VLAN


A virtual local area network is a local area network broadcast domain that is partitioned and isolated in a virtual network at the data link layer. A VLAN behaves like a virtual network switch or network link that can share the same physical structure with other VLANs while staying logically separate from them.
VLANs work by applying tags to network frames that are forwarded within the broadcast domain, creating the appearance and functionality of network traffic that behaves as if it were split between separate networks. In this way, VLANs can keep network applications separate despite being connected to the same physical network, and without requiring multiple sets of cabling and networking devices to be deployed.
VLANs allow network administrators to group hosts together even if the hosts are not directly connected to the same network switch. Because VLAN membership can be configured through software, this can greatly simplify network design and deployment. Without VLANs, grouping hosts according to their resources requires relocating nodes or rewiring data links. VLANs allow devices that must be kept separate to share the cabling of a physical network and yet be prevented from directly interacting with one another. This managed sharing yields gains in simplicity, security, traffic control, and economy.
Many Internet hosting services use VLANs to separate customers' private zones from one another, enabling each customer's servers to be grouped within a single network segment regardless of where the individual servers are located in the data center. Some precautions are needed to prevent traffic "escaping" from a given VLAN, an exploit known as VLAN hopping.
To subdivide a network into VLANs, one configures network equipment. Simpler equipment might partition only each physical port, in which case each VLAN runs over a dedicated network cable. More sophisticated devices can mark frames through VLAN tagging, so that a single interconnect may be used to transport data for multiple VLANs. Since VLANs share bandwidth, a VLAN trunk can use link aggregation, quality-of-service prioritization, or both to route data efficiently.

Uses

VLANs address issues such as scalability, security, and network management. Network architects set up VLANs to provide network segmentation. Routers between VLANs filter broadcast traffic, enhance network security, perform address summarization, and mitigate network congestion.
In a network utilizing broadcasts for service discovery, address assignment and resolution and other services, as the number of peers on a network grows, the frequency of broadcasts also increases. VLANs can help manage broadcast traffic by forming multiple broadcast domains. Breaking up a large network into smaller independent segments reduces the amount of broadcast traffic each network device and network segment has to bear. Switches may not bridge network traffic between VLANs, as doing so would violate the integrity of the VLAN broadcast domain.
VLANs can also help create multiple layer 3 networks on a single physical infrastructure. VLANs are data link layer constructs, analogous to Internet Protocol subnets, which are network layer constructs. In an environment employing VLANs, a one-to-one relationship often exists between VLANs and IP subnets, although it is possible to have multiple subnets on one VLAN.
Without VLAN capability, users are assigned to networks based on geography and are limited by physical topologies and distances. VLANs can logically group networks to decouple the users' network location from their physical location. By using VLANs, one can control traffic patterns and react quickly to employee or equipment relocations. VLANs provide the flexibility to adapt to changes in network requirements and allow for simplified administration.
VLANs can be used to partition a local network into several distinctive segments, for instance:
A common infrastructure shared across VLAN trunks can provide a measure of security with great flexibility for a comparatively low cost. Quality of service schemes can optimize traffic on trunk links for real-time or low-latency requirements. However, VLANs as a security solution should be implemented with great care as they can be defeated unless implemented carefully.
In cloud computing VLANs, IP addresses, and MAC addresses in the cloud are resources that end users can manage. To help mitigate security issues, placing cloud-based virtual machines on VLANs may be preferable to placing them directly on the Internet.
Network technologies with VLAN capabilities include:
After successful experiments with voice over Ethernet from 1981 to 1984, W. David Sincoskie joined Bellcore and began addressing the problem of scaling up Ethernet networks. At 10 Mbit/s, Ethernet was faster than most alternatives at the time. However, Ethernet was a broadcast network and there was no good way of connecting multiple Ethernet networks together. This limited the total bandwidth of an Ethernet network to 10 Mbit/s and the maximum distance between nodes to a few hundred feet.
By contrast, although the existing telephone network's speed for individual connections was limited to 56 kbit/s, the total bandwidth of that network was estimated at 1 Tbit/s.
Although it was possible to use IP routing to connect multiple Ethernet networks together, it was expensive and relatively slow. Sincoskie started looking for alternatives that required less processing per packet. In the process, he independently reinvented transparent bridging, the technique used in modern Ethernet switches. However, using switches to connect multiple Ethernet networks in a fault-tolerant fashion requires redundant paths through that network, which in turn requires a spanning tree configuration. This ensures that there is only one active path from any source node to any destination on the network. This causes centrally located switches to become bottlenecks, limiting scalability as more networks are interconnected.
To help alleviate this problem, Sincoskie invented VLANs by adding a tag to each Ethernet frame. These tags could be thought of as colors, say red, green, or blue. In this scheme, each switch could be assigned to handle frames of a single color, and ignore the rest. The networks could be interconnected with three spanning trees, one for each color. By sending a mix of different frame colors, the aggregate bandwidth could be improved. Sincoskie referred to this as a multitree bridge. He and Chase Cotton created and refined the algorithms necessary to make the system feasible. This color is what is now known in the Ethernet frame as the IEEE 802.1Q header, or the VLAN tag. While VLANs are commonly used in modern Ethernet networks, they are not used in the manner first envisioned here.
In 1998, Ethernet VLANs were described in the first edition of the IEEE 802.1Q-1998 standard. This was extended with IEEE 802.1ad to allow nested VLAN tags in service of provider bridging. This mechanism was improved with IEEE 802.1ah-2008.

Configuration and design considerations

Early network designers often segmented physical LANs with the aim of reducing the size of the Ethernet collision domain—thus improving performance. When Ethernet switches made this a non-issue, attention turned to reducing the size of the data link layer broadcast domain. VLANs were first employed to separate several broadcast domains across one physical medium. A VLAN can also serve to restrict access to network resources without regard to physical topology of the network.
VLANs operate at the data link layer of the OSI model. Administrators often configure a VLAN to map directly to an IP network, or subnet, which gives the appearance of involving the network layer. Generally, VLANs within the same organization will be assigned different non-overlapping network address ranges. This is not a requirement of VLANs. There is no issue with separate VLANs using identical overlapping address ranges. However, it is not possible to route data between two networks with overlapping addresses without delicate IP remapping, so if the goal of VLANs is segmentation of a larger overall organizational network, non-overlapping addresses must be used in each separate VLAN.
A basic switch that is not configured for VLANs has VLAN functionality disabled or permanently enabled with a default VLAN that contains all ports on the device as members. The default VLAN typically uses VLAN identifier 1. Every device connected to one of its ports can send packets to any of the others. Separating ports by VLAN groups separates their traffic very much like connecting each group using a distinct switch for each group.
Remote management of the switch requires that the administrative functions be associated with one or more of the configured VLANs.
In the context of VLANs, the term trunk denotes a network link carrying multiple VLANs, which are identified by labels inserted into their packets. Such trunks must run between tagged ports of VLAN-aware devices, so they are often switch-to-switch or switch-to-router links rather than links to hosts.. A router serves as the backbone for network traffic going across different VLANs. It is only when the VLAN port group is to extend to another device that tagging is used. Since communications between ports on two different switches travel via the uplink ports of each switch involved, every VLAN containing such ports must also contain the uplink port of each switch involved, and traffic through these ports must be tagged.
Switches typically have no built-in method to indicate VLAN to port associations to someone working in a wiring closet. It is necessary for a technician to either have administrative access to the device to view its configuration, or for VLAN port assignment charts or diagrams to be kept next to the switches in each wiring closet.