VLAN - VLAN IDs 0 and 4095

When configuring VLANs, the VLAN ID is a value that is used to designate to which VLAN a particular frame belongs when traversing a trunk. This is part of the trunk encapsulation process. The VLAN ID is a 12 bit field found within the VLAN tag, which is also known as the extended range of VLAN IDs.

VLAN ID values of 0 and 4095 are unusable as actual VLAN IDs. If these values are found in the VLAN ID field of a VLAN tag, then they are interpreted like so:

VLAN ID of 0

This value is used for what is called “priority tagging”. Remember, a VLAN tag, other than the VLAN ID itself, also includes priority fields that contain Class of Service (CoS) values used by QoS mechanisms. If a frame is sent that needs to be handled with particular Layer 2 QoS priorities, but doesn’t need to be associated with a specific VLAN, then a VLAN ID of 0 would be used.

VLAN ID of 4095

On the other hand, VLAN ID 4095 is reserved for implementation-specific use and is often used in conjunction with VTP pruning. It can also be used internally by network devices to designate “untagged” or “forbidden” VLANs in certain network scenarios.






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