H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – QoS
H3C S7500E Series Ethernet Switches Chapter 2 Traffic Classification, TP, and LR Configuration
2-2
scheduling is performed on the packets; when congestion get worse, congestion
avoidance is performed on the packets.
2.1.2 Priority
The following describes several types of precedence:
1) IP precedence, ToS precedence, and DSCP precedence
Figure 2-1 DS field and ToS field
The ToS field in an IP header contains eight bits, which are described as follows:
z The first three bits indicate IP precedence in the range of 0 to 7.
z Bit 3 to bit 6 indicate ToS precedence in the range of 0 to 15.
z RFC2474 re-defines the ToS field in the IP packet header, which is called the DS
field. The first six (bit 0 to bit 5) bits of the DS field indicate DSCP precedence in
the range of 0 to 63. The last two bits (bit 6 and bit 7) are reserved bits.
Table 2-1 Description on IP Precedence
IP Precedence (decimal) IP Precedence (binary) Description
0 000 Routine
1 001 priority
2 010 immediate
3 011 flash
4 100 flash-override
5 101 critical
6 110 internet
7 111 network
In a network providing differentiated services, traffics are grouped into the following four
classes, and packets are processed according to their DSCP values.
z Expedited Forwarding (EF) class: In this class, packets can be forwarded
regardless of link share of other traffic. The class is suitable for preferential