802.1Q Vlans Spanning Multiple Switches; Vlans Over 802.1Q-Compliant Switches - D-Link DES-3624 Series User Manual

Stackable NWay Ethernet Switch User's Guide
The server attached to Port 7 is shared by VLAN 1 and VLAN 2 because Port 7 is a member of both VLANs (it
is listed as a member of VID 1 and 2). Since it can receive packets from both 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 11
and 12 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
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.
Switch Management Concepts
2
3
Table 5-2. VLAN assignments for Figure 5-4
7,11,12
1,2,3,7,11,12
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