PDF(83.9 KB) View with Adobe Reader on a variety of devices
Updated:Jun 13, 2008
Last updated: July 2008
This document describes the new behavioral changes and Command Line Interface (CLI) modifications to the queuing infrastructure on non-distributed hardware (Cisco 7200 Series Router and other lower-end platforms). The changes being applied to Cisco IOS Software Release 12.4(20)T will provide a consistent queuing behavior using the same Modular Quality of Service CLI (MQC) across all Cisco IOS Software platforms. These changes are currently present in the latest versions of Cisco IOS Software Releases 12.0S and 12.2S.
The purpose of this document is to highlight the new queuing changes for customers who are upgrading to the latest T release, starting with Cisco IOS Software Release 12.4(20)T. This document does not intend to discuss all of the new MQC feature capabilities that have been introduced to the Cisco 7200 Series Router and the lower-end platforms. For a complete list of the Quality of Service (QoS) feature capabilities, refer to the appropriate Cisco IOS Software release documentation on Cisco.com.
With the deployment of Hierarchical Queuing Framework (HQF) in the Release 12.4(20)T train, we are bringing together the queuing and shaping mechanisms that are currently used in Cisco IOS Software Releases 12.0S and 12.2S. With this migration, our customers benefit in several different ways:
• Consistent queuing behavior applied with common MQC across all main Cisco IOS Software releases.
• Common functionality for both distributed and non-distributed implementations, providing consistency of QoS feature behavior across all software-forwarding hardware, making implementation of QoS easier and transparent regardless of Cisco IOS Software Release is being used.
• With HQF customers using any of the IOS releases will have:
– The ability to provide multiple levels of packet scheduling
– The ability to support integrated class-based shaping and queuing
– The ability to apply fair queuing and drop policies on a per-class basis
New QoS Feature Functionality
The following HQF features are new in Cisco IOS Software Release 12.4(20)T:
Hierarchical Policy with Queuing Features at Every Level
With this feature you can apply class-based queuing to any traffic class in the parent or child level of a hierarchical policy and obtain different service levels for different sessions or subscribers. In the example shown below, the traffic belonging to class parent-c2 will have more scheduling time than class parent-c1.
Fair Queue in an MQC Class
This feature provides a fair-share of the bandwidth among the flows within any traffic class that has fair queuing enabled. You can apply the
fair-queue command to a user-defined class as shown in the following example:
Shaping in an ATM PVC Policy
The shape feature is now supported at the ATM PVC level through a service policy. Prior to applying the service policy to the PVC, a service category such as vbr-nrt has to be enabled on the PVC. You can then apply a class-based shaping within an ATM PVC as shown in the following example:
shape average 1000000
bandwidth percent 40
bandwidth percent 25
vbr-nrt 2000 2000
service-policy output p1
Strict Priority with No Policing Rate
Only one class is allowed strict priority configuration. Other classes cannot have priority or bandwidth configuration. If minimum bandwidth is required by one of the other classes, the
bandwidth remaining percent command must be used, as shown in the following example:
bandwidth remaining percent 20
Priority with Explicit Policing Rate
When a priority class is configured with an explicit policer, traffic is limited to the policer rate regardless of congestion conditions. In other words, even if bandwidth is available the priority traffic will not be able to exceed the rate specified with the explicit policer.
police cir 1000000 conform-action transmit exceed-action drop
Random-detect Support in Class-default
random-detect command can be configured for class-default to calculate the probability of dropping a packet. An example of applying random-detect to class-default traffic based on precedence bits is shown below.
random-detect precedence 0 40 80
Random-detect Options and Thresholds Support
random-detect command supports the
cos-based options to calculate the probability of dropping a packet.
queue-limit command can also be set in units of bytes or ms, in addition to its default units of packets.
queue-limit 1000 bytes
queue-limit 500 bytes
QoS Behavioral Changes
With the migration of HQF into Cisco IOS Software Release 12.4(20)T, the following behavioral changes occur for some of the QoS features currently available in the T train.
Changes Related to Class-Default
When you do not explicitly configure the class-default class in a policy map, its default queuing behavior is FIFO. You can configure the
service-policy commands in the class-default class to achieve different queuing behaviors.
When fair-queue is applied to class-default, the behavior is that of flow-based. This is a change from the Weighted Fair Queuing (WFQ) behavior in previous releases. With flow-based fair queuing, the flow queues in the class-default class are scheduled equally instead of by weight based on the IP Precedence bits.
The bandwidth assigned to the class-default class is the unused interface bandwidth not consumed by user-defined classes. By default, the class-default class receives a minimum of 1% of the interface or parent shape bandwidth. In the example below, when the following policy-map is attached to a 10Mbps interface,
class-default will get the remaining bandwidth guarantee of 4Mbps (10 - 2 - 4). In the event that less than 4Mbps of class-default traffic were present, class c1 and class c2 will evenly share the available bandwidth not used by class-default.
