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VLAN (Virtual Local Area Network)

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VLAN (Virtual Local Area Network)

VLAN (Virtual Local Area Network) in computer networking, a single layer-2 network may be partitioned to create multiple distinct broadcast domains, which are mutually isolated so that packets can only pass between them via one or more routers; such a domain is referred to as a virtual local area network, virtual LAN or VLAN.

A virtual local area network (VLAN) is a logical group of workstations, servers and network devices that appear to be on the same LAN despite their geographical distribution. A VLAN allows a network of computers and users to communicate in a simulated environment as if they exist in a single LAN and are sharing a single broadcast and multicast domain. VLANs are implemented to achieve scalability, security and ease of network management and can quickly adapt to change in network requirements and relocation of workstations and server nodes.

Higher-end switches allow the functionality and implementation of VLANs. The purpose of implementing a VLAN is to improve the performance of a network or apply appropriate security features.

A VLAN allows several networks to work virtually as an LAN. One of the most beneficial elements of a VLAN is that it removes latency in the network, which saves network resources and increases network efficiency. In addition, VLANs are created to provide segmentation and assist in issues like security, network management and scalability. Traffic patterns can also easily be controlled by using VLANs.

VLANs are configured through software rather than hardware, which makes them extremely flexible. One of the biggest advantages of VLANs is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration.

Physical View of VLAN
Physical View of VLAN
Logical View of VLAN
Logical View of VLAN

VLAN’s also allow broadcast domains to be defined without using routers. Bridging software is used instead to define which workstations are to be included in the broadcast domain. Routers would only have to be used to communicate between two VLAN’s

VLANs can be used to partition a local network into several distinctive segments, for example:

  1. Production
  2. Voice over IP
  3. Network management
  4. Storage area network (SAN)
  5. Guest network
  6. Demilitarized zone (DMZ)
  7. Client separation (ISP)

In a common infrastructure shared across VLAN trunks can provide a very high level of security with great flexibility for a comparatively low cost. Quality of Service schemes can optimize traffic on trunk links for realtime (VoIP) or low-latency requirements (Storage Area Network).

History

After successful experiments with Voice over Ethernet from 1981 to 1984, Dr. 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 any two nodes to a few hundred feet.

Why Use VLAN’s?

VLAN’s offer a number of advantages over traditional LAN’s. They are:

Performance

In networks where traffic consists of a high percentage of broadcasts and multicasts, VLAN’s can reduce the need to send such traffic to unnecessary destinations. For example, in a broadcast domain consisting of 10 users, if the broadcast traffic is intended only for 5 of the users, then placing those 5 users on a separate VLAN can reduce traffic.

Compared to switches, routers require more processing of incoming traffic. As the volume of traffic passing through the routers increases, so does the latency in the routers, which results in reduced performance. The use of VLAN’s reduces the number of routers needed, since VLAN’s create broadcast domains using switches instead of routers.

Formation of Virtual Workgroups

Nowadays, it is common to find cross-functional product development teams with members from different departments such as marketing, sales, accounting, and research. These workgroups are usually formed for a short period of time. During this period, communication between members of the workgroup will be high. To contain broadcasts and multicasts within the workgroup, a VLAN can be set up for them. With VLAN’s it is easier to place members of a workgroup together. Without VLAN’s, the only way this would be possible is to physically move all the members of the workgroup closer together.

However, virtual workgroups do not come without problems. Consider the situation where one user of the workgroup is on the fourth floor of a building, and the other workgroup members are on the second floor. Resources such as a printer would be located on the second floor, which would be inconvenient for the lone fourth floor user.

Another problem with setting up virtual workgroups is the implementation of centralized server farms, which are essentially collections of servers and major resources for operating a network at a central location. The advantages here are numerous, since it is more efficient and cost-effective to provide better security, uninterrupted power supply, consolidated backup, and a proper operating environment in a single area than if the major resources were scattered in a building. Centralized server farms can cause problems when setting up virtual workgroups if servers cannot be placed on more than one VLAN. In such a case, the server would be placed on a single VLAN and all other VLAN’s trying to access the server would have to go through a router; this can reduce performance.

