Cisco Systems WSC4500X16SFP Basic QoS Model

27-5
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter27 Configuring Quality of Service
Overview of QoS
•Classification is the selection of traffic to be marked.
•Marking, according to RFC 2475, is the process of setting a Layer3 DSCP value in a packet; in this
publication, the definition of marking is extended to include setting Layer2 CoS values.
•Scheduling is the assignment of Layer2 frames to a queue. QoS assigns frames to a queue based on
internal DSCP values as shown in Internal DSCP Values, page 27-13.
•Policing is limiting bandwidth used by a flow of traffic. Policing can mark or drop traffic.
Basic QoS Model
Figure 27-2 shows the basic QoS model. Actions at the ingress and egress interfaces include classifying
traffic, policing, and marking:
•Classifying distinguishes one kind of traffic from another. The process generates an internal DSCP
for a packet, which identifies all the future QoS actions to be performed on this packet. For more
information, see the “Classification” section on page27-6.
•Policing determines whether a packet is in or out of profile by comparing the traffic rate to the
configured policer, which limits the bandwidth consumed by a flow of traffic. The result of this
determination is passed to the marker. For more information, see the “Policing and Marking” section
on page 27-10.
•Marking evaluates the policer configuration information regarding the action to be taken when a
packet is out of profile and decides what to do with the packet (pass through a packet without
modification, mark down the DSCP value in the packet, or drop the packet). For more information,
see the “Policing and Marking” section on page 27-10.
Actions at the egress interface include queueing and scheduli ng:
•Queueing evaluates the internal DSCP and determines which of the four egress queues in which to
place the packet.
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0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
16
17
18
19
20
21
22
23
61
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
48
49
50
51
52
53
54
55
30
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
24
25
26
27
28
29
30
31
71
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
56
57
58
59
60
61
62
63
1. MSb = most significant bit
Table27-1 IP Precedence and DSCP Values (continued)
3-bit IP
Precedence
6 MSb1 of ToS 6-bit
DSCP
3-bit IP
Precedence
6 MSb1 of ToS 6-bit
DSCP
876 543 876 543