VLAN tagging and trunking are essential methods for managing traffic in networks. The IEEE 802.1Q standard, also known as Dot1Q, defines the method for identifying VLAN traffic between two switches. In this article, we will explore the terminologies associated with VLAN tagging and trunking, including Dot1Q VLAN tags, trunk ports, access ports, and more.
Suppose we have two switches, Switch 1 and Switch 2, and four computers, A, B, C, and D. We assign computer A and C to VLAN 10 and B and D to VLAN 20 on Switch 1. Similarly, we assign computer C to VLAN 10 and D to VLAN 20 on Switch 2. Since VLANs are local to each switch, we need to link these two switches to enable communication between computers on the same VLAN. This link is called a trunk.
A trunk or Dot1Q link provides VLAN ID for frames traversing between switches. By default, a trunk can carry traffic from all lines to and from the switch, but it can be configured to carry only specified VLAN traffic. The process of transmitting different VLAN traffic over the trunk is called “chunking.” A trunk port adds a VLAN header to an Ethernet frame to indicate which VLAN the frame belongs to. In contrast, an access port carries traffic only for one VLAN.
Suppose computer A wants to talk to computer C on Switch 2. A computer attached to an access port has no idea about a VLAN, and VLAN creation and management are switches’ responsibilities. Switch 1 adds a VLAN tag to the Ethernet frame to indicate that this frame belongs to VLAN 10. The tag consists of four fields: the first field contains the information that allows Ethernet devices to recognize that this is a frame to a VLAN, the second field is used for quality of service, and the third field is one bit of lab, which is always set to zero. The last field is the VLAN identifier, which can be a number from one to four thousand 94.
The VLAN ID tag is inserted into the Ethernet frame, creating a new frame called an “IEEE 802.1Q frame.” The process of adding or inserting a VLAN tag into the Ethernet frame is called “tagging” or “encapsulation.” The trunk port delivers the tagged frame to Switch 2. Switch 2 reads the VLAN ID tag and delivers the original Ethernet frame to computer C.
In conclusion, knowing the basic terminologies associated with VLAN tagging and trunking can help you understand related protocols, such as Cisco’s VLAN trunking protocol and dynamic trunking protocol. Additionally, it’s essential to understand the two most confusing terms in the Cisco world, the default VLAN and the native VLAN.