Virtual LANs

A router between sections of the LAN can help keep broadcast domains to a suitable size, providing efficient network communication.

Virtual LANs help keep broadcast domains small by providing seperation between hosts on the same network switch. This can allow a network switch to be better utilised by providing different subnetworks across the same network switch.

Host devices in one VLAN can not communicate with network devices in another VLAN without the use of a Layer 3 device (such as a multi layer switch or router) to route between the networks.

VLANs are defined as a Institue of Electrical and Electronic Engineers (IEEE) standard of 802.1Q or commonly known as dot1q. 801.1Q states that 32 bits are added to each network packet header in the following fields:

Destination MACSource MACTPID
(16 bits)
(3 bits)
(1 bit)
VLAN ID (12 bit)Destination IPSource IPData Payload
The header of a tagged VLAN packet, additions in bold.

Tag protocol identifier (TPID) is a 16-bit field set to 0x8100 to identify the packet is a 802.1Q packet. The field is set in the same position where the EtherType field would normally sit, so it is easy for network devices to determine if a packet is tagged or not.

Priority code point (PCP) indicates a class of service as part of a quality of service configuration between network switches.

Dropped Eligible Indicator (DEI) this 1 bit field indicates to the recieving network switch if this packet can be dropped when bandwidth is contented.

VLAN identifier (VLAN ID) specifies the VLAN that the network packet is assiociated with. This field is 12 bits in length which can provide a VLAN ID value of up to 4094.

Some VLANs numbers are reserved:

VLAN0 is reserved for 802.1P traffic and can not be modified or deleted.

VLAN1 is the default VLAN and can not be modified or deleted.

VLAN 1002 to 1005 are reserved by the Cisco Catalyst operating system and can not be modified or deleted.

In the VLAN configuration mode on a Cisco switch, it can be given a name of up to 32 characters.

To create a VLAN, in configuration mode enter the following commands.

We’ll create a VLAN of 400 with the name VoIP:

sw(config)# vlan 400
sw(config-vlan)# name voip
sw(config-vlan)# exit

The VLAN will not be created until we exit the VLAN configuraiton mode (config-vlan).

VLANs can be viewed on the network switch via the show vlan command, optional keywords can be added to the end to change the output:

  • show vlan brief – Displays only the relevant port to VLAN mappings.
  • show vlan summary – Displays the count of VLANs, which VLANS take part in VTP and VLANs which are in the extended VLAN range (1006-4094)
  • show vlan id 400 – Displays all output from the show vlan command except filters all other output except for that relevant to vlan 400.
  • show vlan name voip – Similar to show vlan id 400 but filters the output by name rather than an ID number.



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