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104 Intel® Gigabit Ethernet Switch AXXSW1GB User Guide
Quality of Service Overview
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.
CoS Services
After packets are assigned to a specific egress queue, CoS services can be assigned to the
queue. Egress queues are configured with a scheduling scheme by one of the following
methods:
• Strict Priority — Ensures that time-sensitive applications are always forwarded.
Strict Priority (SP) allows the prioritization of mission-critical, time-sensitive traffic
over less time-sensitive applications.
For example, under SP, voice over IP (VoIP) traffic can be prioritized so that it is
forwarded before FTP or e-mail (SMTP) traffic.
• Weighted Round Robin — Ensures that a single application does not dominate the
device forwarding capacity. Weighted Round Robin (WRR) forwards entire queues in
a round robin order. SP queues are serviced before WRR queues. If the traffic flow is
minimal, and SP queues do not occupy the whole bandwidth allocated to a port, the
WRR queues can share the bandwidth with the SP queues. This ensures that the
remaining bandwidth is distributed according to the weight ratio. If WRR is selected,
the following weights are assigned to the queues: 1, 2, 4, 8.