Home > Articles > Certification > Cisco Certification > CCNA

CCNA Self-Study (ICND Exam): Extending Switched Networks with Virtual LANs

Steve McQuerry discusses how VLANs operate to provide more effective networks by controlling broadcasts in your network.
This chapter is from the book VLAN Concepts

After completing this chapter, you will be able to perform the following tasks:

  • Identify what a VLAN is and how it operates.

  • Configure a VLAN to improve network performance.

  • Identify what role the switch plays in the creation of VLANs.

  • Identify how network devices communicate about VLANs.

  • Describe the need and operation of the VLAN Trunking Protocol.

  • Configure the Catalyst Switch for VLAN operation.

The design and function of a bridged/switched network is to provide enhanced network services by segmenting the network into multiple collision domains. The fact remains, however, that without any other mechanism, the bridged/switched network is still a single broadcast domain. A broadcast domain is a group of devices that can receive one another's broadcast frames. For example, if device A sends a broadcast frame and that frame is received by devices B and C, all three devices are said to be in a common broadcast domain. Because broadcast frames are flooded out all ports on a bridge/switch (by default), the devices connected to the bridge/switch are in a common broadcast domain.

Controlling broadcast propagation throughout the network is important to reduce the amount of overhead associated with these frames. Routers, which operate at Layer 3 of the OSI model, provide broadcast domain segmentation for each interface. Switches can also provide broadcast domain segmentation using virtual LANs (VLANs). A VLAN is a group of switch ports, within a single or multiple switches, that is defined by the switch hardware and/or software as a single broadcast domain. A VLAN's goal is to group devices connected to a switch into logical broadcast domains to control the effect that broadcasts have on other connected devices. A VLAN can be characterized as a logical network.

The benefits of VLANs include the following:

  • Security

  • Segmentation

  • Flexibility

VLANs enable you to group users into a common broadcast domain regardless of their physical location in the internetwork. Creating VLANs improves performance and security in the switched network by controlling broadcast propagation and requiring that communications between these broadcast be carried out by a Layer 3 device that is capable of implementing security features such as access control lists (ACLs).

In a broadcast environment, a broadcast sent out by a host on a single segment would propagate to all segments. In normal network operation, hosts frequently generate broadcast/multicast traffic. If hundreds or thousands of hosts each sent this type of traffic, it would saturate the bandwidth of the entire network, as shown in Figure 3-1. Also, without forcing some method of checking at an upper layer, all devices in the broadcast domain would be able to communicate via Layer 2. This severely limits the amount of security you can enforce on the network.

Figure 1Figure 3-1 Broadcast Propagation

Before the introduction of switches and VLANs, internetworks were divided into multiple broadcast domains by connectivity through a router. Because routers do not forward broadcasts, each interface is in a different broadcast domain. Figure 3-2 shows an internetwork broken into multiple broadcast domains using routers. Notice that each segment is an individual IP subnet and that regardless of a workstation's function, its subnet is defined by its physical location.

A VLAN is a logical broadcast domain that can span multiple physical LAN segments. A VLAN can be designed to provide independent broadcast domains for stations logically segmented by functions, project teams, or applications, without regard to the users' physical location. Each switch port can be assigned to only one VLAN. Ports in a VLAN share broadcasts. Ports that do not belong to the same VLAN do not share broadcasts. This control of broadcast improves the internetwork's overall performance.

VLANs enable switches to create multiple broadcast domains within a switched environment, as illustrated in Figure 3-3.

Figure 2Figure 3-2 Multiple Broadcast Domains Using Routers

Notice that now all users in a given group (department in this example) are defined to be in the same VLAN. Any user in this VLAN receives a broadcast from any other member of the VLAN, while users of other VLANs do not receive these broadcasts. Each of the users in a given VLAN is also in the same IP subnet. This is different from the broadcast domains of Figure 3-2, in which the physical location of the device determines the broadcast domain. However, there is a similarity with a legacy, non-VLAN internetwork because a router is still needed to get from one broadcast domain to another, even if a VLAN is used to define the broadcast domain instead of a physical location. Therefore, the creation of VLANs does not eliminate the need for routers.

Within the switched internetwork, VLANs provide segmentation and organizational flexibility. Using VLAN technology, you can group switch ports and their connected users into logically defined communities of interest, such as coworkers in the same department, a cross-functional product team, or diverse user groups sharing the same network application.

A VLAN can exist on a single switch or span multiple switches. VLANs can include stations in a single building or multiple-building infrastructures. In rare and special cases, they can even connect across wide-area networks (WANs).

