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VLANs and Trunks

June 11, 2015
Alan Thomas


When properly utilized, VLANs and trunks provide flexibility, stability and ease of troubleshooting. This paper provides technical details about VLANs and trunks, along with design options at a basic to intermediate level. Recommendations and commands are included throughout.


Virtual Local Area Networks (VLANs) provide several benefits to enterprise networks. VLANs provide a measure of flexibility, improve user mobility, ease the application of security measures, and increase the overall efficiency of the network.

Trunks also provide benefits, specifically the ability to reduce the number of physical connections needed between switches in order to support multiple VLANs.

This paper describes VLANs and trunks. It includes an overview of Ethernet-based computer networks, which includes definitions of several terms. This paper also includes technical details about VLANs and trunks, along with design options at a basic to intermediate level. Recommendations and commands are included throughout.


The Open Systems Interconnect (OSI) communications model is a seven-layer reference model that describes the functions necessary for two endpoints to communicate. The 2nd Layer of the OSI model is called the Data Link Layer. Its functions are to format data for transmission on the physical media and to define how devices access the physical media (twisted pair copper, fiber optic, or wireless).

Ethernet is the Layer 2 protocol used for devices that connect to a Local Area Network (LAN). Ethernet defines how data is formatted for transmission by creating frames. Figure 1 shows a sample Ethernet frame.

When data is transmitted along the media, the data is just a series of bits. Ethernet defines how the sending and receiving device will interpret those bits. The key fields of an Ethernet frame are the destination and source address fields, the type field, and the Frame Check Sequence (FCS) field.

The destination and source address fields contain the Media Access Control (MAC) addresses of the receiving and sending devices respectively as defined by standards from the Institute of Electrical and Electronics Engineers (IEEE). The type field contains a value identifying the next layer protocol. For example, a value of 0x0800 (0x indicates the numbers that follow are hexadecimal numbers) means IPv4 is the next layer protocol; 0x86DD means IPv6; and 0x0806 means Address Resolution Protocol (ARP). The FCS field is used for error detection. Its value is calculated by the sending device and attached to the end of the frame. As the frame is received, the receiving device performs the same calculation. If the values match, the receiving device knows the frame is error free. If the values do not match, the receiving device knows an error occurred during transmission and the frame is discarded.

Another term for LAN is broadcast domain. A broadcast domain is the most basic of computer networks. It is defined as a collection of devices that receive broadcast frames from each other at Layer 2. A broadcast frame is one that is destined (addressed) to every device on the LAN by using the value 0xffff.ffff.ffff in the destination address field. This type of frame is flooded out of every interface of an Ethernet switch, except the interface on which the frame was received. This is very inefficient as the switch has to forward (or replicate) a copy of the frame for every active interface. For example, a fully populated 48-port switch has to forward 47 copies of every broadcast frame it receives. Additionally, the end hosts that receive the broadcast frame are required to perform some processing of the frame, even if the broadcast frame does not contain data that is pertinent to the host.

Originally, broadcast domains (LANs) were implemented based on physical location. The first implementation of Ethernet was designed for use with coaxial cable. In this environment, all devices connected to a single piece of coaxial cable; in other words, the piece of coaxial cable was the LAN media, and the LAN was limited to a single room or maybe a group of two or three rooms. Eventually, Ethernet was updated to transmit data over twisted pair copper wiring, at which point hubs replaced coaxial cable as the LAN. However, whether the LAN was a piece of coaxial cable or a hub, the LAN was limited to devices that shared a physical media.

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