DGS-3100 Series Gigabit Stackable Managed Switch User Manual
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Term Description
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 switch. 802.1Q VLANs require tagging, which enables them to span
the entire network (assuming all switches on the network are IEEE 802.1Q-compliant).
VLANs allow a network to be segmented in order to reduce the size of broadcast domains. All packets entering a VLAN are
only forwarded to the stations (over IEEE 802.1Q enabled switches) that are members of that VLAN, and this includes
broadcast, multicast and unicast packets from unknown sources.
VLANs can also provide a level of security to a network. IEEE 802.1Q VLANs only deliver packets between stations that
are members of the VLAN.
Any port can be configured as either tagged or untagged. 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 in 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.
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 packets 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.
Figure 0–2 IEEE 802.1Q Packet Forwarding
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