The bandwidth command may be configured in class-default to explicitly assign a different bandwidth ratio.
Default Queuing Implementation for the Shape Feature
When you configure the
shape command in a class, the default queuing behavior for the shape queue is FIFO instead of WFQ. You can configure the
service-policy commands in shape class to achieve different queuing behaviors.
Policy Map and Interface Bandwidth
In HQF, a policy map can reserve up to 100% of the interface bandwidth. Up to a maximum of 99% of the interface bandwidth can be assigned to
user-defined classes as by default 1% of the bandwidth is reserved for the class-default class.
Note that when migrating to Release 12.4(20)T , if the configured policy map allocates 100% of the bandwidth to the user-defined classes, an error message will appear in the console after booting the HQF image. The message will indicate that the allocated bandwidth exceeds the allowable amount and the service policy will be rejected. In HQF, the policy map has to be re-configured to account for the minimum 1% bandwidth guaranteed for the class-default. The service policy can then be applied to the interface.
Per-Flow Queue Limit in Fair Queue
In HQF, when you enable fair queuing, the default per-flow queue limit is ¼ of the class queue limit. If you do not enable the queue limit in a class, the default per-flow queue limit is 16 packets (1/4 of 64).
Over-Subscription Support for Multiple Policies on Logical Interfaces
When you attach a shaping policy to multiple logical interfaces including a subinterface, and the sum of shape rate exceeds the physical interface bandwidth, congestion at the physical interface results in backpressure to each logical interface policy. This backpressure causes each policy to reduce the output rate down to its fair share of the interface bandwidth.
Example: 10 subinterface policies each shaped to 2Mbps, physical interface has 10Mbps bandwidth (2:1 oversubscription), when all 10 subinterfaces are sending at 2Mbps, each subinterface gets a throughput of 1Mbps (10 Mbps / 10 subinterfaces).
Shaping Behavior on GRE Tunnel
In HQF, the shape feature can be applied to a GRE tunnel via a hierarchical service policy. Shaping on GRE tunnel will be applied after encapsulation. This means the shape rate is based on packets with tunnel encapsulation and L2 encapsulation.
Shape is the only feature allowed in the service policy applied to a tunnel interface. When configuring the shape feature in the parent policy is applied to the tunnel interface, only the class-default class is permitted. Configuring a user-defined class in the parent policy is not allowed. A typical hierarchical policy applied to a GRE tunnel interface is shown below.
Service-policy output parent
shape average 10000000
Currently, certain QoS deployments include a service policy with queuing features applied at the tunnel or a virtual interface, and a service policy with queuing features applied at the physical interface. In Release 12.4(20)T, a service policy with queuing features can only be supported at one of these interfaces. When migrating to Release 12.4(20)T, a router configuration containing service policies at both interfaces will only keep the one applied to the physical interface.
Change in FRF.12 and FRF.9 Behavior
With HQF implementation, when you enable Frame Relay Fragmentation (FRF.12) on an FR PVC or FR main interface, priority class packets are no longer subject to fragmentation. Priority packets, regardless of the packet size, always interleave among data fragments.
When you enable Frame Relay payload compression (FRF.9) on an FR PVC or main interface, priority class packets are no longer compressed. When you enable both FRF.12 and FRF.9, priority class packets are neither fragmented nor compressed.
Changes in CLI
Some CLI commands are being changed with the integration of HQF.
random-detect prec-based Command
random-detect prec-based command within a policy-map has been replaced by the
random-detect precedence-based command. The function of the command does not change.
shape max-buffers Command
shape max-buffers command currently configured under a class in a policy-map will no longer be supported in HQF. It will be replaced by the
queue-limit command, which provides similar functionality. Upon migration to an IOS release which integrates HQF, a router configured with the shape max-buffer command will be automatically configured with the queue-limit command.
max-reserved-bandwidth command no longer affects the amount of bandwidth available to a service policy. Any policy-map can allocate up to 100% of the bandwidth without the need of the max-reserved-bandwidth command. The max-reserved-bandwidth command was used in previous IOS releases in order to overcome the restriction of allocating 75% of the bandwidth to user-defined classes. In HQF, that restriction does not exist anymore.
Show Command Changes
show queuing and show queue Commands
show queuing and
show queue commands are no longer supported. Instead, you can use the
show policy-map and
show policy-map interface commands to gather QoS-related information and statistics.
show policy-map interface output has additional fields that display the DSCP value, WRED statistics in bytes, new column for number of transmitted packets by WRED, and a counter displaying packets output/bytes output in each class. The
packets queued/bytes queued counter is no longer supported. Table 1 compares the old and new output formats for both commands. The outputs have been lined up for easier comparison and to see what is missing between them.
Table 1. Comparison of Old and New Command Output Formats