48CHAPTER 6: USING TRAFFIC PRIORITIZATION
■Resource planning applications — Used by organizations that require predictable and reliable access to enterprise resource planning applications such as SAP.
■Financial applications — Used by Accounts departments that need immediate access to large files and spreadsheets.
■CAD/CAM design applications — Used by design departments that need priority connections to server farms and other devices for transferring large files.
How Traffic Traffic prioritization ensures that high priority data is forwarded through Prioritization Works the Switch without being delayed by lower priority data. Traffic
prioritization uses the two traffic queues that are present in the hardware of the Switch to ensure that high priority traffic is forwarded on a different queue from lower priority traffic. High priority traffic is given preference over low priority traffic to ensure that the most critical traffic gets the highest level of service.
The Switch employs two methods of classifying traffic for prioritization. Traffic classification is the means of identifying which application generated the traffic, so that a service level can be applied to it.
The two supported methods for classifying traffic are:
■802.1D (classification is done at layer 2 of the OSI model).
■DiffServ code point (classification is done at layer 3 of the OSI model).
802.1D traffic At layer 2, a traffic service class is defined in 802.1Q frame, which is able classification to carry VLAN identification and user priority information. The
information is carried in a header field immediately following the destination MAC address, and Source MAC address.
802.1D Priority Levels
The traffic prioritization feature supported by the Switch at layer 2 is compatible with the relevant sections of the IEEE 802.1D/D17 standard (incorporating IEEE 802.1p). Once a packet has been classified, the level of service relevant to that type of packet is applied to it.
The 802.1D standard specifies eight distinct levels of priority (0 to 7), each of which relates to a particular type of traffic. The priority levels and their traffic types are shown in Figure 11 in order of increasing priority.
