Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches

Document ID: 5234

Introduction

Spanning Tree Protocol (STP) is a Layer 2 (L2) protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network. Loops are deadly to a network.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

Although this document uses Cisco Catalyst 5500/5000 switches, the spanning tree principles that the document presents are applicable to almost all devices that support STP.

For the examples, this document used:

• A console cable that is suitable for the Supervisor Engine in the switch
• Six Catalyst 5509 switches

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Background Theory

The configurations in this document apply to Catalyst 2926G, 2948G, 2980G, 4500/4000, 5500/5000, and 6500/6000 switches that run Catalyst OS (CatOS). For information on the configuration of STP on other switch platforms, refer to these documents:

• Configuring STP and IEEE 802.1s MST on Catalyst 6500/6000 switches that run Cisco IOS® Software
• Understanding and Configuring STP on Catalyst 4500/4000 switches that run Cisco IOS Software
• Configuring STP on Catalyst 2900 XL/3500 XL switches
• Configuring STP on Catalyst 3550 switches
• Configuring STP on Catalyst 2950 switches

Network Diagram

This document uses this network setup:

Concepts

STP runs on bridges and switches that are 802.1D-compliant. There are different flavors of STP, but 802.1D is the most popular and widely implemented. You implement STP on bridges and switches in order to prevent loops in the network. Use STP in situations where you want redundant links, but not loops. Redundant links are as important as backups in the case of a failover in a network. A failure of your primary activates the backup links so that users can continue to use the network. Without STP on the bridges and switches, such a failure can result in a loop. Consider this network:

In this network, a redundant link is planned between Switch A and Switch B. But this setup creates the possibility of a bridging loop. For example, a broadcast or multicast packet that transmits from Station M and is destined for Station N simply continues to circulate between both switches.

However, when STP runs on both switches, the network logically looks like this:

This information applies to the scenario in the Network Diagram:

• Switch 15 is the backbone switch.


• Switches 12, 13, 14, 16, and 17 are switches that attach to workstations and PCs.


• The network defines these VLANs:
  
    ¨1
    ¨200
    ¨201
    ¨202
    ¨203
    ¨204


• The VLAN Trunk Protocol (VTP) domain name is STD-Doc.

In order to provide this desired path redundancy, as well as to avoid a loop condition, STP defines a tree that spans all the switches in an extended network. STP forces certain redundant data paths into a standby (blocked) state and leaves other paths in a forwarding state. If a link in the forwarding state becomes unavailable, STP reconfigures the network and reroutes data paths through the activation of the appropriate standby path.

Description of the Technology

With STP, the key is for all the switches in the network to elect a root bridge that becomes the focal point in the network. All other decisions in the network, such as which port to block and which port to put in forwarding mode, are made from the perspective of this root bridge. A switched environment, which is different from a bridge environment, most likely deals with multiple VLANs. When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.

Note: The selection of the root switch for a particular VLAN is very important. You can choose the root switch, or you can let the switches decide, which is risky. If you do not control the root selection process, there can be suboptimal paths in your network.

All the switches exchange information for use in the root switch selection and for subsequent configuration of the network. Bridge protocol data units (BPDUs) carry this information. Each switch compares the parameters in the BPDU that the switch sends to a neighbor with the parameters in the BPDU that the switch receives from the neighbor.

In the STP root selection process, less is better. If Switch A advertises a root ID that is a lower number than the root ID that Switch B advertises, the information from Switch A is better. Switch B stops the advertisement of its root ID, and accepts the root ID of Switch A.

Refer to Configuring Optional STP Features for details about some of the optional STP features, such as:

• PortFast
• Root guard
• Loop guard
• BPDU guard

STP Operation

Task

Prerequisites: Before you configure STP, select a switch to be the root of the spanning tree. This switch does not need to be the most powerful switch. But choose the most centralized switch on the network. All data flow across the network is from the perspective of this switch. Also, choose the least disturbed switch in the network. The backbone switches often serve as the spanning tree root because these switches typically do not connect to end stations. Also, moves and changes within the network are less likely to affect these switches.

After you decide on the root switch, set the appropriate variables to designate the switch as the root switch.

The only variable that you must set is the bridge priority. If the switch has a bridge priority that is lower than all the other switches, the other switches automatically select the switch as the root switch.

Clients (end stations) on switch ports: You can also issue the set spantree portfast command, on a per-port basis. When you enable the portfast variable on a port, the port immediately switches from blocking mode to forwarding mode. Enablement of portfast helps to prevent timeouts on clients who use Novell Netware or use Dynamic Host Configuration Protocol (DHCP) in order to obtain an IP address. However, do not use this command when you have switch-to-switch connection. In this case, the command can result in a loop. The 30?0 second delay that occurs during the transition from blocking to forwarding mode prevents a temporal loop condition in the network when you connect two switches.

Leave most other STP variables at their default values.

Rules of Operation: This section lists rules for how STP works. When the switches first come up, they start the root switch selection process. Each switch transmits a BPDU to the directly connected switch on a per-VLAN basis.

