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Contents
The Multi-Level Priority Queues (MPQ) feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device. Having multiple priority queues enables the device to place delay-sensitive traffic (for example, voice) on the outbound link before delay-insensitive traffic. As a result, high-priority traffic receives the lowest latency possible on the device.
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
You must configure traffic classes using the class-map command.
Device> enable Device# configure terminal Device(config)# policy-map Map1 Device(config-pmap)# class Bronze Device(config-pmap-c)# priority level 1 Device(config-pmap-c)# exit Device(config-pmap)# class Gold Device(config-pmap-c)# priority rate 1000 Device(config-pmap-c)# end
Device> enable Device# configure terminal Device(config)# policy-map Map1 Device(config-pmap)# class Bronze Device(config-pmap-c)# priority level 1 Device(config-pmap-c)# police cir percent 30 Device(config-pmap-c)# exit Device(config-pmap)# class Gold Device(config-pmap-c)# priority level 1 Device(config-pmap-c)# police cir 10000 Device(config-pmap-c)# end
policy-map P1 class C1 priority level 1 bandwidth 200
policy-map P1 class C1 priority level 1 shape average 56000
policy-map new class prec1 priority level 1 police cir 600000000 class prec2 priority level 2 police cir 700000000
The MPQ feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device.
Previously, devices could have only one strict priority queue per policy map for all delay-sensitive traffic—the device associated all priority traffic with this one single priority queue. However, having only one priority queue can cause significant delay in delivering traffic, especially if the device sends high-priority traffic (for example, voice) behind low-priority traffic (for example, video). Using class-based weighted fair queueing (CBWFQ) to reduce delay by heavily weighting one queue can affect the granularity of bandwidth allocations to the other queues. The MPQ feature addresses these issues and improves latency.
The priority command is used to specify that a class of traffic has latency requirements with respect to other classes. For multiple priority queues, you can use the priority level command to configure a level of priority service on a class in a policy map. Currently, the device supports two priority levels: level 1 (high) and level 2 (low). The device places traffic with a high-priority level on the outbound link ahead of traffic with a low-priority level. High-priority packets, therefore, are not delayed behind low-priority packets.
The device services the high-level priority queues until empty before servicing the next-level priority queues and non-priority queues. While the device services a queue, the service rate is as fast as possible and is constrained only by the rate of the underlying link or parent node in a hierarchy. If a rate is configured and the device determines that a traffic stream has exceeded the configured rate, the device drops the exceeding packets during periods of congestion. If the link is currently not congested, the device places the exceeding packets onto the outbound link.
When configuring MPQ on different traffic classes in a policy map, you must specify different priority levels for the traffic classes. For example, configure one traffic class to have priority level 2 and another class to have priority level 1.
Note | In a hierarchical MPQ configuration in which all traffic is sent through the level-2 priority queue only, the traffic sent through the level-2 priority queue receives the same treatment as the traffic sent through the level-1 priority queue. |
If high-priority traffic is not policed appropriately, bandwidth starvation of low-priority traffic can occur. Therefore, though not required, we recommend that you configure a policer for high-priority traffic using the police command. If you configure the police command for priority queues, the traffic rate is policed to the police rate for each of the priority queues.
You cannot configure the priority command and the priority level command on different classes in the same policy map.
Bandwidth guarantees can be given to other classes only if traffic policing is enabled on the priority queue.
Using the priority and police commands, multi-level priority queues can be configured to police traffic in one of the following ways:
policy-map my_policy class voice priority 400000 <<< Priority queue conditionally policed to 400M class gold bandwidth 400000 <<< 400M minimum guaranteed to class goldWith conditional traffic policing on the queue, you run the risk of sudden degradation in priority service when an interface becomes congested. You can go from an instance of a priority class using the entire link to suddenly traffic being policed to the configured value. You need to know the available bandwidth and use some form of admission control to ensure that your offered loads do not exceed the available bandwidth.
Note | With the conditional policing, traffic policing does not engage unless the interface is congested. |
policy-map my_policy class voice priority <<< Indicates priority scheduling police 400000000 <<< Traffic policed to 400M class gold bandwidth 400000 <<<400M minimum guaranteed to class goldThe priority class is configured with an “always on” (unconditional) policer. The priority class is always policed to the configured value regardless of whether the interface is congested. The advantage of an unconditional policer is that you always know how much priority traffic will be offered to the downstream devices, thus making your bandwidth planning much simpler. This is the recommended choice.
The traffic classes, class maps, and policy maps must exist.
1.
enable
2.
configure
terminal
3.
policy-map
policy-name
4.
class
class-name
5.
priority
level
level
6.
police
cir
bps
7.
police
cir
percent
percent
8.
end
1.
enable
2.
show
policy-map
interface
type
number
3.
exit
Command or Action | Purpose | |
---|---|---|
Step 1 |
enable
Example: Device> enable |
Enables privileged EXEC mode. |
Step 2 |
show
policy-map
interface
type
number
Example: Device# show policy-map interface serial4/0/0 |
Displays the packet statistics of all classes that are configured for all service policies either on the specified interface or subinterface or on a specific PVC on the interface. |
Step 3 |
exit
Example: Device# exit |
(Optional) Exits privileged EXEC mode. |
The following example shows how to configure multiple priority queues. The policy map named Business has two traffic classes: Bronze and Gold. Bronze traffic has a level 2 (low) priority, whereas Gold traffic has a level 1 (high) priority. To prevent bandwidth starvation of Bronze traffic, the Gold traffic is policed at 30 percent of the interface bandwidth.
Device> enable Device# configure terminal Device(config)# policy-map Business Device(config-pmap)# class Bronze Device(config-pmap-c)# priority level 2 Device(config-pmap-c)# police cir 1000 Device(config-pmap-c)# exit Device(config-pmap)# class Gold Device(config-pmap-c)# priority level 1 Device(config-pmap-c)# police cir percent 30 Device(config-pmap-c)# end
Note | Although a policer is not required, configure policing for priority traffic to prevent bandwidth starvation of low-priority traffic. When policing is configured, the traffic rate is policed at the police rate for each of the priority queues. |
The following is partial sample output from the show policy-map interface command.
Device# show policy-map interface serial2/1/0 Serial2/1/0 Service-policy output: P1 Queue statistics for all priority classes: . . . Class-map: Gold (match-all) 0 packets, 0 bytes /*Updated for each priority level configured.*/ 5 minute offered rate 0 bps, drop rate 0 bps Match: ip precedence 2 Priority: 0 kbps, burst bytes 1500, b/w exceed drops: 0 Priority Level 2: 0 packets, 0 bytes
Related Topic |
Document Title |
---|---|
QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples |
Cisco IOS Quality of Service Solutions Command Reference |
Priority queues, creating classes, class maps, and policy maps |
“Applying QoS Features Using the MQC” module |
Description |
Link |
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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Releases |
Feature Information |
---|---|---|
Multi-Level Priority Queues |
Cisco IOS Release XE 2.1 Cisco IOS Release XE 3.7S |
The MPQ feature allows you to configure multiple priority queues for multiple traffic classes by specifying a different priority level for each of the traffic classes in a single service policy map. You can configure multiple service policy maps per device. Having multiple priority queues enables the device to place delay-sensitive traffic (for example, voice) on the outbound link before delay-insensitive traffic. As a result, high-priority traffic receives the lowest latency possible on the device. in Cisco IOS XE Release 2.1, this feature was implemented on the Cisco ASR 1000 Series Routers. In Cisco IOS XE Release 3.7S, support was added for the Cisco ASR 903 Router. The following commands were introduced or modified: priority level, show policy-map interface. |