Home > Articles > Networking

Network Switching Methods: Store-and-Forward Versus Cut-Through

Sean Wilkins, co-author of CCNA Routing and Switching 200-120 Network Simulator, believes every network engineer should understand the differences between the two major switching methods employed in computer networks. This overview discusses the two methods from their initial deployments to modern usage.
Like this article? We recommend

Overview

Network engineers need to understand many things beyond the typical knowledge of a basic technician; for example, how switch or router electronics communicate between different types of hardware. Certain topics are best left to those with the skill set of an electrical engineer; even the most senior network engineer may not need to understand them. However, the line is fuzzy between hardware operations and the deployment, use, and configuration of network devices.

One thing that sits on this fuzzy line is the differences between the switching methods used by various models and makers of switches. The two primary methods are store-and-forward and cut-through. These methods are used on many different platforms, and each has its own way of handling the bits that enter and are processed by the switch. This article takes a look at these two switching methods, exploring how they work and their various advantages and disadvantages.

Store-and-Forward Switches: The Beginning

Switches (and bridges) historically operated using the store-and-forward switching method. This method’s operation is simple: When a frame comes into a switchport, the whole frame is brought into a buffer. When the complete frame has been received, the switch uses the Frame Check Sequence (FCS) value contained within the frame to perform an integrity check, ensuring that the frame has been received exactly as it was transmitted. If this integrity check fails, the frame is dropped; if the check passes, the information in the frame is parsed. This method of switching was used for a long time. For the most part, this is how switching is explained up to this point in most textbooks.

Cut-Through Switches: The Beginning

The early 1990s introduced cut-through switching. The first switches that supported this method were developed for the sheer purpose of forwarding frames as fast as possible. These switches accelerated frame forwarding by processing only the first six bytes of the frame (after the delimiter), where the destination MAC address is located. After processing these first six bytes, the switch had enough information to make a forwarding decision and move the frame to the appropriate switchport.

The advantage of this technique is speed; the disadvantage is that even frames with integrity problems are forwarded. Because of this disadvantage, cut-through switches were limited to specific positions within the network that required pure performance, and typically they were not tasked with performing extended functionality (core).

Store-and-Forward Switches: Evolution

The next evolution in switching came with the implementation of extended functionality—access control lists (ACL), Layer 3 routing, and so on—within the switch platform. These switches, not limited to being “dumb,” are tasked with doing more than simply forwarding frames out the appropriate switchports. Most modern switches are managed/smart, with some level of extended functionality that blends pure switch activities with other features that are closely linked with routers. IP routing and ACLs are typically associated with being implemented on routers, but many modern switches support both to some degree. The simplest of these switches may offer only basic static routing functionality and some limited ACL handling, but advanced models are built as full Layer 3 switches, supporting almost all the standard (Ethernet-based) functions of a router, including dynamic routing, ACLs, policy-based routing (PBR), quality of service (QoS) features, and more.

With the store-and-forward switching method, these capabilities are no problem because the complete frame can be buffered before it is forwarded out another switchport. This design allows the device to parse the contents and determine whether any of the previously mentioned extended features alter the way in which a frame should be processed. If so, the frame is decapsulated to the appropriate level and run through that feature’s process. If the frame is not affected, it is forwarded out the appropriate outbound switchport according to the typical switch mechanisms—MAC tables, STP interface states, and so forth—which is simply impossible using the legacy cut-through switching method.

When configured to use these extended functions, switches have typically been used in parts of the network where this functionality is needed. Over time, switch hardware has also been updated to establish performance close to that of previous cut-through switches, which made them very popular. This performance improvement led to phasing out the legacy cut-through switches in all parts of the network.

Cut-Through Switches: Evolution

Some switch vendors, including Cisco, recently began bringing back cut-through switching on some platforms, with enhancements that include dynamic frame processing. Instead of simply waiting for the first six bytes, these switches wait until at least enough of the frame is received to determine its EtherType. For example, if the frame’s EtherType indicates that an IP packet is encapsulated, and the switch is configured with IP extended functions, the switch will process enough of the frame to determine whether the packet contained within the frame matches one indicated in the configuration.

This design improvement allows a cut-through switch to operate in a way that extends performance, while retaining support for the extended features that many companies want to use.

Summary

At this point, beginning network engineers will wonder why this information is needed by anyone who is just getting into the field. The simplest answer is that this topic is covered on entry-level Cisco exams, which means that entry-level engineers need to understand it before taking their exams. Other new network engineers won’t necessarily need to be familiar with these two types of switches until they are experienced enough to be tasked with evaluating switches to be deployed into the network. I hope this article will give readers a better understanding of how these switching methods work and relate to each other, so that network engineers at any level will know which method to select if the question arises.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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@informit.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 InformIT. 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.informit.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