Q Vlan Tags - D-Link DES-3028 User Manual

DES-3028 DES-3028P DES-3028G DES-3052 DES-3052P Layer 2 Fast Ethernet Managed Switch
Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q VLANs allows VLANs to work
with legacy switches that don't recognize VLAN tags in packet headers. The tagging feature allows VLANs to span multiple
802.1Q-compliant switches through a single physical connection and allows Spanning Tree to be enabled on all ports and work
normally.
The IEEE 802.1Q standard restricts the forwarding of untagged packets to the VLAN of which the receiving port is a member.
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. This switch supports MSTP.

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:

Ingress rules - rules relevant to the classification of received frames belonging to a VLAN.

Forwarding rules between ports - decides whether to filter or forward the packet.

Egress rules - determines if the packet must be sent tagged or untagged.

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 7- 1. IEEE 802.1Q Packet Forwarding
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