Table of Contents
Advertisement
DES-1210G Fast Ethernet/Gigabit Ethernet Switch User's Guide
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 DES-1210G gives you the ability 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 15:7:3:1. This means
that the highest priority queue, Queue 3, will clear 15 packets for every 7 packets cleared from
Queue 2, 15 packets for every 3 packets from Queue 1, and 15 packets for 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.
Another benefit of VLANs is that you can change the network topology without physically moving
stations or changing cable connections. Stations can be 'moved' simply by changing VLAN
settings from one VLAN (the sales VLAN, for example) to another VLAN (the marketing VLAN).
This allows VLANs to accommodate network moves, changes and additions with the utmost
flexibility.
VLANs can also provide a level of security to your network. Port-based VLANs allow you to
configure ports to not accept packets from outside of the VLAN.
17
Advertisement
Table of Contents
