3Com 2928 User Guide - Page 470

Figure 2-9 Line rate implementation With a token bucket used for traffic control, when there are tokens in the token bucket, the bursty packets can be transmitted; if no tokens are available, packets cannot be transmitted until new tokens are generated in the token bucket. In this way, the traffic rate is restricted to the rate for generating tokens, thus limiting traffic rate and allowing bursty traffic. Priority Mapping Concepts When a packet enters a network, it is marked with a certain priority to indicate its scheduling weight or forwarding priority. Then, the intermediate nodes in the network process the packet according to the priority. When a packet enters a device, the device assigns to the packet a set of predefined parameters (including the 802.1p precedence, DSCP values, IP precedence, and local precedence). z For more information about 802.1p precedence, DSCP values, and IP precedence, refer to Packet Precedences. z Local precedence is a locally significant precedence that the device assigns to a packet. A local precedence value corresponds to an output queue. Packets with the highest local precedence are processed preferentially. The device provides two priority trust modes on a port: z Trust packet priority: the device assigns to the packet the priority parameters corresponding to the packet's priority from the mapping table. z Trust port priority: the device assigns a priority to a packet by mapping the priority of the receiving port. You can select one priority trust mode as needed. Figure 2-10 shows the process of priority mapping on a device. 2-10