Simplified Administration

Seventy percent of network costs are a result of adds, moves, and changes of users in the network [ Buerger]. Every time a user is moved in a LAN, recabling, new station addressing, and reconfiguration of hubs and routers becomes necessary. Some of these tasks can be simplified with the use of VLAN’s. If a user is moved within a VLAN, reconfiguration of routers is unnecessary. In addition, depending on the type of VLAN, other administrative work can be reduced or eliminated [ Cisco white paper]. However the full power of VLAN’s will only really be felt when good management tools are created which can allow network managers to drag and drop users into different VLAN’s or to set up aliases.

Despite this saving, VLAN’s add a layer of administrative complexity, since it now becomes necessary to manage virtual workgroups.

Reduced Cost

VLAN’s can be used to create broadcast domains which eliminate the need for expensive routers.

Security

Periodically, sensitive data may be broadcast on a network. In such cases, placing only those users who can have access to that data on a VLAN can reduce the chances of an outsider gaining access to the data. VLAN’s can also be used to control broadcast domains, set up firewalls, restrict access, and inform the network manager of an intrusion.

VLANs also have some disadvantages and limitations as listed below:

  1. High risk of virus issues because one infected system may spread a virus through the whole logical network.
  2. Equipment limitations in very large networks because additional routers might be needed to control the workload.
  3. More effective at controlling latency than a WAN but less efficient than a LAN.

Establishing VLAN Memberships

The two common approaches to assigning VLAN membership are as follows:

Static VLANs

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

Dynamic VLANs are created using software. With a VLAN Management Policy Server (VMPS), an administrator can assign switch ports to VLANs dynamically based on information such as the source MAC address of the device connected to the port or the username used to log onto that device. As a device enters the network, the switch queries a database for the VLAN membership of the port that device is connected to.

Types of VLAN’s

VLAN membership can be classified by port, MAC address, and protocol type:

Layer 1 VLAN: Membership by Port

Membership in a VLAN can be defined based on the ports that belong to the VLAN. For example, in a bridge with four ports, ports 1, 2, and 4 belong to VLAN 1 and port 3 belongs to VLAN 2.

Assignment of ports to different VLAN’s:

Port VLAN
1 1
2 1
3 2
4 1

The main disadvantage of this method is that it does not allow for user mobility. If a user moves to a different location away from the assigned bridge, the network manager must reconfigure the VLAN.

Layer 2 VLAN: Membership by MAC Address

Here, membership in a VLAN is based on the MAC address of the workstation. The switch tracks the MAC addresses which belong to each VLAN (see Figure4). Since MAC addresses form a part of the workstation’s network interface card, when a workstation is moved, no reconfiguration is needed to allow the workstation to remain in the same VLAN. This is unlike Layer 1 VLAN’s where membership tables must be reconfigured.

Assignment of MAC addresses to different VLAN’s:

MAC Address VLAN
1212354145121 1
2389234873743 2
3045834758445 2
5483573475843 1

The main problem with this method is that VLAN membership must be assigned initially. In networks with thousands of users, this is no easy task. Also, in environments where notebook PC’s are used, the MAC address is associated with the docking station and not with the notebook PC. Consequently, when a notebook PC is moved to a different docking station, its VLAN membership must be reconfigured.

Layer 2 VLAN: Membership by Protocol Type

VLAN membership for Layer 2 VLAN’s can also be based on the protocol type field found in the Layer 2 header.

Assignment of protocols to different VLAN’s:

Protocol VLAN
IP 1
IPX 2

Layer 3 VLAN: Membership by IP Subnet Address

Membership is based on the Layer 3 header. The network IP subnet address can be used to classify VLAN membership.

Assignment of IP subnet addresses to different VLAN’s:

IP Subnet VLAN
23.2.24 1
26.21.35 2

Although VLAN membership is based on Layer 3 information, this has nothing to do with network routing and should not be confused with router functions. In this method, IP addresses are used only as a mapping to determine membership in VLAN’s. No other processing of IP addresses is done.

In Layer 3 VLAN’s, users can move their workstations without reconfiguring their network addresses. The only problem is that it generally takes longer to forward packets using Layer 3 information than using MAC addresses.

