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VLAN’s are a very useful tool in an enterprise network,they segment each departments data using a single switch and trunking.

Here we will discuss the basics of VLAN’s in a cisco internetwork.

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.

Benefits of a VLAN
Security – Groups that have sensitive data are separated from the rest of the network, decreasing the chances of confidential information breaches.

Broadcast storm mitigation – Dividing a network into VLANs reduces the number of devices that may participate in a broadcast storm.

Higher performance – Dividing flat Layer 2 networks into multiple logical workgroups (broadcast domains) reduces unnecessary traffic on the network and boosts performance

Improved IT staff efficiency – VLANs make it easier to manage the network because users with similar network requirements share the same VLAN

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.

The 80/20 & 20/80 rule

Today’s networks are engineered, based on traffic flow patterns, to have 80 percent of the traffic contained within a VLAN.
The remaining 20 percent crosses the router to the enterprise servers and to the Internet and WAN.
This is known as the 80/20 rule.
Note:
With today’s traffic patterns, this rule is becoming obsolete.
The 20/80 rule applies to many of today’s networks, with 20% of the traffic within a VLAN, and 80% outside the VLAN.

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