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Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 3 Configuring QoS Policy Actions and Rules
QoS Performance
QoS Performance
The parallel express forwarding (PXF) engine processes QoS traffic. Sometimes the PXF engine cannot
finish processing a packet before the packet completes a single pass through the PXF; the packet requires
additional processing. As a result, the packet is fed back through the PXF and processing continues. This
is referred to as a feedback operation.
Packets that are subject to both inbound and outbound QoS policies require additional PXF processing,
resulting in a feedback. However, packets subject to only one QoS policy (either inbound or outbound)
require only one pass through the PXF; a feedback is not needed.
Extra PXF passes reduce the system forwarding capacity. For example, if x packets per second require y
extra passes, the system forwarding capacity diminishes by xy—the system has xy fewer packets per
second forwarding capacity than before. Although the forwarding capacity diminishes, system
performance is not affected. Packet classification processing affects only the forwarding capacity of the
system, not the speed. Packet delay due to additional PXF passes is negligible. Therefore, system
performance degradation occurs only at high system utilization.
The following describes PXF requirements:
• All releases prior to Cisco IOS Release 12.0(17)SL—For each packet, the PXF requires one pass per
class-map match statement.
• Cisco IOS Release 12.0(17)SL—For each packet, the PXF requires one pass for every four
non-access control list (ACL) class-map match statements. For each packet, the PXF requires one
pass for one ACL class-map match statement.
• Cisco IOS Release 12.0(19)SL and later releases—For each packet, the PXF requires one pass per
policy, regardless of the sum of the match statements in each class of the policy.
Configuring QoS Policies
To create a QoS policy, perform the following required tasks:
• Creating a Policy Map, page 3-19
• Defining QoS Actions in a Policy Map, page 3-20
• Attaching Service Policies, page 3-20 (See Chapter 4, “Attaching Service Policies.”)
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