D-Link DGS-3100 Configuring VLANs, Understanding IEEE 802.1p Priority, VLAN Description

DGS-3100 Series Gigabit Stackable Managed Switch User Manual

Configuring VLANs

Understanding IEEE 802.1p Priority

Priority tagging is an IEEE 802.1p defined standard function designed to provide a means of managing traffic on networks where many different types of data are transmitted simultaneously. It is intended to alleviate problems associated with the delivery of time-critical data over congested networks. The quality of applications dependent on such data, such as video conferencing, can be severely and adversely affected by even very small delays in transmission.

IEEE 802.1p standard-compliant network devices recognize the priority level of data packets and can assign priority labels or tags to packets, as well as strip priority tags from packets. The priority tag determines the packet's degree of expeditiousness and the queue to which it is assigned.

Priority tags are assigned values from 0 to 7, with 0 being assigned to the lowest priority data, and 7 to the highest. Generally, tag 7 is used for data associated with video or audio applications, sensitive to even slight delays, or for data from specified end users whose data transmissions warrant special consideration.

The switch enables increased definition for handling priority tagged data packets on the network. Using queues to manage priority tagged data enables user-specification for the data’s relative priority to suit the needs of the network. Circumstances can arise where it is 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.

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 8:1. This means that the highest priority queue, Queue 3, clears eight packets for every one packet cleared from Queue 0.

It is important that the priority queue settings on the switch are for all ports, and all devices connected to the switch are affected. The priority queuing system is especially beneficial for networks that employ priority tag assignment capable switches.

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 necessarily.

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

A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes that frequently communicate with each other are assigned to the same VLAN, regardless of where they are physically on the network. Logically, a VLAN can be equated to a broadcast domain, because broadcast packets are forwarded to only members of the VLAN on which the broadcast was initiated.

Notes about VLANs on the DGS-3100 Series

No matter what basis is used to uniquely identify end nodes and assign these nodes VLAN membership, packets cannot cross VLANs without a network device performing a routing function between the VLANs.

The DGS-3100 series supports IEEE 802.1Q VLANs. The port untagging function can be used to remove the 802.1Q tag from packet headers to maintain compatibility with devices that are tag-unaware.

The switch's default is to assign all ports to a single 802.1Q VLAN named ‘default.’ The ‘default’ VLAN has a VID = 1.

IEEE 802.1Q VLANs

Some relevant terms:

Term

Tagging

Description

The act of putting 802.1Q VLAN information into the header of a packet.

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