Virtual Local Area Network) header is a small piece of information added to Ethernet frames to identify which VLAN the frame belongs to. It allows a single physical network to be logically divided into multiple separate broadcast domains or virtual networks.
By implementing VLANs, network administrators can enhance security, improve performance, and simplify network management by segregating traffic based on predefined criteria, such as departments, user groups, or application types.
Where is the VLAN Header Located?
The VLAN header is inserted between the Source MAC Address and EtherType/Length fields of the original Ethernet frame. It consists of a Tag Protocol Identifier (TPID) and a Tag Control Information (TCI) field.
The TPID is a 2-byte field that identifies the frame as carrying VLAN information, with a value of 0x8100 for standard 802.1Q frames. The TCI is also a 2-byte field that contains crucial information about the VLAN, as shown in the following table:
| Field | Size | Description |
|---|---|---|
| VLAN Identifier (VID) | 12 bits | Represents the VLAN ID (0-4094) |
| Priority Code Point (PCP) | 3 bits | Prioritizes traffic based on predefined levels |
| Drop Eligible Indicator (DEI) | 1 bit | Indicates frames that can be dropped during congestion |
The presence of the VLAN header within the Ethernet frame allows network devices, such as switches, to identify and process frames based on their VLAN membership, enabling efficient traffic segregation and forwarding.
What is the purpose of a VLAN header?
The VLAN header allows network devices like switches to identify which VLAN a frame belongs to and handle it accordingly. This enables the virtual segregation of a single physical LAN into multiple logical broadcast domains, providing enhanced security, improved performance, and simplified network management.
By examining the VLAN information in the header, switches can make intelligent forwarding decisions, ensuring that frames are delivered only to the appropriate VLAN members.
Why is VLAN segregation useful?
VLAN segregation provides several benefits:
- Improved security by isolating traffic between different groups, preventing unauthorized access, and minimizing the risk of security breaches.
- Better performance by reducing unnecessary traffic and limiting broadcast domains, resulting in more efficient use of network resources.
- Easier network management by logically grouping devices based on organizational structures, applications, or user roles, simplifying configuration and troubleshooting processes.
How do switches handle VLAN-tagged frames?
Switches maintain a VLAN membership table that maps ports to VLAN IDs. When a tagged frame arrives, the switch checks the VLAN ID in the header and forwards the frame only to ports that are members of that VLAN. This prevents the frame from being sent to ports on other VLANs, providing isolation and ensuring that traffic remains within its intended broadcast domain.
Switches can also perform additional processing based on the Priority Code Point (PCP) and Drop Eligible Indicator (DEI) fields, enabling quality of service (QoS) and congestion management capabilities.
What happens at the destination device?
When the frame reaches its destination port, the VLAN header is removed before delivering the original Ethernet frame to the destination device. End devices like computers and servers are typically unaware of VLANs and receive the frames as normal Ethernet traffic, without the additional VLAN information. This process ensures compatibility with devices that are not VLAN-aware and allows for seamless communication within the designated VLAN.
Network administrators can effectively design, configure, and troubleshoot VLAN implementations to meet the organization’s security, performance, and management requirements by understanding where the VLAN header resides within Ethernet frames and its purpose.
The ability to logically segment networks and control traffic flows through VLAN tagging is a crucial aspect of modern network architectures, enabling efficient resource utilization and enhanced network operations.