Layer 2 Features; Vlans - D-Link xStack DES-3800 Series User Manual

Layer 2 Features

VLAN
Trunking
IGMP Snooping
Spanning Tree
Forwarding
The following section will aid the user in configuring security functions for the Switch. The Switch
includes various functions for VLAN, Trunking, IGMP Snooping, Spanning Tree, and Forwarding, all
discussed in detail in the following section.

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 7, 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.
Strick mode and weighted round robin system are 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 7, 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.
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
Section 7
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