Restrictions And Limitations For Hierarchical Input Policing Policies; Hierarchical Policies And Oversubscription - Cisco 10000 Series Configuration Manual

Chapter 13 Defining QoS for Multiple Policy Levels
Figure 13-2
500 packets received
at the interface

Restrictions and Limitations for Hierarchical Input Policing Policies

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Hierarchical Policies and Oversubscription

For releases prior to Cisco IOS Release 12.0(25)SX and Release 12.3(7)XI, the router does not allow
oversubscription of interfaces. If you oversubscribe hierarchical policies, instead of reducing the shape
rate of all policies, the router preserves as many policies as possible and reduces the policy shape rates
of a minimum number of policies to bring the sum of the hierarchical policy shape rates to less than the
physical interface bandwidth.
For Cisco IOS Release 12.0(25)SX and Release 12.3(7)XI and later releases, the router allows you to
oversubscribe interfaces. Oversubscription is always enabled.
For more information about oversubscription, see
Links."
OL-7433-09
Packet Flow Between Hierarchical Input Policing Policies
Top-level parent policy map:
policy_map_level1
100 packets dropped
Packet classification for the bottom-level child policy map occurs before the top-level policer acts
on the traffic classes.
Traffic policing at the top-level parent does not guarantee fairness in sharing bandwidth among the
child classes. If packets from two different traffic classes arrive at the same rate and then go through
a traffic policer, the output rates of the two classes might be different because the hierarchical input
policer acts as an aggregate policer. The parent policer might drop packets in one class in favor of
the other class. This situation can happen when the top-level policer has enough tokens when the
packets for one class arrive, but does not have enough tokens left for the other class. Based on the
arrival pattern of the packets, this pattern could continue indefinitely.
400 packets transmitted
Bottom-level child policy map:
policy_map_level2
200 packets dropped
Chapter 15, "Oversubscribing Physical and Virtual
Cisco 10000 Series Router Quality of Service Configuration Guide
Hierarchical Policies and Oversubscription
200 packets exit
the interface
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