DGS-3048 Gigabit Ethernet Switch Manual
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8
L2 Features
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 DGS-3048
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.
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