Vlans


VLAN Overview
The solution for the community college is to use a networking technology called a virtual LAN (VLAN). A VLAN allows a network administrator to create groups of logically networked devices that act as if they are on their own independent network, even if they share a common infrastructure with other VLANs. When you configure a VLAN, you can name it to describe the primary role of the users for that VLAN. As another example, all of the student computers in a school can be configured in the "Student" VLAN. Using VLANs, you can logically segment switched networks based on functions, departments, or project teams. You can also use a VLAN to geographically structure your network to support the growing reliance of companies on home-based workers. In the figure, one VLAN is created for students and another for faculty. These VLANs allow the network administrator to implement access and security policies to particular groups of users. For example, the faculty, but not the students, can be allowed access to e-learning management servers for developing online course materials.
VLAN Details
A VLAN is a logically separate IP subnetwork. VLANs allow multiple IP networks and subnets to exist on the same switched network. The figure shows a network with three computers. For computers to communicate on the same VLAN, each must have an IP address and a subnet mask that is consistent for that VLAN. The switch has to be configured with the VLAN and each port in the VLAN must be assigned to the VLAN. A switch port with a singular VLAN configured on it is called an access port. Remember, just because two computers are physically connected to the same switch does not mean that they can communicate. Devices on two separate networks and subnets must communicate via a router (Layer 3), whether or not VLANs are used. You do not need VLANs to have multiple networks and subnets on a switched network, but there are definite advantages to using VLANs.
Benefits of a VLAN
User productivity and network adaptability are key drivers for business growth and success. Implementing VLAN technology enables a network to more flexibly support business goals. 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. Faculty computers are on VLAN 10 and completely separated from student and guest data traffic.
Cost reduction - Cost savings result from less need for expensive network upgrades and more efficient use of existing bandwidth and uplinks.
Higher performance - Dividing flat Layer 2 networks into multiple logical workgroups (broadcast domains) reduces unnecessary traffic on the network and boosts performance.
Broadcast storm mitigation - Dividing a network into VLANs reduces the number of devices that may participate in a broadcast storm. As discussed in the "Configure a Switch" chapter, LAN segmentation prevents a broadcast storm from propagating to the whole network. In the figure you can see that although there are six computers on this network, there are only 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 you provision a new switch, 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 20 could be named "Student", VLAN 10 could be named "Faculty", and VLAN 30 "Guest."
Simpler project or 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, for example, an e-learning development platform for faculty. It is also easier to determine the scope of the effects of upgrading network services.
VLAN ID Ranges
Access VLANs are divided into either a normal range or an extended range.
Normal Range VLANs
Used in small- and medium-sized business and enterprise networks.
Identified by a VLAN ID between 1 and 1005.
IDs 1002 through 1005 are reserved for Token Ring and FDDI VLANs.
IDs 1 and 1002 to 1005 are automatically created and cannot be removed. You will learn more about VLAN 1 later in this chapter.
Configurations are stored within a VLAN database file, called vlan.dat. The vlan.dat file is located in the flash memory of the switch.
The VLAN trunking protocol (VTP), which helps manage VLAN configurations between switches, can only learn normal range VLANs and stores them in the VLAN database file.
Extended Range VLANs
Enable service providers to extend their infrastructure to a greater number of customers. Some global enterprises could be large enough to need extended range VLAN IDs.
Are identified by a VLAN ID between 1006 and 4094.
Support fewer VLAN features than normal range VLANs.
Are saved in the running configuration file.
VTP does not learn extended range VLANs.
255 VLANs Configurable
One Cisco Catalyst 2960 switch can support up to 255 normal range and extended range VLANs, although the number configured affects the performance of the switch hardware. Because an enterprise network may need a switch with a lot of ports, Cisco has developed enterprise-level switches that can be joined or stacked together to create a single switching unit consisting of nine separate switches. Each separate switch can have 48 ports, which totals 432 ports on a single switching unit. In this case, the 255 VLAN limit per single switch could be a constraint for some enterprise customers.

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