D-Link DGS-3212SR Layer 2 Gigabit Ethernet Switch
IEEE 802.1Q (tagged) VLANs are implemented on the DES-3226S 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 to filter or forward the packet
•
Egress rules – determines if the packet must be sent tagged or untagged.
Figure 5-9. 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 or 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
contained in the packet originally is retained.
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