Home > Articles > Certification > Cisco Certification > CCNA

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

VLAN Trunking Protocol

To provide VLAN connectivity throughout the switched network, VLANs must be configured on each switch. If you are going to trunk VLAN10 from Switch A to Switch C through Switch B, as shown in Figure 3-11, VLAN10 must exist on Switch B even though none of the access ports on that switch are in VLAN10.

Figure 11Figure 3-11 Purpose for VTP

To ensure that a VLAN exists between every pair of Trunked switches, an administrator must manually create all the needed VLANs on each of the switches individually. Cisco's VLAN Trunking Protocol (VTP) provides an easier method for maintaining consistent VLAN configuration throughout the switched network.

VTP is a protocol used to distribute and synchronize identifying information about VLANs configured throughout a switched network. Configurations made to a single VTP server are propagated across trunk links to all connected switches in the network. VTP enables switched network solutions to scale to large sizes by reducing the network's manual configuration needs.

VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency throughout a common administrative domain by managing the additions, deletions, and name changes of VLANs across networks. VTP minimizes misconfigurations and configuration inconsistencies that can cause problems, such as duplicate VLAN names or incorrect VLAN-type specifications.

A VTP domain is one switch or several interconnected switches sharing the same VTP environment. A switch can be configured only in one VTP domain.

By default, a Catalyst switch is in the no-management-domain (or null domain) state until it is configured with a domain or receives an advertisement for a domain over a trunk link. Configuration changes made to the VLANs on a single VTP server switch are propagated across Trunk links to all trunk-connected switches in the network.

Figure 3-12 illustrates how VLAN configuration information is propagated from switch to switch.

Figure 12Figure 3-12 VTP Operation

Figure 3-12 shows a VLAN added to the switched network. The steps illustrated in the figure are as follows:

  1. A new VLAN is added. At this point, VTP makes your job easier.

  2. The VTP advertisement is sent to the other switches in the VTP domain.

  3. The new VLAN is added to the other switch configurations. The result is consistent VLAN configuration.

VTP Modes

VTP operates in one of three modes:

  • Server mode

  • Client mode

  • Transparent mode

The default VTP mode is server mode, but VLANs are not propagated over the network until a management domain name is specified or learned and trunking has been established. A Catalyst switch operating in the VTP server mode can create, modify, and delete VLANs and other configuration parameters for the entire VTP domain. In server mode, VLAN configurations are saved in the Catalyst nonvolatile random-access memory (NVRAM). When you make a change to the VLAN configuration on a VTP server, the change is propagated to all switches in the VTP domain. VTP messages are transmitted out all trunk connections, such as ISL.

A device operating as a VTP client cannot create, change, or delete VLANs. A switch in client mode does send VTP messages, however. A VTP client does not save VLAN configurations in nonvolatile memory.

In both client and server mode, the switches synchronize their VLAN configuration to that of the switch with the highest revision number in the VTP domain.

A switch operating in VTP transparent mode does not create VTP advertisements or synchronize its VLAN configuration with information received from other switches in the management domain. A switch in transparent mode forwards VTP advertisements received from other switches that are part of the same management domain. A switch configured in VTP transparent mode can create, delete, and modify VLANs, but the changes are not transmitted to other switches in the domain; they affect only the local switch. Table 3-1 offers a comparative overview of the three VTP modes.

Table 3-1 VTP Modes

Server Mode

Client Mode

Transparent Mode

Sends/forwards VTP advertisements.

Sends/forwards VTP advertisements.

Forwards VTP advertisements.

Synchronizes VLAN configuration information with other switches.

Synchronizes VLAN configuration information with other switches.

Does not synchronize VLAN configuration information with other switches.

VLAN configurations are saved in NVRAM.

VLAN configurations are not saved in NVRAM.

VLAN configurations are saved in NVRAM.

Catalyst switch can create VLANs.

Catalyst switch cannot create VLANs.

Catalyst switch can create VLANs.

Catalyst switch can modify VLANs.

Catalyst switch cannot modify VLANs.

Catalyst switch can modify VLANs.

Catalyst switch can delete VLANs.

Catalyst switch cannot delete VLANs.

Catalyst switch can delete VLANs.


When setting up VTP on a switch, choosing the appropriate mode is important. Because VTP is a simple and dangerous tool, it can overwrite VLAN configurations on some switches and create network problems. The next section further explains this phenomenon. Nevertheless, you must be aware that the mode you choose can eliminate the chance of these problems:

  • Choose server mode for the switch that you will use to create, change, or delete VLANs. The server will propagate this information to other switches that are configured as servers or clients.

  • Set client mode on any switch where you do not want to create, change, or delete VLANS.

  • Use transparent mode on a switch that needs to pass VTP advertisements to other switches but also needs the capability to have its VLANs independently administered.

