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.
Links
https://networklessons.com/cisco/ccnp-encor/802-1q-encapsulation-explained
https://networklessons.com/switching/introduction-to-vtp-vlan-trunking-protocol