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Configuring Quality of Service
VPT Classification Information
Page 183
Section 15. Configuring Quality of Service
Quality of Service (QoS) provides the ability to implement QoS and priority queuing within a network. For example,
certain types of traffic that require minimal delay, such as Voice, Video, and real-time traffic can be assigned a
high priority queue, while other traffic can be assigned a lower priority queue. The result is an improved traffic flow
for traffic with high demand. QoS is defined by:
• Classification — Specifies which packet fields are matched to specific values. All packets matching the user-
defined specifications are classified together.
• Action — Defines traffic management where packets are forwarded are based on packet information, and
packet field values such as VLAN Priority Tag (VPT) and DiffServ Code Point (DSCP).
VPT Classification Information
VLAN Priority Tags (VPT) are used to classify packets by mapping packets to one of the egress queues.
VPT-to-queue assignments are user-definable. Packets arriving untagged are assigned a default VPT value,
which is set on a per-port basis. The assigned VPT is used to map the packet to the egress queue. The table
below details the VPT-to-queue default settings:
DSCP values can be mapped to priority queues. DSCP mapping is enabled on a per-system basis. The following
table contains the default DSCP mapping to egress queue values:
Table 16: CoS to Queue Mapping Table Default values
CoS Value Forwarding Queue Values
0 q1 (Lowest Priority)
1 q0 (Lowest Priority)
2 q0 (Lowest Priority)
3 q1 (Lowest Priority)
4q2
5q2
6q3
7q3
Table 17: DSCP to Queue Mapping Table Default Values
DSCP Value Forwarding Queue Values
0-15 q1 (Lowest Priority)
16-31 q2
32-47 q3
48-63 q4