D-Link xStack DXS-3350SR Installation & User Manual - Vlans

DXS-3350SR Gigabit Layer 3 Switch 

VLANs 

Understanding IEEE 802.1p Priority 

Priority tagging is a function defined by the IEEE 802.1p standard designed to provide a means of managing traffic on a 

network where many different types of data may be transmitted simultaneously. It is intended to alleviate problems 

associated with the delivery of time critical data over congested networks. The quality of applications that are dependent on 

such time critical data, such as video conferencing, can be severely and adversely affected by even very small delays in 

transmission. 

Network devices that are in compliance with the IEEE 802.1p standard have the ability to recognize the priority level of data 

packets. These devices can also assign a priority label or tag to packets. Compliant devices can also strip priority tags from 

packets. This priority tag determines the packet's degree of expeditiousness and determines the queue to which it will be 

assigned.  

Priority tags are given values from 0 to 7 with 0 being assigned to the lowest priority data and 7 assigned to the highest. The 

highest priority tag 7 is generally only used for data associated with video or audio applications, which are sensitive to even 

slight delays, or for data from specified end users whose data transmissions warrant special consideration.  

The Switch allows you to further tailor how priority tagged data packets are handled on your network. Using queues to 

manage priority tagged data allows you to specify its relative priority to suit the needs of your network. There may be 

circumstances where it would be advantageous to group two or more differently tagged packets into the same queue. 

Generally, however, it is recommended that the highest priority queue, Queue 1, be reserved for data packets with a priority 

value of 7. Packets that have not been given any priority value are placed in Queue 0 and thus given the lowest priority for 

delivery.  

A weighted round robin system is employed on the Switch to determine the rate at which the queues are emptied of packets. 

The ratio used for clearing the queues is 4:1. This means that the highest priority queue, Queue 1, will clear 4 packets for 

every 1 packet cleared from Queue 0. 

Remember, the priority queue settings on the Switch are for all ports, and all devices connected to the Switch will be 

affected. This priority queuing system will be especially beneficial if your network employs Switches with the capability of 

assigning priority tags. 

VLAN Description 

A Virtual Local Area Network (VLAN) is a network topology configured according to a logical scheme rather than the 

physical layout. VLANs can be used to combine any collection of LAN segments into an autonomous user group that 

appears as a single LAN. VLANs also logically segment the network into different broadcast domains so that packets are 

forwarded only between ports within the VLAN. Typically, a VLAN corresponds to a particular subnet, although not nec-

essarily. 

VLANs can enhance performance by conserving bandwidth, and improve security by limiting traffic to specific domains. 

A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes that frequently communicate 

with each other are assigned to the same VLAN, regardless of where they are physically on the network. Logically, a VLAN 

can be equated to a broadcast domain, because broadcast packets are forwarded to only members of the VLAN on which the 

broadcast was initiated. 

Notes About VLANs on the DXS-3350SR 

•  No matter what basis is used to uniquely identify end nodes and assign these nodes VLAN membership, packets 

cannot cross VLANs without a network device performing a routing function between the VLANs. 

•  The DXS-3350SR supports IEEE 802.1Q VLANs and Port-Based VLANs. The port untagging function can be 

used to remove the 802.1Q tag from packet headers to maintain compatibility with devices that are tag-unaware. 

•  The Switch's default is to assign all ports to a single 802.1Q VLAN named "default."   

•  The "default" VLAN has a VID = 1.  

•  The member ports of Port-based VLANs may overlap, if desired.   

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