Home > Articles > STP


  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Dec 1, 2017.

Chapter Description

Network redundancy is a key to maintaining network reliability. In this sample chapter from Scaling Networks v6 Companion Guide, explore potential redundancy problems, symptoms, and protocols to manage them.

From the Book

Scaling Networks v6 Companion Guide

Scaling Networks v6 Companion Guide

$66.00 (Save 10%)

Spanning Tree Configuration (3.3)

In this section, you will learn how to implement PVST+ and Rapid PVST+ in a switched LAN environment.

PVST+ Configuration (3.3.1)

The focus of this topic is on how to configure PVST+ in a switched LAN environment.

Catalyst 2960 Default Configuration (

Table 3-7 shows the default spanning-tree configuration for a Cisco Catalyst 2960 Series switch. Notice that the default spanning-tree mode is PVST+.

Table 3-7 Default Switch Configuration

Feature Default Setting
Enable state Enabled on VLAN 1
Spanning-tree mode PVST+ (Rapid PVST+ and MSTP are disabled.)
Switch priority 32768
Spanning-tree port priority (configurable on a per-interface basis) 128
Spanning-tree port cost (configurable on a per-interface basis) 1000 Mbps: 4
  100 Mbps: 19
  10 Mbps: 100
Spanning-tree VLAN port priority (configurable on a per-VLAN basis) 128
Spanning-tree VLAN port cost (configurable on a per-VLAN basis) 1000 Mbps: 4
  100 Mbps: 19
  10 Mbps: 100
Spanning-tree timers Hello time: 2 seconds
  Forward-delay time: 15 seconds
  Maximum-aging time: 20 seconds
  Transmit hold count: 6 BPDUs

Configuring and Verifying the Bridge ID (

When an administrator wants a specific switch to become a root bridge, the bridge priority value must be adjusted to ensure that it is lower than the bridge priority values of all the other switches on the network. There are two different methods to configure the bridge priority value on a Cisco Catalyst switch.

Method 1

To ensure that a switch has the lowest bridge priority value, use the spanning-tree vlan vlan-id root primary command in global configuration mode. The priority for the switch is set to the predefined value of 24,576 or to the highest multiple of 4096 less than the lowest bridge priority detected on the network.

If an alternate root bridge is desired, use the spanning-tree vlan vlan-id root secondary global configuration mode command. This command sets the priority for the switch to the predefined value 28,672. This ensures that the alternate switch becomes the root bridge if the primary root bridge fails. This assumes that the rest of the switches in the network have the default 32,768 priority value defined.

In Figure 3-39, S1 has been assigned as the primary root bridge, using the spanning-tree vlan 1 root primary command, and S2 has been configured as the secondary root bridge, using the spanning-tree vlan 1 root secondary command.

Method 2

Another method for configuring the bridge priority value is by using the spanning-tree vlan vlan-id priority value global configuration mode command. This command gives more granular control over the bridge priority value. The priority value is configured in increments of 4096 between 0 and 61,440.

In the example in Figure 3-39, S3 has been assigned a bridge priority value of 24,576, using the spanning-tree vlan 1 priority 24576 command.

Figure 3-39

Figure 3-39 Configuring the Bridge ID

To verify the bridge priority of a switch, use the show spanning-tree command. In Example 3-4, the priority of the switch has been set to 24,576. Also notice that the switch is designated as the root bridge for the spanning-tree instance.

Example 3-4 Verifying the Root Bridge and BID

S3# show spanning-tree
  Spanning tree enabled protocol ieee
  Root ID    Priority     24577
              Address     000A.0033.0033
              This bridge is the root
              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

 Bridge ID   Priority    24577  (priority 24576 sys-id-ext 1)
              Address     000A.0033.3333
              Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
              Aging Time  300
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- ------------------------
Fa0/1               Desg FWD 4         128.1    P2p
Fa0/2               Desg FWD 4         128.2    P2p

PortFast and BPDU Guard (

PortFast is a Cisco feature for PVST+ environments. When a switch port is configured with PortFast, that port transitions from blocking to forwarding state immediately, bypassing the usual 802.1D STP transition states (the listening and learning states). As shown in Figure 3-40, you can use PortFast on access ports to allow these devices to connect to the network immediately rather than wait for IEEE 802.1D STP to converge on each VLAN. Access ports are ports that are connected to a single workstation or to a server.

