3Com 3C17300-US Implementation Manual

Contents

52 C HAPTER 6: U SING T RAFFIC P RIORITIZATION■ Resource planning applications — Used by organizations thatrequire predictable and reliable access to enterprise resource planningapplications such as SAP.■ Financial applications — Used by Accounts departments that needimmediate access to large files and spreadsheets.■ CAD/CAM design applications — Used by design departments thatneed priority connections to server farms and other devices fortransferring large files.How TrafficPrioritization WorksTraffic prioritization ensures that high priority data is forwarded throughthe Switch without being delayed by lower priority data. Trafficprioritization uses the two traffic queues that are present in the hardwareof the Switch to ensure that high priority traffic is forwarded on adifferent queue from lower priority traffic. High priority traffic is givenpreference over low priority traffic to ensure that the most critical trafficgets the highest level of service.The Switch employs two methods of classifying traffic for prioritization.Traffic classification is the means of identifying which applicationgenerated 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 trafficclassificationAt layer 2, a traffic service class is defined in 802.1Q frame, which is ableto carry VLAN identification and user priority information. Theinformation is carried in a header field immediately following thedestination MAC address, and Source MAC address.802.1D Priority LevelsThe traffic prioritization feature supported by the Switch at layer 2 iscompatible with the relevant sections of the IEEE 802.1D/D17 standard(incorporating IEEE 802.1p). Once a packet has been classified, the levelof 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 andtheir traffic types are shown in Figure 13 in order of increasing priority.dua1730-0bAA03.book Page 52 Monday, July 11, 2005 11:14 AM