How VTP Works

VTP advertisements are flooded throughout the management domain every five minutes or whenever a change occurs in VLAN configurations. VTP advertisements are sent over a factory default VLAN (VLAN 1) using multicast frames. Included in a VTP advertisement is a configuration revision number. A higher configuration revision number indicates that the VLAN information being advertised is more current than the stored information.

A device that receives VTP advertisements must check various parameters before incorporating the received VLAN information.

First, the management domain name and the password, which can be configured to prevent unauthorized switches from altering the VTP domain, must match those configured in the local switch before information can be used.

Next, if the configuration revision number indicates that the message was created after the configuration currently in use, the switch overwrites its VLAN database with the advertised VLAN information. To reset the configuration revision number on a Catalyst switch, you must either change the switch mode to transparent then back to server or client with the command vtp mode [server | client | transparent] in global configuration mode, or change the VTP domain name and then set it back using the command vtp domain name in global configuration mode. Example 3-1 demonstrates changing the mode and then setting it back to reset the configuration revision number. The command show vtp status is executed before and after the change to show the configuration number being reset.

Example 3-1 Resetting a Switches VTP Configuration Revision Number

Switch#show vtp status
VTP Version           : 2
Configuration Revision     : 5
Maximum VLANs supported locally : 250
Number of existing VLANs    : 10
VTP Operating Mode       : Server
VTP Domain Name         : switch_domain_1
VTP Pruning Mode        : Disabled
VTP V2 Mode           : Disabled
VTP Traps Generation      : Disabled
MD5 digest           : 0x1E 0xED 0x19 0x49 0x0F 0x37 0x65 0x64 
Configuration last modified by 192.168.255.21 at 3-1-93 00:02:39
Local updater ID is 192.168.255.21 on interface Vl1 (lowest numbered VLAN interface found)
Switch#config t
P2_2950(config)#vtp mode transparent
Setting device to VTP TRANSPARENT mode.
Switch(config)#vtp mode server
Setting device to VTP SERVER mode
Switch(config)#end

Switch#show vtp status
VTP Version           : 2
Configuration Revision     : 0
Maximum VLANs supported locally : 250
Number of existing VLANs    : 10
VTP Operating Mode       : Server
VTP Domain Name         : switch_domain_1
VTP Pruning Mode        : Disabled
VTP V2 Mode           : Disabled
VTP Traps Generation      : Disabled
MD5 digest           : 0x1E 0xED 0x19 0x49 0x0F 0x37 0x65 0x64 
Configuration last modified by 192.168.255.21 at 3-1-93 00:02:39
Local updater ID is 192.168.255.21 on interface Vl1 (lowest numbered VLAN interface found)

NOTE

Underscores are used in the VTP domain name because a domain name cannot contain spaces.

One of the most critical components of VTP is the configuration revision number. Each time a VTP server modifies its VLAN information, it increments the configuration revision number by one. The VTP server then sends out a VTP advertisement with the new configuration revision number. If the configuration revision number being advertised is higher than the number stored on the other switches in the VTP domain, the other switches will overwrite their VLAN configurations with the new information being advertised. The configuration revision number in VTP transparent mode is always 0. Figure 3-13 illustrates how VTP operates in a switched network.

CAUTION

The overwrite process would mean that the VTP server with the highest revision number determines the overall VLAN configuration for the domain. For example, if you deleted all VLANs on a VTP server and that server had the higher revision number, the other devices in the VTP domain would also delete their VLANs. This could create a loss of connectivity.

Figure 13Figure 3-13 VTP Operation

VTP Pruning

Because ISL trunk lines carry VLAN traffic for all VLANs by default, some traffic might be needlessly flooded across links that do not need to carry that traffic. VTP pruning uses VLAN advertisements to determine when a trunk connection is flooding traffic needlessly.

By default, a trunk connection carries traffic for all VLANs in the VTP management domain. Often, some switches in an enterprise network do not have local ports configured in each VLAN. In Figure 3-14, Switches 1 and 4 support ports statically configured in VLAN10. As illustrated, with VTP pruning enabled, when Station A sends a broadcast, the broadcast is flooded only toward any switch with ports assigned to VLAN10. As a result, broadcast traffic from Station A is not forwarded to Switches 3, 5, and 6 because traffic for VLAN10 has been pruned on the links indicated on Switches 2 and 4. Pruning must be enabled on one VTP server, and it will be propagated to all other switches in the VTP domain.

VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate network devices.

NOTE

Because VLAN1 is the management VLAN and is used for administrative functions such as VTP advertisements, it will not be pruned from a trunk line by VTP pruning.

Figure 14Figure 3-14 VTP Pruning

  • + Share This
  • 🔖 Save To Your Account

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