Thursday, November 22, 2012

Tag or untagged?

Fundamentally, there are only two different Ethernet switch port types: access or trunk.

An access switchport can only participate in the VLAN to which it has been assigned and data is not tagged.

A trunk switchport can participate in multiple VLANs but in order to do so expects the data to be 'tagged' so the switch will know where the data is ultimately expected to go. The switch then reads the VLAN ID tagged data and then knows to which ports it should be talking or where it should be going.

VLANs are typically used to provide logical separation for different network segments or subnets but it is important to realize that just because a switch supports layer-2 tagging (802.1q VLAN tagging) does not mean that it provides routing between them, which is a layer-3 function.

A trunk switchport would be employed if, for instance, your router supported trunking/tagging. Instead of simply being on VLAN 13, port 1 could be configured on the switch as a trunk port and both VLAN 13 and VLAN 459 would be set as tagged. Data from both networks could traverse the same physical wire back to your router but would not be aware of one another. Guests would still be isolated on VLAN 459 while the office was safe and sound on VLAN 13. Trunk switchports are typically employed in virtualization, where one network interface is expected to support systems on multiple VLAN IDs or network segments.

Tagged and untagged describe how a frame on a VLAN is transmitted from a port. The frame can be tagged in which case it will contains an 802.1q VLAN Tag Control information field, or not tagged. Untagged doesn't mean "not on a VLAN". In a VLAN-aware network, every packet is forwarded on a VLAN. In a VLAN-aware network frames need not contain a tag identifying the VLAN they are travelling on. Switches can use other mechanisms, such as policy to decide that.

VLAN is completely on wire side, wireless doesn't have this concept.

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