Home > Articles > Cisco Certification > CCNP > CCNP Self-Study: Understanding and Implementing Quality of Service in Cisco Multilayer Switched Networks

CCNP Self-Study: Understanding and Implementing Quality of Service in Cisco Multilayer Switched Networks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: May 13, 2004.

Chapter Description

This chapter describes how QoS is an integral part of any multilayer switched network deployment. With QoS, you can build a network of predictable behavior for latency, jitter, and packet loss. In addition, QoS mitigates anomalous network behavior and provides for differentiation of traffic flows.


The QoS configuration and application differs between high-speed interfaces on Catalyst switches and low-speed WAN on Cisco IOS routers or WAN modules of Catalyst switches. This section highlights a few QoS configurations and features that are applicable to low-speed serial interfaces. Specifically, this section introduces weighted fair queuing (WFQ) and low-latency queuing (LLQ).

Weighted Fair Queuing

Flow-based and class-based WFQ applies priority (or weights) to identified traffic to classify traffic into conversations and to determine how much bandwidth each conversation is allowed relative to other conversations. WFQ classifies traffic into different flows based on such characteristics as source and destination address, protocol, and port and socket of the session. WFQ is the default queuing mechanism for E1 and slower links.

Class-based WFQ (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes. This enables you to specify the exact amount of bandwidth to be allocated for a specific class of traffic. Taking into account available bandwidth on the interface, you can configure up to 64 classes and control distribution among them.

Low-Latency Queuing

The distributed LLQ feature brings the ability to specify low-latency behavior for a traffic class. LLQ allows delay-sensitive data to be dequeued and sent first, before packets in other queues are dequeued, giving delay-sensitive data preferential treatment over other traffic.

The priority command is used to allow delay-sensitive data to be dequeued and sent first. LLQ enables use of a single priority queue within which individual classes of traffic are placed.

LLQ offers these features:

  • LLQ supports multiple traffic types over various Layer 2 technologies, including High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), ATM, and Frame Relay.

  • All classes are policed to bandwidth to ensure that other traffic is serviced.

  • The rate limit is per class, even if multiple classes point traffic to a priority queue.

  • Oversubscription of bandwidth is not allowed for the priority class.

  • No WRED support is provided on priority classes. WRED is allowed only on bandwidth classes.

  • Bandwidth and priority are mutually exclusive.

The next two sections discuss two additional WAN QoS features, IP RTP Priority and link-efficiency mechanisms.

IP RTP Priority

The IP Real-Time Transport Protocol Priority (IP RTP Priority) feature provides a strict-priority queuing scheme that allows delay-sensitive data such as voice to be dequeued and sent before packets in other queues. This feature is similar to strict-priority queuing on Catalyst Ethernet interfaces but is applicable to low-speed serial or WAN interfaces on Cisco IOS routers. IP RTP Priority is especially useful on links whose speed is less than 1.544 Mbps.

Use this feature on serial interfaces or other similar WAN interfaces in conjunction with either WFQ or CBWFQ on the same outgoing interface. In either case, traffic matching the range of User Datagram Protocol (UDP) ports specified for the priority queue is guaranteed strict priority over other CBWFQ classes or WFQ flows; packets in the priority queue are always serviced first.

Voice traffic can be identified by its RTP port numbers and classified into a priority queue. The result of using this feature is that voice traffic is serviced as strict priority in preference to nonvoice traffic. Figure 10-16 illustrates the behavior of IP RTP priority (PQ stands for priority queuing in the figure).

Figure 16Figure 10-16 IP RTP Priority

When configuring the priority queue with the IP RTP Priority feature, you specify a strict bandwidth limitation. This switch or router guarantees the amount of bandwidth to traffic queued in the priority queue. IP RTP Priority polices the flow every second. IP RTP Priority prohibits transmission of additional packets once the allocated bandwidth is consumed. If it discovered that the configured amount of bandwidth is exceeded, IP RTP Priority drops packets. The sum of all bandwidth allocation for voice and data flows on an interface cannot exceed 75 percent of the total available bandwidth. Bandwidth allocation takes into account the payload plus the IP, RTP, and UDP headers, but again, not the Layer 2 header. Allowing 25 percent bandwidth for other overhead is conservative and safe.

