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Gigabit Ethernet Switch User's Guide
Untagging The act of stripping 802.1Q VLAN information out of the packet header. Ports with untagging
enabled will take all VLAN information out of all packets that flow out of a port. If the packet doesn't have
a VLAN tag, the port will not alter the packet, thus keeping the packet free of VLAN information.
Untagging is used to send packets from an 802.1Q-compliant switch to a non-compliant device.
Ingress port A port on a switch where packets are flowing into the switch. If an ingress port has the
Ingress Filter enabled, the switch will examine each packet to determine whether or not it is a VLAN
member and then take one of two actions: if the port is not a member of a VLAN, the packet will be
dropped; if the port is a member of a VLAN, then the packet will be forwarded. Otherwise, if the Ingress
Filter is disabled, then the switch will process any packet received at this port in its normal fashion.
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. If an egress port is connected to an 802.1Q-compliant
switch, tagging should be enabled so the other device can take VLAN data into account when making
forwarding decisions (this allows VLANs to span multiple switches). If an egress connection is to a non-
compliant switch or end-station, tags should be stripped so the (now normal Ethernet) packet can be read
by the receiving device.
VLANs Over 802.1Q-compliant Switches
When switches maintaining the same VLANs are 802.1Q-compliant, it is possible to use tagging. Tagging puts
802.1Q VLAN information into each packet header, enabling other 802.1Q-compliant switches that receive the
packet to know how to treat it. Upon receiving a tagged packet, an 802.1Q-compliant switch can use the
information in the packet header to maintain the integrity of VLANs, carry out priority forwarding, etc.
Data transmissions between 802.1Q-compliant switches take place as shown below.
In the above example, step 4 is the key element. Because the packet has 802.1Q VLAN data encoded in its
header, the ingress port can make VLAN-based decisions about its delivery: whether server #2 is attached to a
port that is a member of VLAN 2 and, thus, should the packet be delivered; the queuing priority to give to the
packet, etc. It can also perform these functions for VLAN 1 packets as well, and, in fact, for any tagged packet
it receives regardless of the VLAN number.
Figure 5-5. Data transmissions between 802.1Q-compliant Switches
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