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D-Link DES-1210G [25/36] Ieee 802 p priority

DES-1210G Fast Ethernet/Gigabit 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 DES-1210G gives you the ability 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.

Another benefit of VLANs is that you can change the network topology without physically moving

stations or changing cable connections. Stations can be ‘moved’ simply by changing VLAN

settings from one VLAN (the sales VLAN, for example) to another VLAN (the marketing VLAN).

This allows VLANs to accommodate network moves, changes and additions with the utmost

flexibility.

VLANs can also provide a level of security to your network. Port-based VLANs allow you to

configure ports to not accept packets from outside of the VLAN.

D-Link DES-1210G [25/36] Ieee 802 p priority

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