DGS-3312SR Stackable Gigabit Layer 3 Switch
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IEEE 802.1Q VLANs
Some relevant terms:
•
Tagging - The act of putting 802.1Q VLAN information into the header of a packet.
•
Untagging - The act of stripping 802.1Q VLAN information out of the packet header.
•
Ingress port - A port on a Switch where packets are flowing into the Switch and VLAN decisions must be made.
•
Egress port - A port on a Switch where packets are flowing out of the Switch, either to another Switch or to an
end station, and tagging decisions must be made.
IEEE 802.1Q (tagged) VLANs are implemented on the DGS-3312SR Switch. 802.1Q VLANs require tagging, which
enables the VLANs to span an entire network (assuming all Switches on the network are IEEE 802.1Q-compliant).
Any port can be configured as either
tagging
or
untagging
. The
untagging
feature of IEEE 802.1Q VLANs allow 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 VLAN compliant Switches through a single physical connection and allows Spanning Tree to be
enabled on all ports and work normally.
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 filter or forward the packet
•
Egress rules – determines if the packet must be sent tagged or untagged.
Figure 4- 48. 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
three bits or user priority, one bit of Canonical Format Identifier (CFI – used for encapsulating Token Ring packets so they
can be carried across Ethernet backbones) and twelve bits of VLAN ID (VID). The three 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 twelve bits long, 4094
unique VLANs can be identified.
The tag is inserted into the packet header making the entire packet longer by four octets. All of the information contained
in the packet originally is retained.