Figure 3Figure 3-3 VLAN Overview

VLAN Concepts

As mentioned previously, prior to the VLAN, the only way to control broadcast traffic was through segmentation using routers. VLANs are an extension of a switched and routed internetwork. By having the ability to place segments (ports) in individual broadcast domains, you can control where a given broadcast is forwarded. The sections that follow expand on these concepts. Basically, each switch acts independently of other switches in the network. With the concept of VLANs, a level of interdependence is built into the switches themselves. The characteristics of a typical VLAN setup are as follows:

  • Each logical VLAN is like a separate physical bridge.

  • VLANs can span multiple switches.

  • Trunk links carry traffic for multiple VLANs.

With VLANs, each switch can distinguish traffic from different broadcast domains. Each forwarding decision is based on which VLAN the packet came from; therefore, each VLAN acts like an individual bridge within a switch. To bridge/switch between switches, you must either connect each VLAN independently (that is, dedicate a port per VLAN) or have some method of maintaining and forwarding the VLAN information with the packets. A process called trunking allows this single connection. Figure 3-4 illustrates a typical VLAN setup in which multiple VLANs span two switches interconnected by a Fast Ethernet trunk.

Figure 4Figure 3-4 Multiple VLANs Can Span Multiple Switches

How VLANs Operate

A Catalyst switch operates in your network like a traditional bridge. Each VLAN configured on the switch implements address learning, forwarding/filtering decisions, and loop avoidance mechanisms as if it were a separate physical bridge. This VLAN might include several ports, possibly on multiple switches.

Internally, the Catalyst switch implements VLANs by restricting data forwarding to destination ports in the same VLAN as originating ports. In other words, when a frame arrives on a switch port, the Catalyst must retransmit the frame only to a port that belongs to the same VLAN as that of the incoming port. The implication is that a VLAN operating on a Catalyst switch limits transmission of unicast, multicast, and broadcast traffic. Flooded traffic originating from a particular VLAN floods out only other ports belonging to that VLAN. Each VLAN is an individual broadcast domain because a broadcast in a given VLAN will never reach any ports in other VLANs.

Normally, a port carries traffic only for the single VLAN to which it belongs. For a VLAN to span multiple switches on a single connection, a trunk is required to connect two switches. A trunk carries traffic for all VLANs by identifying the originating VLAN as the frame is carried between the switches. Figure 3-4 shows a Fast Ethernet trunk carrying multiple VLANs between the two switches. Most ports on Catalyst switches are capable of being trunk ports. Any port on a Catalyst 2950 can be a trunk port.

VLAN Membership Modes

VLANs are a Layer 2 implementation in your network's switching topology. Because they are implemented at the data link layer, they are protocol-independent. To put a given port (segment) into a VLAN, you must create a VLAN on the switch and then assign that port membership on the switch. After you define a port to a given VLAN, broadcast, multicast, and unicast traffic from that segment will be forwarded by the switches only to ports in the same VLAN. If you need to communicate between VLANs, you must add a router (or Layer 3 switch) and a Layer 3 protocol to your network.

The ports on a Layer 2 Catalyst switch, such as a 2950, all function as Layer 2 ports. In Cisco IOS Software, a Layer 2 port is known as a switchport. A switchport can either be a member of a single VLAN or be configured as a trunk link to carry traffic for multiple VLANs. When a port is in a single VLAN, the port is called an access port. Access ports are configured with a VLAN membership mode that determines to which VLAN they can belong. The membership modes follow:

  • Static—When an administrator assigns a single VLAN to a port, it is called static assignment. By default, all Layer 2 switchports are statically assigned to VLAN 1 until an administrator changes this default configuration.

  • Dynamic—The IOS Catalyst switch supports the dynamic assignment of a single VLAN to a port by using a VLAN Membership Policy Server (VMPS). The VMPS must be a Catalyst Operating System switch, such as a Catalyst 5500 or 6500, running the set-based operating system. An IOS-based Catalyst switch cannot operate as the VMPS. The VMPS contains a database that maps MAC addresses to VLAN assignment. When a frame arrives on a dynamic port, the switch queries the VMPS for the VLAN assignment based on the arriving frame's source MAC address.

A dynamic port can belong to only one VLAN at a time. Multiple hosts can be active on a dynamic port only if they all belong to the same VLAN. Figure 3-5 demonstrates the static and dynamic VLAN membership modes.

Figure 5Figure 3-5 VLAN Membership Modes

For an access port, the VLAN identity is not known by the sender or receiver attached to the access port. Frames going into and out of access ports are standard Ethernet frames, as discussed in Chapter 2, "Configuring Catalyst Switch Operations." The VLAN identity is used only within the switch to provide broadcast domain boundaries.

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