D-Link DES-6300 User Manual - 802.1Q Vlans

Modular L3 Ethernet Switch User’s Guide 

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802.1Q VLANs 

The Switch supports up to 2048 802.1Q VLANs. 802.1Q VLANs limit traffic that flows into and out of 

switch ports. Thus, all devices connected to a port are members of the VLAN(s) the port belongs to, 

whether there is a single computer directly connected to a switch, or an entire department. 

On 802.1Q VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers. NICs send 

and receive normal Ethernet packets. If the packet’s destination lies on the same segment, 

communications take place using normal Ethernet protocols. Even though this is always the case, when 

the destination for a packet lies on another Switch port, VLAN considerations come into play to decide if 

the packet gets dropped by the Switch or delivered. 

There are two key components to understanding 802.1Q VLANs; Port VLAN ID numbers (PVIDs) and 

VLAN ID numbers (VIDs). Both variables are assigned to a switch port, but there are important 

differences between them. A user can only assign one PVID to each switch port. The PVID defines which 

VLAN a packet belongs to when packets need to be forwarded to another switch port or somewhere else 

on the network. On the other hand, a user can define a port as a member of multiple VLANs (VIDs), 

allowing the segment connected to it to receive packets from many VLANs on the network. These two 

variables control a port’s ability to transmit and receive VLAN traffic, and the difference between them 

provides network segmentation, while still allowing resources to be shared across more than one VLAN.  

802.1Q VLAN Segmentation 

The following example is helpful in explaining how 802.1Q VLAN segmentation works. Take a packet 

that is transmitted by a machine on Port 1 that is a member of VLAN 2 and has the Port VLAN ID 

number 2 (PVID=2). If the destination lies on another port (found through a normal forwarding table 

lookup), the Switch then looks to see if the other port (Port 10) is a member of VLAN 2 (and can 

therefore receive VLAN 2 packets). If port 10 is not a member of VLAN 2, then the packet will be 

dropped by the Switch and will not reach its destination. If Port 10 is a member of VLAN 2, the packet 

will go through. This selective forwarding feature based on VLAN criteria is how VLANs segment 

networks. The key point being that Port 1 will only transmit on VLAN 2, because it’s Port VLAN ID 

number is 2 (PVID=2). 

Sharing Resources Across 802.1Q VLANs 

Network resources such as printers and servers however, can be shared across 802.1Q VLANs. This is 

achieved by setting up overlapping VLANs as shown in the diagram below. 

Figure 5- 3.  Example of typical VLAN configuration  

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: