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.

Weighted Random Early Detection (WRED)

Whereas queuing provides congestion management, mechanisms such as WRED provide congestion avoidance. Specifically, WRED can prevent an output queue from ever filling to capacity, which would result in packet loss for all incoming packets. This section examines the need for and the configuration of WRED on Cisco routers.

How TCP Handles Drops

Recall from your early studies of networking technology how Transport Control Protocol (TCP) windowing functions. A sender sends a single segment, and if the sender receives a successful acknowledgment from the receiver, it then sends two segments (that is, a “windows size” of 2). If those two segments were acknowledged successfully, the sender sends four segments, and so on, increasing the window size exponentially.

However, if one of the segments is dropped, the TCP flow goes into TCP slow start, where the window size is reduced to 1. The TCP flow then exponentially increases its window size until the window size reaches half of the window size when congestion originally occurred. At that point, the TCP flow’s window size increases linearly.

TCP slow start is relevant to QoS, because when an interface’s output queue is full, all newly arriving packets are discarded (that is, “tail dropped”), and all of those TCP flows simultaneously go into TCP slow start.

Note that the process of multiple TCP flows simultaneously entering TCP slow start is called global synchronization or TCP synchronization. When TCP synchronization occurs, the link’s bandwidth is underutilized, resulting in wasted bandwidth.

RED Basics

The purpose of Random Early Detection (RED) is to prevent TCP synchronization by randomly discarding packets as an interface’s output queue begins to fill. How aggressively RED discards packets depends on the current queue depth.

The following three parameters influence when a newly arriving packet is discarded:

  • Minimum threshold

  • Maximum threshold

  • Mark Probability Denominator (MPD)

The minimum threshold specifies the number of packets in a queue before the queue considers discarding packets. The probability of discard increases until the queue depth reaches the maximum threshold. After a queue depth exceeds the maximum threshold, all other packets that attempt to enter the queue are discarded.

However, the probability of packet discard when the queue depth equals the maximum threshold is 1/(MPD). For example, if the mark probability denominator were set to 10, when the queue depth reached the maximum threshold, the probability of discard would be 1/10 (that is, a 10 percent chance of discard).

Figure 21Figure 21

The minimum threshold, maximum threshold, and MPD comprise the RED profile. The following figure shows the three distinct ranges in a RED profile: no drop, random drop, and full drop.

Figure 22Figure 22

RED is most useful on router interfaces where congestion is likely. For example, a WAN interface might be a good candidate for RED.

CB-WRED

Cisco does not support RED, but fortunately it supports something better: Weighted Random Early Detection (WRED). Unlike RED, WRED has a profile for each priority marking. For example, a packet with an IP Precedence value of 0 might have a minimum threshold of 20 packets, whereas a packet with an IP Precedence of 1 might have a minimum threshold of 25 packets. In this example, packets with an IP Precedence of 0 would start to be discarded before packets with an IP Precedence of 1.

Although WRED can be configured from interface-configuration mode or from virtual-circuit-configuration mode, these Quick Reference Sheets focus on an MQC-based WRED configuration. To enable WRED and to specify the marking that WRED pays attention to (that is, IP Precedence or DSCP), issue the following policy-map-class configuration-mode command:

Router(config-pmap-c)#random-detect [dscp-based | prec-based]

If neither dscp-based nor prec-based is specified, WRED defaults to prec-based. After WRED is configured, the IOS assigns default minimum threshold, maximum threshold, and MPD values. However, you can alter those default parameters with the following commands:

Router(config-pmap-c)#random-detect precedence precedence_value minimum-threshold maximum-threshold mark-probability-denominator

(Used for prec-based WRED)

Router(config-pmap-c)#random-detect dscp dscp_value minimum-threshold maximum-threshold mark-probability-denominator

(Used for dscp-based WRED)

To reinforce this syntax, consider the following example, where the goal is to configure WRED for the WREDTEST class-map. After the class-map’s queue depth reaches 25 packets, a DSCP value of AF13 might be discarded. Packets that are marked with a DSCP value of AF12 should not be discarded until the queue depth reaches 30 packets, and finally, packets that are marked with a DSCP value of AF11 should have no chance of discard until the queue depth reaches 35 packets. If the queue depth exceeds 100 packets, there should be a 100 percent chance of discard for these three DSCP values. However, when the queue depth is exactly 100 packets, the chance of discard for these various packet types should be 25 percent. Also, CB-WRED requires that CB-WFQ be configured for the interface. So, as an additional requirement, you make 25 percent of the interface’s bandwidth available to the WREDTEST class of traffic.

Figure 23Figure 23

Router(config-pmap)#class WREDTEST
Router(config-pmap-c)#bandwidth percent 25
Router(config-pmap-c)#random-detect dscp-based
Router(config-pmap-c)#random-detect dscp af13 25 100 4
Router(config-pmap-c)#random-detect dscp af12 30 100 4
Router(config-pmap-c)#random-detect dscp af11 35 100 4

Examine the solution, and notice that the MPD is 4. This value was chosen to meet the requirement of a 25 percent chance of discard when the queue depth equals the maximum threshold (that is, 1/4 = .25). Also, notice that a DSCP value of AF13 is dropped before a DSCP value of AF12, which is dropped before a DSCP value of AF11. This approach is consistent with the definition of the per-hop behaviors (PHBs), because the last digit in the Assured Forwarding (AF) DSCP name indicates its drop preference. For example, a value of AF13 would drop before a value of AF12.

To view the minimum threshold, maximum threshold, and MPD settings for the various IP Precedence or DSCP values, you can issue the show policy-map interface interface-identifier command.

ECN Configuration

WRED discards packets, and that is one way for the router to indicate congestion. However, routers can now indicate a congestion condition by signaling, using an approach called Explicit Congestion Notification (ECN).

ECN uses the 2 last bits in the ToS byte to indicate whether a device is ECN capable, and if so, whether congestion is being experienced.

Figure 24Figure 24

Cisco routers can use ECN as an extension to WRED and mark packets that exceed a specified value, instead of dropping the packets. If the queue depth is at or below the WRED minimum threshold, the packets are sent normally, just as with WRED. Also, if the queue depth is above the WRED maximum threshold, all packets are dropped, just as with WRED.

But if the queue depth is currently in the range from the minimum threshold through the maximum threshold, one of the following things can happen:

  • If both endpoints are ECN capable, the ECT and CE bits are set to a 1 and sent to the destination, indicating that the transmission rate should be reduced.

  • If neither endpoints supports ECN, the normal WRED behavior occurs.

  • A packet with its ECN and CE bits marked can reach a destination router that already has a full queue. In such an instance, the notification is dropped.

Use the following command to enable ECN:

Router(config-pmap-c)#random-detect ecn

Note that although WRED also can be configured in interface-configuration mode, ECN must be configured through MQC. Because ECN is configured by the three-step MQC process, the same verification and troubleshooting commands apply. Specifically, you can use the show policy-map and show policy-map interface interface-identifier commands to verify the ECN configuration.

9. Traffic Conditioners | 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