Home > Articles > Cisco Network Technology > IP Communications/VoIP > Cisco IP Telephony Flash Cards: Weighted Random Early Detection (WRED)

Cisco IP Telephony Flash Cards: Weighted Random Early Detection (WRED)

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 24, 2004.

Chapter Description

This chapter provides an overvew of Weighted Random Early Detection (WRED) for Cisco IP Telephony, including Question & Answer flash cards to help you prepare for the Cisco IP Telephony Exam.

QoS on Slow-Speed Links

In this section, you make the most of your limited bandwidth on lower-speed WAN interfaces. Specifically, you are introduced to compression technologies, which send fewer bits across the link, and link fragmentation and interleaving technologies, which fragment large payloads to reduce the serialization delay that is experienced by smaller payloads.

Tools for Using Bandwidth Efficiently

The two broad categories of compression are as follows:

  • Payload compression—Reduces the payload size, using approaches such as STAC, Predictor, or MPPC.

  • Header compression—Reduces the size of the TCP and RTP headers.

The goal of compression technologies is to increase the throughput over a WAN link while reducing the delay. However, particularly with payload-compression approaches, the time that is required by lower-end routers to run the compression algorithm can increase the overall delay. Fortunately, these routers can have hardware acceleration modules that you can add to dramatically improve the router’s ability to perform compression in a timely manner. For example, a Compression Advanced Integration Module (CAIM) is available to offload compression tasks from 2600 Series routers.

These Quick Reference Sheets, however, focus on header compression. With header compression, a header typically is reduced from approximately 40 bytes in size to approximately 3 to 5 bytes [for Transport Control Protocol (TCP) header compression] or 2 to 4 bytes [for Real-Time Transport Protocol (RTP) header compression]. However, the routers technically are not doing compression. Rather, these routers cache information that does not change during a conversation, such as source and destination IP addresses and TCP/UDP port numbers. The compressed header then carries such information as UDP checksums and a session context ID (CID), which identifies which flow the packet is a part of.

Figure 32Figure 32

Another QoS mechanism that is useful for slower link speeds is Link Fragmentation and Interleaving (LFI). Consider a 1500-byte data frame that is being sent out of a 64-kbps serial interface. The interface, in this case, needs 187 ms just to place that data frame on the wire. If a smaller packet were sitting behind that data frame (for example, a voice frame), that frame might have already experienced excessive “serialization” delay before it was ever placed on the wire. LFI mechanisms fragment larger payloads to specified fragment sizes and then interleave the smaller payloads in among the fragments, greatly reducing the serialization delay that is experienced by the smaller payloads.

Figure 33Figure 33

The three primary LFI mechanisms supported by Cisco are as follows:

  • Multilink PPP (MLP)—Used on PPP links

  • FRF.12—Used on Voice over IP over Frame Relay (VoIPovFR) links

  • FRF.11 Annex C—Used on Voice over Frame Relay (VoFR) links

TCP and RTP Header Compression

Although header compression has been supported in the IOS for some time, IOS 12.2(13)T introduced Class-Based Header (CB-Header) Compression, which allows you to configure compression using the three-step MQC approach. CB-Header Compression is the focus of these Quick Reference Sheets.

Before configuring header compression, realize that header compression is most effective for slow links that are carrying packets with relatively small payloads, such as voice or Telnet traffic.

CB-Header Compression can be configured from policy-map-class configuration mode with the following command:

Router(config-pmap-c)#compression header ip [tcp | rtp]

Note that if you do not specify tcp or rtp, this command performs both TCP and RTP header compression. Unlike previous versions of header compression, you do not need to specify the maximum number of simultaneous compressed sessions supported. With CB-Header Compression, the number of connections is determined automatically by the IOS.

Consider the following CB-Header Compression example:

Router(config)#class-map VOICE
Router(config-cmap)#match protocol rtp
Router(config-cmap)#exit
Router(config)#policy-map COMPRESS
Router(config-pmap)#class VOICE
Router(config-pmap-c)#compression header ip rtp
Router(config-pmap-c)#exit
Router(config-pmap)#exit
Router(config)#interface serial 0/1
Router(config-if)#service-policy output COMPRESS

In this example, you are matching voice traffic (that is, RTP packets) using NBAR. Then, you are applying CB-Header Compression to those RTP packets with the COMPRESS policy-map. The policy-map is then applied outbound to interface serial 0/1. Because you configured header compression using the MQC approach, the same verification commands that you learned earlier (that is, show policy-map and show policy-map interface interface-identifier) are still applicable.

Using MLP and FRF.12 for LFI

The serialization delay goal that you have when configuring an LFI mechanism is in the range of 10 to 15 ms. To determine the serialization delay for a specific frame size on a specific link speed, use the following formula:

Serialization Delay = (Frame_Size * 8) / Link_Speed

The reason that you multiply the frame size by 8 is to convert bytes into bits. Consider a frame size of 512 bytes on a link speed of 128 kbps, as follows:

Serialization Delay = (512 * 8) / 128 = 32 ms

Although Cisco supports FRF.11 Annex C as an LFI mechanism for VoFR networks, these Quick Reference Sheets focus on the configuration of Multilink PPP (MLP) and FRF.12. First, consider the configuration of MLP.