There are two basic commands for configuring IP RTP Priority:

ip rtp priority starting-rtp-port-number port-number-range bandwidth
max-reserved-bandwidth percent

The ip rtp priority command specifies a starting RTP destination port number (starting-rtp-port-number) with a port range (port-number-range). The bandwidth option specifies the maximum reserved bandwidth. The percent option of the max-reserved-bandwidth command specifies the percent of bandwidth allocated for LLQ and IP RTP Priority. Example 10-12 illustrates a sample configuration of IP RTP Priority with a starting RTP port number of 16,384, a range of 16,383 UDP ports, a maximum bandwidth of 25 kbps, and a maximum bandwidth allocated between LLQ and IP RTP priority from the default (75 percent) to 80 percent.

Example 10-12 Sample IP RTP Configuration

Switch(config)#multilink virtual-template 1
Switch(config)#interface virtual-template 1
Switch(config-if)#ip address
Switch(config-if)#no ip directed-broadcast
Switch(config-if)#ip rtp priority 16384 16383 25
Switch(config-if)#service-policy output policy1
Switch(config-if)#ppp multilink
Switch(config-if)#ppp multilink fragment-delay 20
Switch(config-if)#ppp multilink interleave
Switch(config-if)#max-reserved-bandwidth 80

Link-Efficiency Mechanisms

Cisco IOS software offers the following three link-efficiency mechanisms that work in conjunction with queuing and traffic shaping to improve efficiency and predictability of the application service levels:

  • Payload compression

  • Header compression

  • Link fragmentation and interleaving (LFI)

These features are applicable to low-speed WAN interfaces and are emerging for use on high-speed Ethernet interfaces.

Payload Compression

Although many mechanisms exist for optimizing throughput and reducing delay in network traffic within the QoS portfolio, QoS does not create bandwidth. QoS optimizes the use of existing resources and enables the differentiation of traffic according to the operator policy. Payload compression does create additional bandwidth, because it squeezes packet payloads, and therefore increases the amount of data that can be sent through a transmission resource in a given time period. Payload compression is mostly performed on Layer 2 frames and, as a result, compresses the entire Layer 3 packet.

Note that IP Payload Compression Protocol (PCP) is a fairly new technique for compressing payloads on Layer 3, and can handle out-of-order data. As compression squeezes payloads, it both increases the perceived throughput and decreases perceived latency in transmission, because smaller packets with compressed payloads take less time to transmit than the larger, uncompressed packets.

Compression is a CPU-intensive task that may add per-packet delay due to the application of the compression method to each frame. The transmission (serialization) delay, however, is reduced, because the resulting frame is smaller. Depending on the complexity of the payload compression algorithm, overall latency might be reduced, especially on low-speed links. Cisco IOS supports three different compression algorithms used in Layer 2 compression:

  • STAC (or Stacker)

  • Microsoft Point-to-Point Compression (MPPC)

  • Predictor

These algorithms differ slightly in their compression efficiency, and in utilization of router resources. Catalyst switches support compression with specialized WAN modules or security modules.

Header Compression

All compression methods are based on eliminating redundancy when sending the same or similar data over a transmission medium. One piece of data, which is often repeated, is the protocol header. In a flow, the header information of packets in the same flow does not change much over the lifetime of that flow. Therefore, most header information is sent only at the beginning of the session, then stored in a dictionary, and later referenced by a short dictionary index.

The IETF standardized on two compression methods for use with IP protocols:

  • TCP header compression (also known as the Van Jacobson or VJ header compression)—Used to compress the packet TCP headers over slow links, thus considerably improving the interactive application performance.

  • RTP header compression—Used to compress UDP and RTP headers, thus lowering the delay for transporting real-time data, such as voice and video, over slower links.

It is important to note that network devices perform header compression on a link-by-link basis. Network devices do not support header compression across multiple routers, because routers need full Layer 3 header information to be able to route packets to the next hop.

Link Fragmentation and Interleaving

Link fragmentation and interleaving (LFI) is a Layer 2 technique in which all Layer 2 frames are broken into small, equal-size fragments, and transmitted over the link in an interleaved fashion. When fragmentation and interleaving are in effect, the network device fragments all frames waiting in the queuing system where it prioritizes smaller frames. Then, the network device sends the fragments over the link. Small frames may be scheduled behind larger frames in the WFQ system. LFI fragments all frames, which reduces the queuing delay of small frames because they are sent almost immediately. Link fragmentation reduces delay and jitter by normalizing packet sizes of larger packets in order to offer more regular transmission opportunities to the voice packets.

The following LFI mechanisms are implemented in Cisco IOS:

  • Multilink PPP with interleaving is by far the most common and widely used form of LFI.

  • FRF.11 Annex C LFI is used with Voice over Frame Relay (VoFR).

  • FRF.12 Frame Relay LFI is used with Frame Relay data connections.

5. QoS in the Multilayer Switched Network | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020