As the BPDU goes out through the network, each switch compares the BPDU that the switch sends to the BPDU that the switch receives from the neighbors. The switches then agree on which switch is the root switch. The switch with the lowest bridge ID in the network wins this election process.

Note: Remember that one root switch is identified per VLAN. After the root switch identification, the switches follow these rules:

• STP Rule 1: All ports of the root switch must be in forwarding mode.

Note: In some corner cases, which involve self-looped ports, there is an exception to this rule.

Next, each switch determines the best path to get to the root. The switches determine this path by a comparison of the information in all the BPDUs that the switches receive on all ports. The switch uses the port with the least amount of information in the BPDU in order to get to the root switch; the port with the least amount of information in the BPDU is the root port. After a switch determines the root port, the switch proceeds to Rule 2.

• STP Rule 2: The root port must be set to forwarding mode.

In addition, the switches on each LAN segment communicate with each other to determine which switch is best to use in order to move data from that segment to the root bridge. This switch is called the designated switch.

• STP Rule 3: In a single LAN segment, the port of the designated switch that connects to that LAN segment must be placed in forwarding mode.
• STP Rule 4: All the other ports in all the switches (VLAN-specific) must be placed in blocking mode. The rule only applies to ports that connect to other bridges or switches. STP does not affect ports that connect to workstations or PCs; these ports remain forwarded.

Step-by-Step Instructions

Complete these steps:

Note: All switches run the same software version.

In this scenario, Switch 15 is the best choice for the root switch of the network for all the VLANs because Switch 15 is the backbone switch.

Issue the set spantree root vlan_id command in order to set the priority of the switch to 8192 for the VLAN or VLANs that the vlan_id specifies.

Note: The default priority for switches is 32768. When you set the priority with this command, you force the selection of Switch 15 as the root switch because Switch 15 has the lowest priority.

The shorter version of the command has the same effect, as this example shows:

Note: In this scenario, all the switches started with cleared configurations. So all the switches started with a bridge priority of 32768. If you are not certain that all the switches in your network have a priority that is greater than 8192, set the priority of your desired root bridge to 1.

Note: Only configure this setting on ports that connect to workstations or PCs. Do not enable PortFast on any port that connects to another switch.

This example only configures Switch 12. You can configure other switches in the same way. Switch 12 has these port connections:

    
• Port 2/1 connects to Switch 13.

    
• Port 2/2 connects to Switch 15.

    
• Port 2/3 connects to Switch 16.

    
• Port 3/1 through 3/24 connects to PCs.

    
• Port 4/1 through 4/24 connects to UNIX workstations.

With this information as a basis, issue the set spantree portfast command on ports 3/1›/24 and on ports 4/1oe/24:

From the output from this command, compare the MAC address of the switch that is the root switch to the MAC address of the switch from which you issued the command. If the addresses match, the switch that you are in is the root switch of the VLAN. A root port that is 1/0 also indicates that you are at the root switch. Here is the sample command output:

This output shows that Switch 15 is the designated root on the spanning tree for VLAN 1. The MAC address of the designated root switch (00-10-0d-b1-78-00) is the same as the bridge ID MAC address of Switch 15 (00-10-0d-b1-78-00). Another indicator that this switch is the designated root is that the designated root port is 1/0.

In this output from Switch 12, the switch recognizes Switch 15 as the Designated Root for VLAN 1:

Note: The output of the show spantree vlan_id command for the other switches and VLANs would also indicate that Switch 15 is the designated root for all VLANs.

Verify

This section provides information you can use to confirm that your configuration works properly.

Certain show commands are supported by the Output Interpreter Tool ( registered customers only) , which allows you to view an analysis of show command output.

• show spantree vlan_id Shows the current state of the spanning tree for this VLAN ID, from the perspective of the switch on which you issue the command.
• show spantree summaryProvides a summary of connected spanning tree ports by VLAN.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Troubleshooting Commands

Certain show commands are supported by the Output Interpreter Tool ( registered customers only) , which allows you to view an analysis of show command output.

• show spantree vlan_id Shows the current state of the spanning tree for this VLAN ID, from the perspective of the switch on which you issue the command.
• show spantree summaryProvides a summary of connected spanning tree ports by VLAN.
• show spantree statisticsShows spanning tree statistical information.
• show spantree backbonefastDisplays whether the spanning tree BackboneFast Convergence feature is enabled.
• show spantree blockedportsDisplays only the blocked ports.
• show spantree portstateDetermines the current spanning tree state of a Token Ring port within a spanning tree.
• show spantree portvlancostShows the path cost for the VLANs on a port.
• show spantree uplinkfastShows the UplinkFast settings.

Command Summary

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Related Information

• Spanning Tree Protocol Problems and Related Design Considerations
• Understanding Spanning-Tree Protocol Topology Changes
• Configuring Spanning Tree for Catalyst 4500/4000 Switches
• Configuring Spanning Tree for Catalyst 5500/5000 Switches
• Configuring Spanning Tree for Catalyst 6500/6000 Switches
• LAN Product Support Pages
• LAN Switching Support Page
• Technical Support & Documentation - Cisco Systems

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