Multilink PPP, by default, fragments traffic. You can leverage that fact and run MLP, even over a single link. You perform the MLP configuration under a virtual multilink interface, and then you can assign one or more physical interfaces to the multilink group. The physical interface does not have an IP address assigned, but the virtual multilink interface does. Typically, you use a single interface as a member of the multilink group. Following is the syntax to configure MLP:

Router(config)#interface multilink multilink_interface_number

(Creates a virtual multilink interface.)

Router(config-if)#ip address ip_address subnet_mask

(Assigns an IP address to the virtual multilink interface.)

Router(config-if)#ppp multilink

(Configures fragmentation on the multilink interface.)

Router(config-if)#ppp multilink interleave

(Shuffles the fragments together.)

Router(config-if)#ppp fragment-delay serialization_delay

(Specifies how long it will take for a fragment to exit the interface, in milliseconds. Note that the IOS automatically calculates the appropriate packet size to meet the specified serialization delay.)

Router(config-if)#encapsulation ppp

(Enables ppp encapsulation on the physical interface.)

Router(config-if)#no ip address

(Removes the IP address from the physical interface.)

Router(config-if)#multilink-group multilink_group_number

(Associates the physical interface with the multilink group.)

In the following example, the goal is to configure MLP on routers R1 and R2 so that you are achieving a serialization delay of 10 ms on their serial 0/0 interfaces.

Figure 34Figure 34

R1(config)#interface multilink 1
R1(config-if)#ip address 10.1.1.1 255.255.255.0
R1(config-if)#ppp multilink
R1(config-if)#ppp multilink interleave
R1(config-if)#ppp fragment-delay 10
R1(config-if)#exit
R1(config)#interface serial 0/0
R1(config-if)#encapsulation ppp
R1(config-if)#no ip address
R1(config-if)#multilink-group 1

R2(config)#interface multilink 1
R2(config-if)#ip address 10.1.1.2 255.255.255.0
R2(config-if)#ppp multilink
R2(config-if)#ppp multilink interleave
R2(config-if)#ppp fragment-delay 10
R2(config-if)#exit
R2(config)#interface serial 0/0
R2(config-if)#encapsulation ppp
R2(config-if)#no ip address
R2(config-if)#multilink-group 1

To verify the MLP configuration, you can use the show interfaces multilink interface-identifier command. The output from this command shows how many interleaves have been performed. Therefore, this is an excellent command to verify that MLP is indeed functioning.

FRF.12 is configured as part of a Frame Relay map-class. Unlike MLP, where you can specify a desired serialization delay, with FRF.12, you must specify the size that you want to fragment frames to. As a rule of thumb, divide the line speed by 800 to get a fragment size that results in a 10-ms serialization delay. For example, on a 64,000-bps link, divide 64,000 by 800 to get 80. This means that if you specify a fragment size of 80, your fragments will have a serialization delay of 10 ms.

Following is the syntax to configure FRF.12:

Router(config)#map-class frame-relay name

(Creates the map-class and enters map-class configuration mode.)

Router(config-map-class)#frame-relay fragment fragment-size

(Specifies the size to which FRF.12 will fragment frames. Note that the IOS does not automatically calculate the fragment size based on a specified delay, as the MLP mechanism did.)


Router(config-if)#frame-relay traffic-shaping

(Enables Frame Relay traffic shaping on the physical interface.)

Router(config-if | config-subif)#frame-relay class name

(Associates the map-class with an interface or a subinterface.)

Router(config-fr-dlci)#class name

(Associates the map-class with a Frame Relay DLCI.)

In the following example, you configure FRF.12 to create a serialization delay of 10 ms on a link that is clocked at a rate of 64 kbps. The map-class then is applied to DLCI 101. Because FRF.12 is configured as a part of Frame Relay traffic shaping, you also specify a CIR of 64 kbps and a Bc of 640.

Figure 35Figure 35

R1(config)#map-class frame-relay FRF12-EXAMPLE
R1(config-map-class)#frame-relay cir 64000
R1(config-map-class)#frame-relay bc 640
R1(config-map-class)#frame-relay fragment 80
R1(config-map-class)#exit
R1(config)#interface serial 0/1
R1(config-if)#frame-relay traffic-shaping
R1(config-if)#interface serial 0/1.1 point-to-point
R1(config-subif)#frame-relay interface-dlci 101
R1(config-fr-dlci)#class FRF12-EXAMPLE

You can use the show frame-relay fragment command to view the fragment size that is being used. Also, use the show frame-relay pvc command to view the fragment size that is used on a particular DLCI.

11. QoS Design Guidelines | 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.

Overview

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.

Surveys

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.

Newsletters

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.

Security

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

Children

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

Marketing

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.

Choice/Opt-out

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.

Links

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