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xStack DGS-3400 Series Fast Ethernet Switch

Section 7

Layer 2 Features

VLANs

Trunking

IGMP Snooping

Spanning Tree

Forwarding and Filtering

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.

VLAN Description

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 nec-
essarily.

VLANs can enhance performance by conserving bandwidth, and improve security by limiting traffic to specific domains.