3Com 3824 Implementation Guide - Page 54

54 CHAPTER 6: USING TRAFFIC PRIORITIZATION Figure 14 shows how traffic prioritization works at layer 2. The Switch will check a packet received at the ingress port for IEEE 802.1D traffic classification, and then prioritize it based upon the IEEE 802.1p value (service levels) in that tag. It is this 802.1p value that determines which traffic queue the packet is mapped to. Advanced Traffic Prioritization Incoming traffic can be classified based on packet attributes at different layers of the OSI 7 layer model. The Switch can look in the packet for layer 2, 3 and 4 attributes to identify incoming traffic. Most of the current applications, for example Microsoft Word, Lotus Notes and NetMeeting, are not QoS-aware and do not apply a service level to the traffic that they send. Being an intelligent Switch, your Switch can use its own rules to classify and mark the traffic. If the incoming traffic has pre-defined service level markings, the advanced traffic prioritization of your Switch will assign the appropriate DSCP and 802.1D service level markings to that incoming traffic. The advanced traffic prioritization in the Switch will: ■ Classify traffic based on different packet attributes. The four common methods of classification are DSCP, TCP/UDP ports, IP Address and Protocol. ■ Mark traffic as it enters the Switch with the appropriate DSCP and 802.1D markings. Figure 15 shows how traffic is treated using the advanced traffic prioritization in the Switch. The DSCP field in the IP packet header can be used to classify (identify) traffic as well as carrying the priority markings, as shown in Figure 15.