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xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch User Manual
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 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
will only be 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 your network. IEEE 802.1Q VLANs will only deliver packets between
stations that are members of the VLAN.
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.
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Assumes the presence of a single global spanning tree.
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Uses an explicit tagging scheme with one-level tagging.
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802.1Q VLAN Packet Forwarding
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Packet forwarding decisions are made based upon the following three types of rules:
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Ingress rules - rules relevant to the classification of received frames belonging to a VLAN.
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Forwarding rules between ports - decides whether to filter or forward the packet.
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Egress rules - determines if the packet must be sent tagged or untagged.
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Figure 6- 35. IEEE 802.1Q Packet Forwarding
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