Restrictions And Limitations For Nested Hierarchical Policies - Cisco 10000 Series Configuration Manual

Chapter 13 Defining QoS for Multiple Policy Levels
Nested policy maps specify QoS policies at the following two levels of hierarchy:
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For releases prior to Cisco IOS Release 12.0(25)SX, the sum of the nested policy shape rates you specify
can be no more than 64 kbps less than the physical interface bandwidth. For example, the sum of the
nested policy shape rates for a DS1 Frame Relay interface must be no more than 1472 kbps, calculated
as follows:
If you specify a non-supported rate, the router uses the next lower supported rate instead.
Note The above restriction does not apply to Cisco IOS Release 12.0(25)SX and later releases.
For releases prior to Cisco IOS Release 12.0(25)SX and Release 12.3(7)XI, the router does not limit the
number of nested policies you can configure on a physical network interface as long as the sum of the
nested policy shape rates is 64 kbps less than the total bandwidth of the interface. In Cisco IOS Release
12.0(25)SX and Release 12.3(7)XI, and later releases, the router allows oversubscription. For more
information, see
The router reserves the shape rate you specify in the parent policy for the child traffic classes. The router
does not allocate unused (or excess) bandwidth to other traffic. For example, consider a nested policy
with a shape rate of 64 kbps. If the nested policy traffic rate is 32 kbps, the router does not allocate the
remaining 32 kbps to the other traffic on the network interface.
In some cases, the nested policy shape rate that the system uses might be lower than the shape rate you
specify. Use the show policy-map interface command to verify the actual shape rate.
For Frame Relay PVCs, instead of using a nested policy map to specify the multiqueue shape rate, you
can use the frame-relay traffic-shape command to specify a fair queue policy map.

Restrictions and Limitations for Nested Hierarchical Policies

This section lists restrictions for nested hierarchical policies. These restrictions might not apply to other
Note
types of hierarchical policies.
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Child policy (bottom-level)—Identifies one or more classes of traffic and defines QoS behavior for
the individual traffic streams. If you specify a class bandwidth in a child policy as a percentage, the
router uses the top-level parent shape rate as the bandwidth reference (100 percent) rather than the
bandwidth of the network interface. For example, in a nested policy shaped at 2 Mbps with a
bottom-level child policy configured for 50 percent bandwidth, the router allocates 1 Mbps of
bandwidth to the child policy (50 percent of the parent shape rate).
Parent policy (top-level)—Shapes the output of the traffic classes into a single shape rate. The parent
policy can contain only the class-default class with only the shape command specified.
1536 kbps – 64 kbps = 1472 kbps
Chapter 15, "Oversubscribing Physical and Virtual Links."
Nested hierarchical policies can have no more than two levels of hierarchy.
Only the top-level parent policy can have the class-default class defined.
The parent class-default class can have only the shape command configured; you cannot specify any
other policy action. The class-default class can also have the service-policy command configured to
attach a child policy to the parent policy. You must specify the shape command before you specify
the service-policy command.
Queuing services must exist at a single hierarchy level, except for the shape command, which is
defined in the parent policy's class-default class.
Cisco 10000 Series Router Quality of Service Configuration Guide
Types of Hierarchical Policies
13-7