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DES-1218/DES-1226 10/100/1000 Mbps Ethernet Switch User’s Guide
21
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 effected 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 being given 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 3,
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
15:7:3:1. This means that the highest priority queue, Queue 3, will clear 15 packets for
every 7 packets cleared from Queue 2, 15 packets for every 3 packets from Queue 1, and
15 packets for every 1 packet from Queue 0.
Remember, the priority queue settings on the Switch are for all ports, and all devices
connected to the Switch will be effected. This priority queuing system will be especially
beneficial if your network employs switches with the capability of assigning priority
tags.
VLANs
VLANs are a collection of users or ports grouped together in a secure, autonomous
broadcast and multicast domain. Membership to a VLAN is not restricted by a physical
location and can be defined across multiple LAN switches.
Port-based VLANs allow a network to be segmented in order to reduce the size of
broadcast domains. All packets entering a VLAN will only be forwarded to the ports
that are members of that VLAN. This even includes Multicast frames and unknown
unicast frames.