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VLANs (Virtual Local Area Networks)


VLANs(Virtual Local Area Networks) are the virtual networks that you can seperate big networks into smaller networks. This can be done for reducing broadcast traffic, network performance improvement, security purpose or to seperate different departments each other and for network flexibility.

vlan(virtual local area network)

Basically VLAN (Virtual Local Area Network)

VLANs are logical networks. In the first place VLANs are defined on the switches and then the ports are assigned to that VLANs. By doing this, VLANs and their member ports appear.

vlan(virtual local area network)

VLAN (Virtual Local Area Network)

Generally fristly two terms are learned by new engineers about computer networks. These terms are collision domain and broadcast domain. It can be good to define these terms again. Because in the VLAN lesson, these terms are ciritically important.

Collision domain : A single physical line that a colision can occur. Example: Hubs have one collision domain and only one connected node can make a transfer at any time. Switches collision domain number is like their port number by default.

Broadcast domain : A logical division of networks that all nodes can reach eachother at data link layer(layer 2). Example: Switches are one broadcast domain. Because without any restriction, if one node sends something from one port, all other ports receive it. Routers’ each port is one broadcast domain.

Let’s return our main lesson again. VLANs helps you to build new child broadcast domains in one switch or in one broadcast domain. After configuring VLANs, each VLAN become a single broadcast domain and without routing, there is no communication between VLANs.

There are also collision domains in the VLANs again. Each VLAN has collision domain as the number of their assigned ports.

Here, VLANs can be thinked like small switches in the main switch.

On Cisco switches, all the ports are the member of VLAN 1 by default. So if no VLAN configuration done, all the ports are in the same VLAN, VLAN 1. And they are in the same broadcast domain as mentioned above.

Native VLAN



By default Native VLAN is VLAN 1. All untagged frames are member of this VLAN. This Native VLAN can be changed by a trunk port. For example, one trunk’s native VLAN can be VLAN 5. And all the untagged and VLAN 5 tagged frames are belong to that Native VLAN, VLAN 5. Here the important point is, each end of the connection must be configured with the same Native VLAN.

ISL trunks does not support the Native VLAN and untagged frames. But dot1.q trunks support Native VLAN and untagged frames.

Native VLAN is a security risk at the same time. To avoid this risk, Native VLAN can be assigned to an unused port or disabled port. You can also make the trunk ports to tag the Native VLAN.

To learn mode about VLAN(Vİrtual Local Area Network)s, continue with the following posts ;)

VLAN – Part 1
VLAN – Part 2 (VLAN Assignments and VLAN Port Types)
VLAN – Part 3 (VLAN Frame Tagging Protocols, ISL and Dot1.q)
VLAN – Part 4 (How to Configure Cisco VLANs)
VLAN – Part 5 (Packet Tracer VLAN Configuration Example)




About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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