D-Link DES-1218 [31/42] Ieee 802 p priority

DES-1218/DES-1226 10/100/1000 Mbps Ethernet Switch User’s Guide

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.