Something about VLAN you should know

What is a VLAN

A VLAN is a virtual LAN, it is a broadcast domain created by switches. Normally, it is a router creating that broadcast domain. With VLAN’s, a switch can create the broadcast domain. This works by, you, the administrator, putting some switch ports in a VLAN other than 1, the default VLAN. All ports in a single VLAN are in a single broadcast domain.

Because switches can talk to each other, some ports on switch A can be in VLAN 10 and other ports on switch B can be in VLAN 10. Broadcasts between these devices will not be seen on any other port in any other VLAN, other than 10. However, these devices can all communicate because they are on the same VLAN. Without additional configuration, they would not be able to communicate with any other devices, not in their VLAN.

It is important to point out that you don’t have to configure a VLAN until your network gets so large and has so much traffic that you need one. Many times, people are simply using VLAN’s because the network they are working on was already using them. Another important fact is that, on a Cisco switch, VLAN’s are enabled by default and ALL devices are already in a VLAN. The VLAN that all devices are already in is VLAN 1.

When do I need a VLAN

You need to consider using VLAN in any of the following situations:

  • You have more than 200 devices on your LAN
  • You have a lot of broadcast traffic on your LAN
  • Groups of users need more security or are being slowed down by too many broadcasts?
  • Groups of users need to be on the same broadcast domain because they are running the same applications. An example would be a company that has VoIP phones. The users using the phone could be on a different VLAN, not with the regular users.
  • Or, just to make a single switch into multiple virtual switches.

What is a trunk port

Devices on different VLAN’s can communicate with a router or a Layer 3 switch. As each VLAN is its own subnet, a router or Layer 3 switch must be used to route between the subnets.

When there is a link between two switches or a router and a switch that carries the traffic of more than one VLAN, that port is a trunk port. A trunk port must run a special trunking protocol. The protocol used would be Cisco’s proprietary Interswitch link (ISL) or the IEEE standard 802.1q.

What does VLAN offer

VLAN’s offer higher performance for medium and large LAN’s because they limit broadcasts. As the amount of traffic and the number of devices grow, so does the number of broadcast packets. By using VLAN’s you are containing broadcasts. VLAN’s also provide security because you are essentially putting one group of devices, in one VLAN, on their own network.

NOTE: This is a copied & modified version from the network.

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