xStack DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual
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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 rec-
ommended that the highest priority queue, Queue 1, 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 4:1. This means that the highest priority queue, Queue 1, 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 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 xStack DES-3500 Series switches
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 DES-3500 Series switches supports IEEE 802.1Q VLANs and Port-Based 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.
The member ports of Port-based VLANs may overlap, if desired.
IEEE 802.1Q VLANs
Some relevant terms:
•
Tagging
- The act of putting 802.1Q VLAN information into the header of a packet.
•
Untagging
- The act of stripping 802.1Q VLAN information out of the packet header.
•
Ingress port
- A port on a switch where packets are flowing into the Switch and VLAN decisions must
be made.
•
Egress port
- A port on a switch where packets are flowing out of the Switch, either to another switch
or to an end station, and tagging decisions must be made.
