D-Link DGS-3120-24TC Product Manual - Page 65

xStack® DGS-3120 Series Managed Switch Web UI Reference Guide The main characteristics of IEEE 802.1Q are as follows: • Assigns packets to VLANs by filtering. • Assumes the presence of a single global spanning tree. • Uses an explicit tagging scheme with one-level tagging. • 802.1Q VLAN Packet Forwarding • Packet forwarding decisions are made based upon the following three types of rules: o Ingress rules - rules relevant to the classification of received frames belonging to a VLAN. o Forwarding rules between ports - decides whether to filter or forward the packet. o Egress rules - determines if the packet must be sent tagged or untagged. Figure 4-1 IEEE 802.1Q Packet Forwarding 802.1Q VLAN Tags The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address. Their presence is indicated by a value of 0x8100 in the EtherType field. When a packet's EtherType field is equal to 0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3 bits of user priority, 1 bit of Canonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across Ethernet backbones), and 12 bits of VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID is the VLAN identifier and is used by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique VLANs can be identified. The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally contained in the packet is retained. Figure 4-2 IEEE 802.1Q Tag The EtherType and VLAN ID are inserted after the MAC source address, but before the original EtherType/Length or Logical Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be recalculated. 57