VLAN or Virtual Local Area Network is a phenomenon which is used to logically separate or combine a network. It is used to configured one or more devices, so that they can communicate, as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.
VLAN is a concept of partitioning of a physical network, so that distinct broadcast domains are created. This is usually achieved on switch or router devices. Simpler devices only support partitioning on a port level, so sharing VLANs across devices requires running dedicated cabling for each VLAN.
Grouping hosts with a common set of requirements regardless of their physical location by VLAN can greatly simplify network design. A VLAN has the same attributes as a physical local area network (LAN), but it allows for end stations to be grouped together more easily even if they are not on the same network switch. Without VLANs, a switch considers all interfaces on the switch to be in the same broadcast domain.To physically replicate the functions of a VLAN would require a separate, parallel collection of network cables and equipment separate from the primary network.
Benefits of VLANs
User productivity and network adaptability are important for business growth and success. VLANs make it easier to design a network to support the goals of an organization. The primary benefits of using VLANs are as follows:
- Security – Groups that have sensitive data are separated from the rest of the network, decreasing the chances of confidential information breaches. As shown in the figure, faculty computers are on VLAN 10 and completely separated from student and guest data traffic.
- Cost reduction – Cost savings result from reduced need for expensive network upgrades and more efficient use of existing bandwidth and uplinks.
- Better performance – Dividing flat Layer 2 networks into multiple logical workgroups (broadcast domains) reduces unnecessary traffic on the network and boosts performance.
- Shrink broadcast domains – Dividing a network into VLANs reduces the number of devices in the broadcast domain. As shown in the figure, there are six computers on this network but there are three broadcast domains: Faculty, Student, and Guest.
- Improved IT staff efficiency – VLANs make it easier to manage the network because users with similar network requirements share the same VLAN. When a new switch is provisioned, all the policies and procedures already configured for the particular VLAN are implemented when the ports are assigned. It is also easy for the IT staff to identify the function of a VLAN by giving it an appropriate name. In the figure, for easy identification VLAN 10 has been named “Faculty”, VLAN 20 is named “Student”, and VLAN 30 “Guest.”
- Simpler project and application management – VLANs aggregate users and network devices to support business or geographic requirements. Having separate functions makes managing a project or working with a specialized application easier; an example of such an application is an e-learning development platform for faculty.
Types of VLANs
There are a number of distinct types of VLANs used in modern networks. Some VLAN types are defined by traffic classes. Other types of VLANs are defined by the specific function that they serve.
A data VLAN is a VLAN that is configured to carry user-generated traffic. A VLAN carrying voice or management traffic would not be part of a data VLAN. It is common practice to separate voice and management traffic from data traffic. A data VLAN, is sometimes referred to as a user VLAN. Data VLANs are used to separate the network into groups of users or devices.
All switch ports become a part of the default VLAN after the initial boot up of a switch loading the default configuration. Switch ports that participate in the default VLAN are part of the same broadcast domain. This allows any device connected to any switch port to communicate with other devices on other switch ports. The default VLAN for Cisco switches is VLAN 1. In the figure, the show vlan brief command was issued on a switch running the default configuration. Notice that all ports are assigned to VLAN 1 by default.
VLAN 1 has all the features of any VLAN, except it cannot be renamed or deleted. By default, all Layer 2 control traffic is associated with VLAN 1.
A native VLAN is assigned to an 802.1Q trunk port. Trunk ports are the links between switches that support the transmission of traffic associated with more than one VLAN. An 802.1Q trunk port supports traffic coming from many VLANs (tagged traffic), as well as traffic that does not come from a VLAN (untagged traffic). The 802.1Q trunk port places untagged traffic on the native VLAN, which by default is VLAN 1.
Native VLANs are defined in the IEEE 802.1Q specification to maintain backward compatibility with untagged traffic common to legacy LAN scenarios. A native VLAN serves as a common identifier on opposite ends of a trunk link.
It is a best practice to configure the native VLAN as an unused VLAN, distinct from VLAN 1 and other VLANs. In fact, it is not unusual to dedicate a fixed VLAN to serve the role of the native VLAN for all trunk ports in the switched domain.
A management VLAN is any VLAN configured to access the management capabilities of a switch. VLAN 1 is the management VLAN by default. To create the management VLAN, the switch virtual interface (SVI) of that VLAN is assigned an IP address and subnet mask, allowing the switch to be managed via HTTP, Telnet, SSH, or SNMP. Because the out-of-the-box configuration of a Cisco switch has VLAN 1 as the default VLAN, VLAN 1 would be a bad choice for the management VLAN.
In the past, the management VLAN for a 2960 switch was the only active SVI. On 15.x versions of the Cisco IOS for Catalyst 2960 Series switches, it is possible to have more than one active SVI. With Cisco IOS 15.x, the particular active SVI assigned for remote management must be documented. While theoretically a switch can have more than one management VLAN, having more than one increases exposure to network attacks.
A separate VLAN is needed to support Voice over IP (VoIP). VoIP traffic requires:
- Assured bandwidth to ensure voice quality
- Transmission priority over other types of network traffic
- Ability to be routed around congested areas on the network
- Delay of less than 150 ms across the network
To meet these requirements, the entire network has to be designed to support VoIP. The details of how to configure a network to support VoIP are beyond the scope of this course, but it is useful to summarize how a voice VLAN works between a switch, a Cisco IP phone, and a computer.
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