DES-1218R/DES-1226R 10/100/1000 Mbps Ethernet Switch User ’s Guide
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packets in a data stream to arrive in the same order they were sent. A trunk connection can be
made with any other switch that maintains host-to-host data streams over a single trunk port.
Switches that use a load-balancing scheme and send packets of a host-to-host data stream over
multiple trunk ports cannot have a trunk connection with the Switch.
Note:
If the two Gigabit Ethernet ports are used as a trunk group and either port is disconnected,
packets intended for the disconnected port will be dropped.
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 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 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 12:6:2:1. This means that the
highest priority queue, Queue 3, will clear 12 packets for every 6 packets cleared from Queue 2,
every 2 packets from Queue 1, and 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.
