Assigning Ip Network Addresses And Subnet Masks To Vlans; Q Vlan Packet Forwarding - D-Link DGS-3312SR Product Manual

A Switch that implements layer 3 (or 'subnet') VLANs without performing any routing function between these VLANs is
referred to as performing 'IP Switching'.
Planning VLAN Layout
VLANs on the DGS-3312SR and DES-3226S series Switches have considerably more functions and are more complex
than on a traditional layer 2 Switch, and must therefore be laid-out and configured with a bit more forethought. VLANs
with an IP interface assigned to them could be thought of as network links – not just as a collection of associated end users.
Further, VLANs assigned an IP network address and subnet mask enables IP routing between them.
VLANs must be configured on the Switch before they can be assigned IP subnets. Furthermore, the static VLAN
configuration is specified on a per port basis. On the DGS-3312SR, a VLAN can consist of end-nodes – just like a
traditional layer 2 Switch, but a VLAN can also consist of one or more Switches – each of which is connected to multiple
end-nodes or network resources.
So, the IP subnets for a network must be determined first, and the VLANs configured on the Switch to accommodate the IP
subnets. Finally, the IP subnets can be assigned to the VLANs.

Assigning IP Network Addresses and Subnet Masks to VLANs

The DGS-3312SR allows the assignment of IP subnets to individual VLANs. This is the fundamental advantage of VLANs
in IP Switching.
Developing an IP addressing scheme is a complex subject, but it is sufficient here to mention that the total number of
anticipated end nodes – for each IP interface – must be accommodated with a unique IP address. It should be noted that the
Switch regards a VLAN with an IP network address and corresponding subnet mask assigned as an IP interface.
Understanding 802.1Q VLANs
This review of 802.1Q VLANs presents some basic background about how VLANs work according to the IEEE 802.1Q
standard. VLANs operate according to the same rules regardless of whether the Switching environment is Layer 2 or Layer
3. The difference is primarily that in a Layer 3 Switch there is an added capability of unique association between a VLAN
and an IP interface or subnet group.
A VLAN is a collection of end nodes grouped by logic rather than physical location. End nodes that frequently
communicate with each other are assigned to the same VLAN, regardless of where they are located physically on the
network. Logically, a VLAN can be equated to a broadcast domain, because broadcast packets are forwarded only to
members of the VLAN on which the broadcast was initiated.
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 DGS-3312SR Switch. 802.1Q VLANs require tagging, which
enables the VLANs to span an entire network (assuming all Switches on the network are IEEE 802.1Q-compliant).
Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q VLANs allow 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 VLAN compliant Switches through a single physical connection and allows Spanning Tree to be
enabled on all ports and work normally.

802.1Q VLAN Packet Forwarding

Packet forwarding decisions are made based upon the following three types of rules:
•
Ingress rules – rules relevant to the classification of received frames belonging to a VLAN.
•
Forwarding rules between ports – decides filter or forward the packet
•
Egress rules – determines if the packet must be sent tagged or untagged.
DGS-3312SR Gigabit Layer 3 Switch
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