DGS-3224TGR Gigabit Ethernet Switch User’s Guide
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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 filter or forward the packet
• Egress rules – determines if the packet must be sent tagged or untagged.
Figure 5- 1. IEEE 802.1Q Packet Forwarding
