802.1Q

IEEE 802.1Q, often referred to simply as "dot1q" or "VLAN tagging," is a standard that defines how VLAN information can be carried within Ethernet frames. Here’s an overview of IEEE 802.1Q:

1. Purpose: IEEE 802.1Q addresses the limitations of traditional VLANs (which are based on the 12-bit VLAN ID in the Ethernet header) by providing a method to identify VLAN membership within Ethernet networks using tags.

2. VLAN Tagging: 802.1Q introduces a 4-byte tag (32 bits) inserted into the Ethernet frame between the Source MAC Address and the EtherType/Length fields. This tag includes:

   • Tag Protocol Identifier (TPID): Identifies the frame as an 802.1Q-tagged frame (0x8100).
   • Priority Code Point (PCP): Specifies the priority level for the frame, using 3 bits.
   • Drop Eligible Indicator (DEI): Indicates whether a frame is eligible to be dropped in case of congestion, using 1 bit.
   • VLAN Identifier (VID): Specifies the VLAN to which the frame belongs, using 12 bits, allowing for up to 4096 VLANs.

3. Trunking: 802.1Q tagging is commonly used on trunk links (links that carry traffic for multiple VLANs) to distinguish frames belonging to different VLANs. This allows switches to correctly forward and manage traffic based on VLAN membership.

4. Native VLAN: Frames from the native VLAN (untagged frames) are not tagged with 802.1Q and are sent over trunk links as regular Ethernet frames. By default, VLAN 1 is often the native VLAN, but this can be configured.

5. Interoperability: 802.1Q tagging is widely supported in networking equipment and is essential for building large-scale VLAN-based networks in enterprises and data centers. It allows for efficient use of network resources by segregating traffic into VLANs based on organizational or security requirements.

6. 802.1ad (Q-in-Q): An extension to 802.1Q, 802.1ad allows for multiple levels of VLAN tagging, useful for service provider networks where multiple customers require separate VLANs over the same physical infrastructure.

In summary, IEEE 802.1Q provides a standardized method for VLAN tagging within Ethernet frames, enabling flexible and scalable network designs that support virtualization, traffic segmentation, and improved network management.