Higher Layer VLAN’s

It is also possible to define VLAN membership based on applications or service, or any combination thereof. For example, file transfer protocol (FTP) applications can be executed on one VLAN and telnet applications on another VLAN.

The 802.1Q draft standard defines Layer 1 and Layer 2 VLAN’s only. Protocol type based VLAN’s and higher layer VLAN’s have been allowed for, but are not defined in this standard. As a result, these VLAN’s will remain proprietary.

Types of Connections

Devices on a VLAN can be connected in three ways based on whether the connected devices are VLAN-aware or VLAN-unaware. Recall that a VLAN-aware device is one which understands VLAN memberships (i.e. which users belong to a VLAN) and VLAN formats.

Trunk Link

All the devices connected to a trunk link, including workstations, must be VLAN-aware. All frames on a trunk link must have a special header attached. These special frames are called tagged frames.

Trunk link between two VLAN-aware bridges.
Trunk link between two VLAN-aware bridges

Access Link

An access link connects a VLAN-unaware device to the port of a VLAN-aware bridge. All frames on access links must be implicitly tagged (untagged). The VLAN-unaware device can be a LAN segment with VLAN-unaware workstations or it can be a number of LAN segments containing VLAN-unaware devices (legacy LAN).

Access link between a VLAN-aware bridge and a VLAN-unaware device
Access link between a VLAN-aware bridge and a VLAN-unaware device

Hybrid Link

This is a combination of the previous two links. This is a link where both VLAN-aware and VLAN-unaware devices are attached. A hybrid link can have both tagged and untagged frames, but allthe frames for a specific VLAN must be either tagged or untagged.

Hybrid link containing both VLAN-aware and VLAN-unaware devices
Hybrid link containing both VLAN-aware and VLAN-unaware devices

It must also be noted that the network can have a combination of all three types of links.

Protocol-based VLANs

In a switch that supports protocol-based VLANs, traffic is 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.

For example, it is possible to connect the following to a given switch:

  1. A host generating ARP traffic to port 10
  2. A network with IPX traffic to port 20
  3. A router forwarding IP traffic to port 30

If a protocol-based VLAN is created that supports IP and contains all three ports, this prevents IPX traffic from being forwarded to ports 10 and 30, and ARP traffic from being forwarded to ports 20 and 30, while still allowing IP traffic to be forwarded on all three ports.

Protocols and Design

  1. IEEE 802.1Q
  2. Cisco VLAN Trunking Protocol (VTP)
  3. Multiple VLAN Registration Protocol
  4. Shortest Path Bridging

Default Ethernet VLAN Configuration

The switch module supports only Ethernet interfaces. Table shows the default configuration for Ethernet VLANs.

Parameter Default Range
VLAN ID 1 1 to 4094.
Note: Extended-range VLANs (VLAN IDs 1006 to 4094) are not saved in the VLAN database.
VLAN name VLANxxxx, where xxxx represents four numeric digits (including leading zeros) equal to the VLAN ID number No range
802.10 SAID 100001 (100000 plus the VLAN ID) 1 to 4294967294
MTU size 1500 1500 to 9198
Translational bridge 1 0 0 to1005
Translational bridge 2 0 0 to1005
VLAN state active active, suspend
Remote SPAN disabled enabled, disabled
Private VLANs none configured 2 to 1001, 1006 to 4094
UNI-ENI VLAN UNI-ENI isolated VLAN 2 to 1001, 1006 to 4094.
VLAN 1 is always a UNI-ENI isolated VLAN.

Creating or Modifying an Ethernet VLAN

To access VLAN configuration mode, enter the vlan global configuration command with a VLAN ID. Enter a new VLAN ID to create a VLAN, or enter an existing VLAN ID to modify that VLAN. You can use the default VLAN configuration or enter commands to configure the VLAN.

Switch# configure terminal
Switch(config)# vlan 20
Switch(config-vlan)# name test20
Switch(config-vlan)# end

Assigning Static-Access Ports to a VLAN

You can assign a static-access port to a VLAN.

Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface fastethernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 2
Switch(config-if)# end

Configuring a Trunk Port

Beginning in privileged EXEC mode, follow these steps to configure a port as an 802.1Q trunk port:

Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface fastethernet0/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 33
Switch(config-if)# end

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