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Table Of Contents
Guaranteeing Bandwidth with the priority Command
Configuring Low Latency Queueing
Verifying Low Latency Queueing
Monitoring and Maintaining Low Latency Queueing
Low Latency Queueing
This feature module describes the Low Latency Queueing feature for the Cisco 7200 and 7500 series routers.
Note:
This feature was formerly called:
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Low Latency Queueing for the Versatile Interface Processor
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Some Cisco documentation might use these titles.
Feature Specifications for Low Latency Queueing
Finding Support Information for Platforms and Cisco IOS Software Images
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Contents
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Feature Overview
The Low Latency Queueing feature brings the ability to specify low latency behavior for a traffic class for both VIP and non-VIP platforms. Low Latency Queuing allows delay-sensitive data such as voice to be dequeued and sent first (before packets in other queues are dequeued), giving delay-sensitive data preferential treatment over other traffic. Low Latency Queuing also introduces transmission ring limit tuning.
The Low Latency Queueing feature also introduces the ability to limit the depth of a device transmission ring. Before the introduction of Low Latency Queuing, the maximum transmission ring depth was not a user-configurable parameter. Therefore, particles could accumulate on a transmission ring without limitation, which could result in unavoidable high latencies. The Low Latency Queuing feature allows users to limit the number of particles that may exist on a transmission ring, effectively lowering the latency incurred by packets sitting on that transmission ring.
The priority command is used to allow delay-sensitive data to be dequeued and sent first. Low Latency Queueing enables use of a single priority queue within which individual classes of traffic can be placed. To enqueue class traffic to the priority queue, you configure the priority command for the class after you specify the named class within a policy map. Within a policy map, you can give one or more classes priority status. When multiple classes within a single policy map are configured as priority classes, all traffic from these classes is enqueued to the same, single, priority queue.
Guaranteeing Bandwidth with the priority Command
When you specify the priority command for a traffic class, it takes a bandwidth argument that gives maximum bandwidth in kilobits per second (kbps). You use this parameter to specify the maximum amount of bandwidth allocated for packets belonging to the class configured with the priority command. The bandwidth parameter guarantees the configured bandwidth to the priority class under worst-case congestion scenarios. If excess bandwidth is available, the priority class will be allowed to utilize the bandwidth. If no excess bandwidth is available, the priority traffic will be constrained to the configured rate via packet drops. Each individual class that is configured for priority behavior will have its traffic constrained to its individual rate. When a class is constrained to its individual rate, the traffic is permitted a certain amount of burstiness because of the token bucket mechanism policing the stream. This amount of burstiness is controlled by the optional bytes parameter in the priority command. The bytes parameter specifies, in bytes, the amount of traffic allowed to pass through the token bucket as a one-time burst in excess of the token bucket drop parameters. The default burst value is 200 milliseconds of traffic at the configured token bucket drop parameters.
It is important to note that because bandwidth for the priority class is specified as a parameter to the priority command, you cannot also configure the bandwidth command for a priority class. To do so is a configuration violation that introduces confusion in relation to the amount of bandwidth to allocate.
The bandwidth allocated for a priority queue always includes the Layer 2 encapsulation header. However, it does not include other headers, such as ATM cell tax overheads. When you calculate the amount of bandwidth to allocate for a given priority class, you must account for the fact that the Layer 2 headers are included. When ATM is used, you must account for the fact that ATM cell tax overhead is not included. You must also allow bandwidth for the possibility of jitter introduced by routers in the voice path.
Consider this case that uses ATM: Suppose a voice stream of 60 bytes emitting 50 packets per second is encoded using G.729. Prior to converting the voice stream to cells, the meter for the priority queue used for the voice stream assesses the length of the packet after the Layer 2 LLC headers have been added.
Given the 8-byte Layer 2 LLC header, the meter will take into account a 68-byte packet. Because ATM cells are a standard 53 bytes long, before the 68-kbps packet is emitted on the line, it is divided into two 53-byte ATM cells. Thus, the bandwidth consumed by this flow is 106 bytes per packet.
For this case, then, you must configure the bandwidth to be at least 27.2 kbps (68*50*8 = 27.2 kbps). However, recall that you must also allow for the ATM cell tax overhead, which is not accounted for by the configured bandwidth. In other words, the sum of the bandwidths for all classes must be less than the interface bandwidth by at least (106-68)*50*8 = 15.2 kbps. You should also remember to allow bandwidth for router-introduced jitter.
Benefits
Provides Priority Service on ATM VCs and Serial Interface
The priority queueing scheme allows delay-sensitive data such as voice to be dequeued and sent first—that is, before packets in other queues are dequeued. Delay-sensitive data is given preferential treatment over other traffic. This feature provides priority queueing on ATM virtual circuits (VCs);
Admission Control
By configuring the maximum amount of bandwidth allocated for packets belonging to a class, you can avoid starving non-priority traffic.
Limiting Particles on a Transmission Ring
The Low Latency Queueing feature also introduces particle limiting for transmission rings. Before the introduction of Low Latency Queuing, the transmission ring depth was not user-configurable. Therefore, a user could experience unavoidable high latencies on a transmission ring.
The Low Latency Queuing feature allows users to limit the number of particles on a transmission ring to a predefined limit, effectively lowering the latency on transmission rings.
Restrictions
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Prerequisites
To use this feature, you should be familiar with the following:
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Configuration Tasks
See the following sections for configuration tasks for the Low Latency Queueing feature. Each task in the list indicates if the task is optional or required.
