Configuring Vlans; Understanding Ieee 802.1P Priority; Vlans - D-Link DES-6500 User Manual

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

Configuring 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.

VLANs

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.
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