VLAN
This article needs additional citations for verification. (January 2014) |
OSI model by layer |
---|
A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2).[2][3] In this context, virtual refers to a physical object recreated and altered by additional logic, within the local area network. VLANs work by applying tags to network frames and handling these tags in networking systems – creating the appearance and functionality of network traffic that is physically on a single network but acts as if it is 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
To subdivide a network into VLANs, one configures
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
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.[3]
VLANs can be used to partition a local network into several distinctive segments, for instance:[4]
- Production
- Voice over IP
- Network management
- Storage area network (SAN)
- Guest Internet access
- Demilitarized zone (DMZ)
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 (e.g. VoIP) or low-latency requirements (e.g. SAN). However, VLANs as a security solution should be implemented with great care as they can be defeated unless implemented carefully.[5]
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.[6]
Network technologies with VLAN capabilities include:[citation needed]
- Asynchronous Transfer Mode (ATM)
- Fiber Distributed Data Interface (FDDI)
- Ethernet
- HiperSockets
- InfiniBand
History
After successful experiments with voice over Ethernet from 1981 to 1984,
By contrast, although the existing telephone network's speed for individual connections was limited to 56 kbit/s (less than one hundredth of Ethernet's speed), the total bandwidth of that network was estimated at 1 Tbit/s[citation needed] (100,000 times greater than Ethernet).
Although it was possible to use
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.[8] 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.[clarification needed]
In 1998, Ethernet VLANs were described in the first edition of the
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 (because each switch port is a collision domain), 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.[a]
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 (e.g. two VLANs each use the private network 192.168.0.0/16). 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.[3] 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 (or tags) 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-
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.
Protocols and design
The protocol most commonly used today to support VLANs is
Both ISL and IEEE 802.1Q perform explicit tagging – the frame itself is tagged with VLAN identifiers. ISL uses an external tagging process that does not modify the Ethernet frame, while 802.1Q uses a frame-internal field for tagging, and therefore does modify the basic Ethernet frame structure. This internal tagging allows IEEE 802.1Q to work on both access and trunk links using standard Ethernet hardware.
IEEE 802.1Q
Under IEEE 802.1Q, the maximum number of VLANs on a given Ethernet network is 4,094 (4,096 values provided by the 12-bit VID field minus reserved values at each end of the range, 0 and 4,095). This does not impose the same limit on the number of IP subnets in such a network since a single VLAN can contain multiple IP subnets. IEEE 802.1ad extends the number of VLANs supported by adding support for multiple, nested VLAN tags. IEEE 802.1aq (Shortest Path Bridging) expands the VLAN limit to 16 million. Both improvements have been incorporated into the IEEE 802.1Q standard.
Cisco Inter-Switch Link
Inter-Switch Link (ISL) is a Cisco proprietary protocol used to interconnect switches and maintain VLAN information as traffic travels between switches on trunk links. ISL is provided as an alternative to IEEE 802.1Q. ISL is available only on some Cisco equipment and has been deprecated.[11]
Cisco VLAN Trunking Protocol
VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol that propagates the definition of VLANs on the whole local area network. VTP is available on most of the
Multiple VLAN Registration Protocol
Multiple VLAN Registration Protocol is an application of Multiple Registration Protocol that allows automatic configuration of VLAN information on network switches. Specifically, it provides a method to dynamically share VLAN information and configure the needed VLANs.
Membership
VLAN membership can be established either statically or dynamically.
Static VLANs are also referred to as port-based VLANs. Static VLAN assignments are created by assigning ports to a VLAN. As a device enters the network, the device automatically assumes the VLAN of the port. If the user changes ports and needs access to the same VLAN, the network administrator must manually make a port-to-VLAN assignment for the new connection.
Dynamic VLANs are created using software or by protocol. With a
Protocol-based VLANs
In a switch that supports protocol-based VLANs, traffic may be handled on the basis of its protocol. Essentially, this segregates or forwards traffic from a port depending on the particular protocol of that traffic; traffic of any other protocol is not forwarded on the port. This allows, for example, IP and IPX traffic to be automatically segregated by the network.
VLAN cross connect
VLAN cross connect (CC or VLAN-XC) is a mechanism used to create Switched VLANs, VLAN CC uses IEEE 802.1ad frames where the S Tag is used as a Label as in
See also
- HVLAN, hierarchical VLAN
- GARP VLAN Registration Protocol
- Network virtualization
- Private VLAN
- Software-defined networking
- Switch virtual interface
- Virtual Extensible LAN (VXLAN)
- Virtual Private LAN Service
- Virtual private network
- VLAN access control list
- Wide area network
Notes
- ^ The strength of VLAN security can be compromised by VLAN hopping. VLAN hopping can be mitigated with proper switchport configuration.[10]
References
- ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
- ^ IEEE 802.1Q-2011, 1. Overview
- ^ a b c IEEE 802.1Q-2011, 1.4 VLAN aims and benefits
- ^ Sharma, Komal; Yadav, Meenu; Pundir, Megha; Malhotra, Isha; Singh, Jaskaran (Nov 2013). "VLAN & its implementation over ATM by using IP: a communication" (PDF). Discovery Engineering. 2 (8). Tamilnadu, India: Discovery Publication: 106–7. Archived from the original (PDF) on 2015-06-18. Retrieved 6 January 2024.
- ^ "Virtual LAN Security: weaknesses and countermeasures", SANS Institute InfoSec Reading Room, SANS Institute, archived from the original on 2017-11-18, retrieved 2018-05-18
- ^ Amies A; Wu C F; Wang G C; Criveti M (21 June 2012), "Networking on the cloud" (PDF), IBM developerWorks, archived from the original (PDF) on 2013-11-01
- ^ Sincoskie, WD (2002) "Broadband packet switching: a personal perspective." IEEE Commun 40: 54-66
- ^ W. D. Sincoskie and C. J. Cotton, "Extended Bridge Algorithms for Large Networks" Archived 2020-01-26 at the Wayback Machine IEEE Network, Jan. 1988.
- ^ IEEE Std. 802.1Q-1998, Virtual Bridged Local Area Networks. 1998. Archived from the original on 2021-01-21. Retrieved 2021-01-14.
- ^ Rik Farrow. "VLAN Insecurity". Archived from the original on 2014-04-21.
- ^ CCNA Exploration LAN Switching and Wireless course, v 4.0, sec 3.2.3
Further reading
- Andrew S. Tanenbaum, 2003, "Computer Networks", Pearson Education International, New Jersey.