802.1Q Vlan Packet Forwarding; 802.1Q Vlan Tags - D-Link DGS-3324SR User Manual

D-Link DGS-3324SR Layer 3 Stackable Gigabit Ethernet 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 the receiving port is a
member of.
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:
•
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
Figure 4- 18. IEEE 802.1Q Packet Forwarding
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