DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual
68
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 f lowing into the Switch and VL AN 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 VL ANs r equire taggin g, w hich en ables 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. Al l pac k ets 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 pack ets between
stations that are members of the VLAN.
Any port can be configured as either tagging or untagging. The untaggin g feature of IEEE 802.1Q VLANs allows
VLANs to work with legacy switches that don't recognize VLAN tags in packet headers. T he tagging feature allows
VLANs to span multiple 802.1Q-compliant switches through a single physical connec tion an d allo ws Spann ing T ree to
be enabled on all ports and work normally.
The IEEE 802.1Q standard restricts the forwarding of untagged packets to the VLAN of which the rec eiving port is a
member.
Figure 3 - 2 IEEE 802.1Q Packet Forwarding
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 – r ules 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 m ust be sent
tagged or untagged.