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In the above example, there are three different 802.1Q VLANs and each port can transmit packets on one of
them according to their Port VLAN ID (PVID). However, a port can receive packets on all VLANs (VID) that it
belongs to. The assignments are as follows:
The server attached to Port 4 is shared by VLAN 1, VLAN 2, and VLAN 3 because Port 4 is a member of all
three VLANs (it is listed as a member of VID 1, VID 2, and VID 3). Since it can receive packets from three
VLANs, all ports can successfully send packets to it to be printed. Ports 1, 2, and 3 send these packets on
VLAN 1 (their PVID=1), and Ports 6 and 7 send these packets on VLAN 2 (PVID=2). The third VLAN
(PVID=3) is used by the server to transmit files that had been requested on VLAN 1 or 2 back to the
computers. All computers that use the server will receive transmissions from it since they are all located on
ports which are members of VLAN 3 (VID=3).
802.1Q VLANs Spanning Multiple Switches
802.1Q VLANs can span multiple switches as well as your entire network. Two considerations to keep in mind
while building VLANs of this sort are whether the switches are IEEE 802.1Q-compliant and whether VLAN
packets should be tagged or untagged.
Definitions of relevant terms are as follows:
Tagging The act of putting 802.1Q VLAN information into the header of a packet. Ports with tagging
enabled will put the VID number, priority, and other VLAN information into all packets that flow out it. If
a packet has previously been tagged, the port will not alter the packet, thus keeping the VLAN information
intact. Tagging is used to send packets from one 802.1Q-compliant device to another.
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
PVID
(Port VLAN ID)
1
1
1
2
2
3
VID
(VLAN ID)
1
2
3
Table 5-2. VLAN assignments for Figure 5-4
21
Ports
Port 1
Port 2
Port 3
Port 6
Port 7
Port 4
Member Ports
1, 2, 3, 4
4, 6, 7
1, 2, 3, 4, 6, 7
Gigabit Ethernet Switch User's Guide
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