ZyXEL VMG4380 User Guide - Page 175
IP Precedence, Automatic Priority Queue Assignment
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Chapter 10 Quality of Service (QoS) DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ compliant, ToS-enabled network device will not conflict with the DSCP mapping. DSCP (6 bits) Unused (2 bits) The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be marked for different kinds of forwarding. Resources can then be allocated according to the DSCP values and the configured policies. IP Precedence Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to prioritize packets in a layer-3 network. IP precedence uses three bits of the eight-bit ToS (Type of Service) field in the IP header. There are eight classes of services (ranging from zero to seven) in IP precedence. Zero is the lowest priority level and seven is the highest. Automatic Priority Queue Assignment If you enable QoS on the Device, the Device can automatically base on the IEEE 802.1p priority level, IP precedence and/or packet length to assign priority to traffic which does not match a class. The following table shows you the internal layer-2 and layer-3 QoS mapping on the Device. On the Device, traffic assigned to higher priority queues gets through faster while traffic in lower index queues is dropped if the network is congested. Table 53 Internal Layer2 and Layer3 QoS Mapping LAYER 2 LAYER 3 PRIORITY QUEUE 0 1 2 3 4 5 IEEE 802.1P USER PRIORITY (ETHERNET PRIORITY) 1 2 0 3 TOS (IP PRECEDENCE) 0 0 1 4 2 5 3 DSCP 000000 000000 001110 001100 001010 001000 010110 010100 010010 010000 011110 011100 011010 011000 IP PACKET LENGTH (BYTE) >1100 250~1100
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