DGS-3100 Series Gigabit Stackable Managed Switch User Manual
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Configuring VLANs
Understanding IEEE 802.1p Priority
Priority tagging is an IEEE 802.1p defined standard function designed to provide a means of managing traffic on networks
where many different types of data are transmitted simultaneously. It is intended to alleviate problems associated with the
delivery of time-critical data over congested networks. The quality of applications dependent on such data, such as video
conferencing, can be severely and adversely affected by even very small delays in transmission.
IEEE 802.1p standard-compliant network devices recognize the priority level of data packets and can assign priority labels
or tags to packets, as well as strip priority tags from packets. The priority tag determines the packet's degree of
expeditiousness and the queue to which it is assigned.
Priority tags are assigned values from 0 to 7, with 0 being assigned to the lowest priority data, and 7 to the highest.
Generally, tag 7 is used for data associated with video or audio applications, sensitive to even slight delays, or for data from
specified end users whose data transmissions warrant special consideration.
The switch enables increased definition for handling priority tagged data packets on the network. Using queues to manage
priority tagged data enables user-specification for the data’s relative priority to suit the needs of the network. Circumstances
can arise where it is advantageous to group two or more differently tagged packets into the same queue. Generally, however,
it is recommended that the highest priority queue, Queue 3, be reserved for data packets with a priority value of 7.
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 8:1. This means that the highest priority queue, Queue 3, clears eight packets for
every one packet cleared from Queue 0.
It is important that the priority queue settings on the switch are for all ports, and all devices connected to the switch are
affected. The priority queuing system is especially beneficial for networks that employ priority tag assignment capable
switches.
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
necessarily.
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-3100 Series
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 DGS-3100 series supports IEEE 802.1Q 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.
IEEE 802.1Q VLANs
Some relevant terms:
Term
Description
Tagging
The act of putting 802.1Q VLAN information into the header of a packet.
