VLANmerkinnöillä

VLANmerkinnöillä, often translated as VLAN tagging or VLAN encapsulation, refers to the process of adding a specific tag to network frames that identifies which Virtual Local Area Network (VLAN) the frame belongs to. This tagging is a fundamental mechanism for implementing and managing VLANs within an Ethernet network.

The most common standard for VLAN tagging is IEEE 802.1Q. When a frame is tagged according to this standard, a 4-byte tag control information field is inserted into the Ethernet frame header, just after the destination and source MAC addresses. This field contains crucial information, including a 3-bit Priority Code Point (PCP) for Quality of Service (QoS) and a 1-bit Drop Eligible Indicator (DEI). The most important part for VLAN functionality is the 12-bit VLAN Identifier (VID), which can uniquely identify up to 4096 VLANs.

When an Ethernet switch receives a frame, it examines the VLAN tag. If the tag is present and the VID matches the VLAN configured for the port the frame arrived on, the switch forwards the frame to other ports that are also part of that same VLAN. If the frame arrives on a port not configured for that VLAN, or if the tag is missing on a port expecting tagged traffic, the switch will typically drop the frame. This ensures that traffic remains segregated within its designated VLAN, enhancing network security, performance, and manageability. Untagged traffic on ports configured for VLANs is usually assigned to a default VLAN.