Figure 3-40

Figure 3-40 PortFast and BPDU Guard Topology

In a valid PortFast configuration, BPDUs should never be received because that would indicate that another bridge or switch is connected to the port, potentially causing a spanning-tree loop. Cisco switches support a feature called BPDU guard. When it is enabled, BPDU guard puts the port in an errdisabled (error-disabled) state on receipt of a BPDU. This effectively shuts down the port. The BPDU guard feature provides a secure response to invalid configurations because you must manually put the interface back into service.

Cisco PortFast technology is useful for DHCP. Without PortFast, a PC can send a DHCP request before the port is in forwarding state, denying the host from getting a usable IP address and other information. Because PortFast immediately changes the state to forwarding, the PC always gets a usable IP address (if the DHCP server has been configured correctly and communication with the DHCP server has occurred).

To configure PortFast on a switch port, enter the spanning-tree portfast interface configuration mode command on each interface on which PortFast is to be enabled, as shown in Example 3-5.

Example 3-5 Configuring PortFast

S2(config)# interface FastEthernet 0/11
S2(config-if)# spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single
   host. Connecting hubs, concentrators, switches, bridges, etc... to this
   interface when portfast is enabled, can cause temporary bridging loops.
   Use with CAUTION

%Portfast has been configured on FastEthernet0/11 but will only
   have effect when the interface is in a non-trunking mode.


The spanning-tree portfast default global configuration mode command enables PortFast on all non-trunking interfaces.

To configure BPDU guard on a Layer 2 access port, use the spanning-tree bpduguard enable interface configuration mode command, as shown in Example 3-6.

Example 3-6 Configuring and Verifying BPDU Guard

S2(config-if)#  spanning-tree bpduguard enable
S2(config-if)# end
S2# show running-config interface f0/11
interface FastEthernet0/11
spanning-tree portfast
spanning-tree bpduguard enable


The spanning-tree portfast bpduguard default global configuration command enables BPDU guard on all PortFast-enabled ports.

Notice in Example 3-6 how the show running-config interface command can be used to verify that PortFast and BPDU guard have been enabled for a switch port. PortFast and BPDU guard are disabled, by default, on all interfaces.

PVST+ Load Balancing (

The topology in Figure 3-41 shows three switches with 802.1Q trunks connecting them.

Figure 3-41

Figure 3-41 PVST+ Configuration Topology

Two VLANs, 10 and 20, are being trunked across these links. The goal is to configure S3 as the root bridge for VLAN 20 and S1 as the root bridge for VLAN 10. Port F0/3 on S2 is the forwarding port for VLAN 20 and the blocking port for VLAN 10. Port F0/2 on S2 is the forwarding port for VLAN 10 and the blocking port for VLAN 20.

In addition to establishing a root bridge, it is also possible to establish a secondary root bridge. A secondary root bridge is a switch that may become the root bridge for a VLAN if the primary root bridge fails. Assuming that the other bridges in the VLAN retain their default STP priority, this switch becomes the root bridge if the primary root bridge fails.

Configuring PVST+ on this topology involves the following steps:

  • Step 1. Select the switches you want for the primary and secondary root bridges for each VLAN. For example, in Figure 3-41, S3 is the primary bridge for VLAN 20, and S1 is the secondary bridge for VLAN 20.

  • Step 2. As shown in Example 3-7, configure S3 to be a primary bridge for VLAN 10 and the secondary bridge for VLAN 20 by using the spanning-tree vlan number root { primary | secondary } command.

Example 3-7 Configuring Primary and Secondary Root Bridges for Each VLAN on S3

S3(config)#  spanning-tree vlan 20 root primary
S3(config)# spanning-tree vlan 10 root secondary
  • Step 3. As shown in Example 3-8, configure S1 to be a primary bridge for VLAN 20 and the secondary bridge for VLAN 10.

Example 3-8 Configuring Primary and Secondary Root Bridges for Each VLAN on S1

S1(config)# spanning-tree vlan 10 root primary
S1(config)# spanning-tree vlan 20 root secondary

Another way to specify the root bridge is to set the spanning-tree priority on each switch to the lowest value so that the switch is selected as the primary bridge for its associated VLAN, as shown in Example 3-9.

Example 3-9 Configuring the Lowest Possible Priority to Ensure That a Switch Is Root

S3(config)#  spanning-tree vlan 20 priority 4096

Switch stacks help maintain or reduce the impact of diameter on STP reconvergence. In a switch stack, all switches use the same bridge ID for a given spanning-tree instance. This means that, if the switches are stacked, as shown in Figure 3-51, the maximum diameter becomes 3 instead of 9.

Figure 3-51

Figure 3-51 Switch Stacking Reduces STP Diameter

7. Summary (3.4) | 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