Home > Articles > Cisco Certification > CCNP > Switch Operation for the CCNP BCMSN Exam

Switch Operation for the CCNP BCMSN Exam

Chapter Description

This sample chapter covers the following topics that you need to master for the CCNP BCMSN exam: Layer 2 Switch Operation, Multilayer Switch Operation, Tables Used in Switching, and Troubleshooting Switching Tables.

Foundation Topics

Layer 2 Switch Operation

Recall that with shared Ethernet networks using hubs, many hosts are connected to a single broadcast and collision domain. In other words, shared Ethernet media operates at OSI Layer 1.

Each host must share the available bandwidth with every other connected host. When more than one host tries to talk at one time, a collision occurs, and everyone must back off and wait to talk again. This forces every host to operate in half-duplex mode, by either talking or listening at any given time. In addition, when one host sends a frame, all connected hosts hear it. When one host generates a frame with errors, everyone hears that, too.

At its most basic level, an Ethernet switch provides isolation from other connected hosts in several ways:

  • The collision domain's scope is severely limited. On each switch port, the collision domain consists of the switch port itself and the devices directly connected to that port—either a single host or if a shared-media hub is connected, the set of hosts connected to the hub.

  • Host connections can operate in full-duplex mode because there is no contention on the media. Hosts can talk and listen at the same time.

  • Bandwidth is no longer shared. Instead, each switch port offers dedicated bandwidth across a switching fabric to another switch port. (These connections change dynamically.)

  • Errors in frames are not propagated. Each frame received on a switch port is checked for errors. Good frames are regenerated when they are forwarded or transmitted. This is known as store-and-forward switching technology, where packets are received, stored for inspection, and then forwarded.

  • You can limit broadcast traffic to a volume threshold.

  • n Other types of intelligent filtering or forwarding become possible.

Transparent Bridging

A switch is basically a multiport transparent bridge, where each switch port is its own Ethernet LAN segment, isolated from the others. Frame forwarding is based completely on the MAC addresses contained in each frame, such that the switch won't forward a frame unless it knows the destination's location. (In cases where the switch doesn't know where the destination is, it makes some safe assumptions.) Figure 3-1 shows the progression from a two-port to a multiport transparent bridge, and then to a switch.

Figure 1Figure 3-1 A Comparison of Transparent Bridges and Switches

The entire process of forwarding Ethernet frames then becomes figuring out what MAC addresses connect to which switch ports. A switch must either be told explicitly where hosts are located, or it must learn this information for itself. You can configure MAC address locations through a switch's command-line interface, but this quickly gets out of control when there are many stations on the network or when stations move around.

To dynamically learn about station locations, a switch listens to incoming frames and keeps a table of address information. As a frame is received on a switch port, the switch inspects the source MAC address. If that address is not in the address table already, the MAC address, switch port, and Virtual LAN (VLAN) on which it arrived are recorded in the table. Learning the address locations of the incoming packets is easy and straightforward.

Incoming frames also include the destination MAC address. Again, the switch looks this address up in the address table, hoping to find the switch port and VLAN where the address is attached. If it is found, the frame can be forwarded on out that switch port. If the address is not found in the table, the switch must take more drastic action—the frame is forwarded in a "best effort" fashion by flooding it out all switch ports assigned to the source VLAN. This is known as unknown unicast flooding, where the unicast destination location is unknown. Figure 3-2 illustrates this process, using only a single VLAN for simplification.

A switch constantly listens to incoming frames on each of its ports, learning source MAC addresses. However, be aware that the learning process is allowed only when the Spanning Tree Protocol (STP) algorithm has decided a port is stable for normal use. STP is concerned only with maintaining a loop-free network, where frames will not be recursively forwarded. If a loop were to form, a flooded frame could follow the looped path, where it would be flooded again and again.

In a similar manner, frames containing a broadcast or multicast destination address are also flooded. These destination addresses are not unknown—the switch knows them well. They are destined for multiple locations, so they must be flooded by definition. In the case of multicast addresses, flooding is performed by default. Other more elegant means of determining the destination locations are available and are discussed in Chapter 15, "Multicast."

Follow That Frame!

You should have a basic understanding of the operations that a frame undergoes as it passes through a Layer 2 switch. This helps you get a firm grasp on how to configure the switch for complex functions. Figure 3-3 shows a typical Layer 2 Catalyst switch and the decision processes that take place to forward each frame.

Figure 2Figure 3-2 Unknown Unicast Flooding

When a frame arrives at a switch port, it is placed into one of the port's ingress queues. The queues can each contain frames to be forwarded, each queue having a different priority or service level. The switch port can then be fine-tuned so that important frames get processed and forwarded before less-important frames. This can prevent time-critical data from being "lost in the shuffle" during a flurry of incoming traffic.

Figure 3Figure 3-3 Operations Within a Layer 2 Catalyst Switch

As the ingress queues are serviced and a frame is pulled off, the switch must figure out not only where to forward the frame but also if it should be forwarded and how. There are three fundamental decisions to be made—one concerned with finding the egress switch port, and two concerned with forwarding policies. All of these decisions are made simultaneously by independent portions of switching hardware and can be described as follows:

  • L2 Forwarding Table—The frame's destination MAC address is used as an index, or key, into the Content Addressable Memory (CAM) or address table. If the address is found, the egress switch port and the appropriate VLAN ID are read from the table. (If the address is not found, the frame is marked for flooding so that it is forwarded out every switch port in the VLAN.)

  • Security ACLs—Access control lists (ACLs) can be used to identify frames according to their MAC addresses, protocol types (for non-IP frames), IP addresses, protocols, and Layer 4 port numbers. The Ternary Content Addressable Memory (TCAM) contains ACLs in a compiled form, such that a decision can be made on whether to forward a frame in a single table lookup.

  • QoS ACLs—Other ACLs can classify incoming frames according to quality of service (QoS) parameters, to police or control the rate of traffic flows, and to mark QoS parameters in outbound frames. The TCAM is also used to make these decisions in a single table lookup.

The CAM and TCAM tables are discussed in greater detail in the "CAM" and "TCAM" sections later in this chapter. After the CAM and TCAM table lookups have occurred, the frame is placed into the appropriate egress queue on the appropriate outbound switch port. The egress queue is determined by QoS values either contained in the frame or passed along with the frame. Like the ingress queues, the egress queues are serviced according to importance or time criticality; frames are sent out without being delayed by other outbound traffic.

3. Multilayer Switch Operation | 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