What is a VLAN ID? How it works, and VLAN ID Importance

VLAN ID

Each virtual LAN (VLAN) is given a unique numerical identifier, sometimes called a Virtual LAN Identifier (VLAN ID), which is used to divide a physical network into logical, separate broadcast domains. This 12-bit number, which is defined by the IEEE 802.1Q standard, is incorporated as a VLAN tag into the Ethernet frame header, enabling network switches to recognize and direct traffic to the appropriate VLAN segment. Up to 4,096 distinct VLANs can be supported by a 12-bit ID in a network.

To put it briefly, a VLAN ID is the special number between 1 and 4094 that is used to identify each VLAN within a network.

Definition and Purpose

For groupings of devices on a Local Area Network (LAN), VLAN IDs are distinct identifiers that specify a network segment.

In order for switches and routers to identify which VLAN a data packet belongs to, they are mostly used to tag network frames. In order to preserve broadcast domain separation even when using the same physical infrastructure, this procedure makes sure that traffic from one VLAN does not mix with traffic from another.

Switches would be unable to distinguish between traffic from various VLANs without Virtual LAN Identifiers.

You can also read What Is VLAN Tagging Cisco? An Introduction To IEEE 802.1Q

How does VLAN ID Work

• The open standard for VLAN tagging is IEEE 802.1Q (Dot1Q).
• When a data packet or Ethernet frame enters a switch, it is tagged with a VLAN ID. This tag, a 4-byte (32-bit) header, is inserted into the original Ethernet frame, typically after the Source Address field.
• The VLAN ID itself is a 12-bit field within the 802.1Q tag’s Tag Control Information field.
• Switches use these IDs to route traffic between different segments. The tag contains specific information to guide the switch in segregating traffic, ensuring only intended recipients can access the data.
• The VLAN ID is carried inside the 802.1Q VLAN tag when traffic moves across a trunk link. Trunk links are switch ports configured to carry traffic for multiple VLANs.
• When a frame exits an access port connected to an end device, the Virtual LAN Identifier tag is typically removed, making the VLAN membership transparent to the host device (e.g., PCs or printers).
• This mechanism effectively divides a large LAN into smaller, manageable segments, each behaving as if independently connected to its own network.

VLAN ID Ranges and Their Significance

Virtual LAN Identifiers can be anywhere between 0 and 4095 because they are 12-bit values.

However, 4094 VLANs are functional, but Virtual LAN Identifiers 0 and 4095 are reserved and typically not used. Thus, the practical range is 1 to 4094.

These ranges are usually divided into the following categories:

Default VLAN (VLAN ID 1): This VLAN is usually reserved and utilized by default; unless otherwise specified, all switch ports are automatically members of this VLAN. Most switches don’t let you remove it. Moreover, VLAN 1 is frequently the native VLAN.

Normal Range VLANs (IDs 2-1001 or 1-1005): In everyday business and organizational networks, these are utilized for the most practical purposes and are highly adaptable. VLANs 1002–1005 on Cisco switches are for old Token Ring and Fiber Distributed Data Interface (FDDI) and cannot be removed.

Extended Range VLANs (IDs 1006-4094 or 1002-4094): These are frequently employed for particular purposes, like FDDI VLANs and Token Rings, and are less frequently utilized in conventional networking settings but are essential in certain situations. They are employed by internet service providers (ISPs) to handle a large number of clients and for larger networks.

You can also read Types Of VLANs, How VLAN Works, Advantages & Applications

Native VLAN and Untagged Traffic

• On 802.1Q trunk links, the native VLAN is a unique VLAN that transports untagged traffic.
• The native VLAN is VLAN 1 by default.
• Native VLAN frames are usually transmitted without an 802.1Q tag. A switch believes a frame on an 802.1Q trunk port that is untagged is part of the native VLAN set up for that port.
• An unused or “dummy” VLAN should be utilized as the native VLAN instead of the default VLAN 1, according to security best practices. Frames may “jump” from one VLAN to another due to mismatched native Virtual LAN Identifiers on opposite ends of a trunk link, a security flaw known as VLAN hopping. It’s also a good idea to configure trunk ports to tag native VLAN traffic.

Key Benefits of Implementing VLAN ID

Enhanced Security: By isolating sensitive data within its VLAN, network managers can lower the possibility of unwanted access. It is possible to keep data from overlapping by separating different departments (such as finance, human resources, and guest WiFi).

Improved Network Performance and Traffic Management: By keeping broadcast traffic inside clearly defined limits, VLANs stop unnecessary data from spreading throughout the network, greatly lowering traffic loads and minimizing bottlenecks. The user experience and response times improve as a result.

Broadcast Domain Reduction: In order to prevent network broadcast storms, which can otherwise overload bandwidth, Virtual LAN Identifiers limit broadcasts to devices that are part of the same VLAN.

Flexibility and Simplified Management: With the use of VLANs, administrators can logically arrange devices based on their functions rather than their physical locations, which facilitates additions, updates, and migrations. They allow a network to be logically divided, providing traffic isolation and segmentation.

Configuration and Verification

• VLANs are typically created using the vlan vlan-id command in global configuration mode.
• An interface can be assigned to a VLAN using the switchport access vlan vlan-id command in interface configuration mode.
• The native VLAN on a trunk link is configured using the switchport trunk native vlan vlan-id command.
• Commands like show vlan, show vlan brief, and show vlan id <vlan-id> are used to verify VLAN IDs and their associated ports.

Virtual LAN Identifier in Other Protocols

Spanning Tree Protocol (STP): In Per-VLAN Spanning Tree Plus (PVST+), Virtual LAN Identifier (VID) information must be carried in the Bridge ID (BID) field of Bridge Protocol Data Units (BPDUs). The VLAN ID is carried by reusing a piece of the Bridge Priority field as an extended system ID, which is a 12-bit field.

Inter-VLAN Routing: An encapsulation identification (VLAN ID) must be supplied to each subinterface in order to specify which VLAN it serves when a router is used for inter-VLAN routing (such as in a “router on a stick” configuration). Logical subinterfaces are created on a physical interface.

In summary, by logically dividing traffic and devices, Virtual LAN Identifiers are a crucial part of network infrastructure that allows for strong, well-organized, and secure network configurations.

You can also read What Is Mean By Troubleshooting Interface And Cable Issues?