Home > Articles > Cisco > CCNP Routing and Switching

  • Print
  • + Share This
This chapter is from the book

Study Tips

The following bullets review important BCMSN exam preparation points of this chapter. The bullets only briefly highlight the main points of this chapter related to the BCMSN exam. Consult the text of this chapter for additional information regarding these topics. Table 9-3 lists important commands to review for the BCMSN exam:

  • In describing CEF-based MLS in a few sentences, CEF-based MLS is the Layer 3 forwarding mechanism deployed currently on all shipping Catalyst switches. CEF-based MLS consists of two logical devices: a control-plane and a data-plane. In CEF, the control-plane is generally referred to as the software-switching engine or Layer 3 forwarding engine. In addition, the data-plane carries several common names, including the hardware-switching component, "in hardware", hardware forwarding engine, and ASICs.
  • The control-plane is responsible for building the IP routing table and ARP table. The IP routing table and ARP table ultimately build the IP CEF and CEF adjacency table. The CEF table contains prefixes of subnets, hosts, and so on, which are used to quickly search for an adjacency index. The index into the adjacency table provides the next-hop address, interface, and Layer 3 rewrite information.
  • The control-plane downloads its CEF and adjacency table to the data-plane (hardware forwarding engine). The hardware forwarding engine consists of hardware components including TCAM that enable high-speed switching and routing of frames. All changes, updates, additions, and so on to the CEF and adjacency table are controlled by the Layer 3 forwarding engine.
  • With the Catalyst 6500 family of switches, the control-plane and data-plane are easily distinguishable (MSFC and PFC, respectively). However, the other Catalyst switches do not have separate components from an end-user standpoint, yet these switches still have a distinct control-plane and data-plane.
  • Layer 3 switching (or multilayer switching) is the term used to describe the ability of a Cisco Catalyst switch to perform Layer 3 routing in hardware (hardware-switching) using high-speed hardware components.
  • Layer 3 switching is beneficial in the campus network as it provides versatility, high-availability, large port density, and high-performance.
  • Current shipping Layer 3 switches use CEF to populate IP routing tables and Layer 2 rewrite information in hardware.
  • The terms "CEF-based MLS" or just plain "CEF" are used to describe the routing architecture of Catalyst switches.
  • No specific configuration is needed for CEF-based MLS because it is enabled by default.
  • ARP throttling is a feature that prevents ARP-based DoS attacks on Catalyst switches. The feature works by installing a throttling adjacency in the hardware CEF table such that hardware-switching drops subsequent identical ARP requests for a specific time period until the respective ARP response is received.
  • With CEF, when a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for the individual host prefixes. The subnet prefix points to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix.
  • With CEF, punt adjacencies in TCAM on Catalyst switch hardware are used to send packets to the Layer 3 engine (for example, MSFC) that require special handling or that are used for special situations where the hardware does not yet support the packet flow in question (for example, NAT).
  • Catalyst switches support two methods for CEF-based MLS: centralized switching and distributing switching.
  • Centralized switching describes hardware-switching using a centralized hardware-switching engine found on the Supervisor Engine of a Cisco Catalyst switch. With centralized switching, data to be routed or switched must pass through the switching fabric of the Catalyst switch to the centralized hardware-switching engine for processing. After the forwarding decision has been made and the packet rewritten with new Layer 2 information, the centralized switching engine forwards the packet to the correct destination port and VLAN.
  • The Catalyst 4500 family of switches is based on centralized switching.
  • Distributed switching describes the hardware-switching method used by Catalyst switches that utilize hardware-switching engines on individual line cards and port groups. With distributed switching, individual line cards determine the routing destination and rewrite information for a packet. Therefore, with distributed switching line cards, a Catalyst switch is able to route a packet between source and destination without sending the packet to the Supervisor Engine for processing. In addition, the packet might never leave the line card if the source and destination of the line card are on the same switch.
  • Review the "Sample CEF-Based MLS Operation" section and Figure 9-8, which illustrates the path of a packet during CEF-based MLS operation.
  • Review the "CEF-Based MLS Troubleshooting Methodology" section of this chapter for details on how to approach CEF-based MLS troubleshooting.

Table 9-3 Commands to Review

Command

Description

debug ip cef

Enables debugging with IP CEF. Recommended practice is to use this debug option with optional parameters including ACLs to limit output. In addition, use this debug command in production only in coordination with a Cisco TAC engineer.

show adjacency detail

Displays the IP CEF Layer 2 rewrite information and statistical information from the perspective of the Layer 3 engine.

show arp

Displays the ARP table contents. This command is the first step in troubleshooting adjacency issues.

show ip cef

Displays the IP CEF information from the Layer 3 engine perspective. Displays the prefix, the next hop, and next-hop interface (outgoing interface).

show ip route

Displays the IP routing table. This command is the first step in troubleshooting IP routing or IP CEF issues.

  • + Share This
  • 🔖 Save To Your Account

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification 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 Pearson IT Certification 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 Adobe 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.pearsonitcertification.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