What is meant by VLAN?

VLAN stands for Virtual Local Area Network, and it is created from one or more local area networks. A Virtual Local Area Network (VLAN) is a customized network made up of one or more LANs. It allows a set of devices from different networks to be converted into a single logical network. As a result, a virtual LAN that is managed similarly to a physical LAN emerges.

VLAN Characteristics 

  • Even if the networks are dissimilar, virtual LANs provide structure for forming groups of devices.
  • This is accomplished by creating a separate virtual LAN for only the hosts that have sensitive data.
  • It utilizes a flexible networking approach that divides users into departments rather than network locations.
  • On a VLAN, changing hosts/users is relatively simple. Only a new port-level setup is required.
  • It can alleviate network congestion by sharing traffic because every VLAN functions as a distinct LAN.
  • A VLAN can be shared across many switches.


As networks grow in size, it becomes important to make up many broadcast domains to divide traffic for execution of security and for theoretical reasons. Each network segment would ordinarily require its switch structure, with one/more than one routers directing communication between them if VLANs were not used.

A broadcast domain is denoted by a virtual LAN. VLAN is allotted to one port on a switch/router. For example, traffic delivered to one VLAN port on one switch can be conveyed to any other VLAN port on the switch or to all VLAN ports on another switch via the trunk port. However, traffic will not be dispatched to ports with different VLAN IDs.

VLAN configuration

  • Because VLANs belongs to layer 2 protocol, layer-3 routing is needed to facilitate communication between them. This is analogous to how a router manages traffic between two subnets on directly opposite positions. On some switches of layer-3, VLAN routing is also supported, which allows you to route traffic across core switches rather than via the router, which improves performance.
  • To enable VLANs, a subnet of routers/switches must aid them. Although other proprietary methods for configured VLANs exist, IEEE 802.1Q is the most extensively utilized. Although VLAN-supporting switches are frequently referred to as "managed" switches, this is a marketing term that does not always imply VLAN support. Almost all routers, wireless solutions, and most subnets of switches aid the 802.1Q VLAN protocol, which is powered by third-party hardware.
  • Depending on the router model and physical interface type, the router can be detached into 4095 different VLANs by combining analytical interfaces with suitable VLAN IDs. The VLAN ID 0 is used to denote the law's priority. Normal VLANs are allotted VLAN IDs 1 through 511. Circuit cross-linking between VLANs is engaged for VLAN IDs 512 and higher (CCCs).
  • For Gigabit Ethernet PICs with Gigabit Ethernet IQ interfaces and SFPs, flexible ethernet services integration can be specified in the physical interface. The maximum number of user-configured VLANs on each port of the DENS-FE PIC (8-port / 12-port / 48-port) is 15.

VLAN Tagging

Depending on the hardware vendor, VLAN tagging has a wide range of definitions and applications. VLAN tagging is a technique for managing many VLANs on a single port. VLAN tagging is used to designate which VLAN that same machine belongs to. The VLAN tag is placed in the ethernet frame to aid authentication. A specific VLAN tag is placed into an ethernet network when it crosses the trunk connection and is sent throughout the trunk connection.

Untagged VLAN, also known as native VLAN, is an IEEE 802.1Q concept in which the trunk connection is not tagged for backward compatibility with IEEE 802.1Q unknown devices. The untagged VLAN removes tags in VLAN. The untagged packets don't carry VLAN ID. In untagged VLAN, only switch port is configured in access port. The untagged VLAN uses a cisco access port. The VLAN ID is specified in an IEEE 802.1Q header included in the Ethernet network, allowing 802.1Q capable devices to know which VLAN data packet belongs to.

This VLAN ID tag can be added/removed by the host, router, or switch. The network's physical ports are either tagged/untagged for every VLAN, deciding whether it accepts and sends communication for that VLAN ID.

  • Untagged: "Native VLAN" is a term used to describe untagged VLAN. Any communication that sends the VLAN ID from the host to the undefined switchboard will be allotted to the unlabeled VLAN.
  • This possibility is utilized to link hosts that do not depict their traffic and must interact through a VLAN, like workstations/IP cameras. A port can have one unlabeled VLAN system at a time. 
  • The port is attached to the VLAN when a VLAN tag is allocated to it, but all entrance and exit traffic must be a VLAN tag with the VLAN ID. The host attaching to the switchboard must utilize the same VLAN ID for all of its traffic.
  • When attaching to a host that requires simultaneous access to various networks via a single convergence, like a server offering services to multiple departments in an office, VLAN tagged on one port is commonly utilized. When copulating two switches, it can also be used to curtail access to VLANs to hosts attached to a downlink switch for security deeds.
  • Trunk: The trunk port is usually used for uplink and downlink ports between switches and routers, and it is a constituent of all VLANs. It can accept and transmit traffic to and from any VLAN ID. 

Difference between LAN and VLAN

A local area network (LAN) is a collection of computers and peripherals connected to a single physical location. A virtual local area network (VLAN) is a customized network made up of one/ more LANs. LAN has too much delay. The latency of a VLAN is quite low. The LAN has a big price tag. It is less expensive to utilize a VLAN.

Context and Applications

This topic is important for postgraduate and undergraduate courses, particularly for, 

  • Bachelors in Computer Science Engineering.
  • Associate of Science in Computer Science.

Practice Problems

Question 1: Which VLANs are allowed over a new trunk link when configured on an IOS-based switch?

  1. No VLANs are allowed; each VLAN must be configured manually.
  2. Default VLANs are allowed on the truck.
  3. Only configured VLANs are allowed.
  4. None of these

Answer: Option B is correct.

Explanation: By default, all VLANs are allowed on the trunk connection, and VLANs that do not want to cross the trunked connection must be manually removed.

Question 2: Network speeds should be boosted by giving hosts additional bandwidth and limiting the number of broadcast domains. Which of the following options will assist you in achieving your objective?

  1. Controlled hubs
  2. Bridges 
  3. Switches
  4. None of these

Answer: Option C is correct.

Explanation: By building and activating VLANs on your switched network, you can break down broadcast domains in Layer-2. To connect between hosts on different VLANs, a router or Layer 3 switches are required.

Question 3: Which of the following allows VLAN connection between switches?

  1. ISL
  2. VTP
  3. 802.1q
  4. 802.3Z

  1. only 2
  2. only 2 and 4
  3. only 1 and 3
  4. None of these

Answer: Option C is correct.

Explanation: Cisco's proprietary frame-tagging technique is called ISL. The non-proprietary version of frame tagging is IEEE 802.1Q.

Question 4: In one switch, three distinct VLANs are configured: VLAN2, VLAN3, and VLAN4. A router is used to communicate between VLANs. What type of router interface is necessary if only one connection is to be created between the router and the switch?

  1. Ethernet 10 Mbps
  2. Ethernet 100Mbps
  3. Serial 56Kbps 
  4. None of these

Answer: Option B is correct.

Explanation: Although 100Mbps or 1Gbps Ethernet can be used, 100Mbps is the ideal option for this issue because it is the smallest. You must unplug the router's connection from the switch to enable this connection with the VLAN inter-VLAN connection.

Question 5: Which VTP mode on the switch permits VLAN data to be transferred?

  1. Server
  2. STP 
  3. Client
  4. None of these

Answer: Option A is correct.

Explanation: VTP data on a switch can be changed in server mode only.

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