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Configuring Low Latency Queueing
To give priority to a class within a policy map, use the following commands in the policy map class configuration mode:
Verifying Low Latency Queueing
To view the contents of the priority queue (such as queue depth and the first packet queued), use the following commands in EXEC mode:
The priority queue is the queue in which the conversation ID is equal to the number of dynamic queues plus 8. The packets in the priority queue have a weight of 0.
Monitoring and Maintaining Low Latency Queueing
To tune your Real-Time Transport Protocol (RTP) bandwidth or to decrease RTP traffic if the priority queue is experiencing drops, use one or more of the following commands:
Configuration Example
In the following example, a strict priority queue with a guaranteed allowed bandwidth of 50 kbps is reserved for traffic that is sent from the source address 10.10.10.10 to the destination address 10.10.10.20, in the range of ports 16384 through 20000 and 53000 through 56000.
The following commands configure an access list called 102 to match the desired voice traffic:
Router(config)# access-list 102 permit udp host 10.10.10.10 host 10.10.10.20 range 16384 20000Router(config)# access-list 102 permit udp host 10.10.10.10 host 10.10.10.20 range 53000 56000The following commands perform the following functions:
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Router(config)# class-map voiceRouter(config-cmap)# match access-group 102Router(config)# policy-map policy1Router(config-pmap)# class voiceRouter(config-pmap-c)# priority 50Router(config-pmap)# class barRouter(config-pmap-c)# bandwidth 20Router(config-pmap)# class class-defaultRouter(config-pmap-c)# fair-queueRouter(config)# interface atm1/0.2Router(config-subif)# pvc 0/102Router(config-subif-vc)# service-policy output policy1Additional References
Related Documents
Standards
MIBs
Class-Based Quality of Service MIB
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
This section documents the priority command. All other commands used with this feature are documented in the Cisco IOS Release 12.3 command reference publications or Cisco IOS Release 12.xT or 12.0S feature module publications.
priority
To give priority to a class of traffic belonging to a policy map, use the priority command in policy-map class configuration mode. To remove a previously specified priority for a class, use the no form of this command.
priority {bandwidth-kbps | percent percentage} [burst]
no priority {bandwidth-kbps | percent percentage} [burst]
Syntax Description
Defaults
No default behavior or values
Command Modes
Policy-map class configuration
Command History
Usage Guidelines
This command configures low latency queueing (LLQ), providing strict priority queueing (PQ) for class-based weighted fair queueing (CBWFQ). Strict PQ allows delay-sensitive data such as voice to be dequeued and sent before packets in other queues are dequeued.
The priority command allows you to set up classes based on a variety of criteria (not just User Datagram Ports (UDP) ports) and assign priority to them, and is available for use on serial interfaces and ATM permanent virtual circuits (PVCs). A similar command, the ip rtp priority command, allows you to stipulate priority flows based only on UDP port numbers and is not available for ATM PVCs.
When the device is not congested, the priority class traffic is allowed to exceed its allocated bandwidth. When the device is congested, the priority class traffic above the allocated bandwidth is discarded.
The bandwidth and priority commands cannot be used in the same class, within the same policy map. These commands can be used together in the same policy map, however.
Within a policy map, you can give one or more classes priority status. When multiple classes within a single policy map are configured as priority classes, all traffic from these classes is queued to the same, single, priority queue.
When the policy map containing class policy configurations is attached to the interface to stipulate the service policy for that interface, available bandwidth is assessed. If a policy map cannot be attached to a particular interface because of insufficient interface bandwidth, the policy is removed from all interfaces to which it was successfully attached.
For more information on bandwidth allocation, refer to the chapter "Congestion Management Overview" in the Cisco IOS Quality of Service Solutions Configuration Guide.
Examples
The following example configures PQ with a guaranteed bandwidth of 50 kbps and a one-time allowable burst size of 60 bytes for the policy map called policy1:
Router(config)# policy-map policy1Router(config-pmap)# class voiceRouter(config-pmap-c)# priority 50 60In the following example, 10 percent of the available bandwidth is reserved for the class called voice on interfaces to which the policy map called policy1 has been attached:
Router(config)# policy-map policy1Router(config-pmap)# class voiceRouter(config-pmap-c)# priority percent 10Related Commands
Glossary
CBWFQ—Class-Based Weighted Fair Queueing. Extends the standard WFQ functionality to provide support for user-defined traffic classes.
Class-Based Weighted Fair Queueing—See CBWFQ.
LLC—logical link control. Higher of the two data link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow control, framing, and MAC-sublayer addressing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants.
logical link control—See LLC.
PPP—Point-to-Point Protocol. Successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP.
Random Early Detection—See RED.
RED—Random Early Detection. A congestion avoidance mechanism that takes advantage of TCP's congestion control mechanism. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it will decrease its transmission rate until all the packets reach their destination, indicating that the congestion is cleared.
RTP—Real-Time Transport Protocol. One of the IPv6 protocols. RTP is designed to provide end-to-end network transport functions for applications sending real-time data such as audio, video, or simulation data over multicast or unicast network services. RTP provides services such as payload type identification, sequence numbering, time-stamping, and delivery monitoring to real-time applications.
SNA—Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences.
UDP—User Datagram Protocol. Connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.
weighted fair queueing—See WFQ.
WFQ—weighted fair queueing. Congestion management algorithm that identifies conversations (in the form of traffic streams), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations. WFQ is an automatic way of stabilizing network behavior during congestion and results in increased performance and reduced retransmission.
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