您的当前位置：首页正文

stp

来源：小奈知识网

Understanding and Configuring Spanning TreeProtocol (STP) on Catalyst Switches

Document ID: 5234

Contents

IntroductionPrerequisites Requirements Components Used Conventions

Background Theory Network DiagramConcepts

Description of the TechnologySTP Operation Task

Step−by−Step InstructionsVerify

Troubleshoot

STP Path Cost Automatically Changes When a Port Speed/Duplex Is Changed Troubleshoot Commands Command SummaryRelated Information

Introduction

Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches. The specification forSTP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you haveredundant 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 thedocument 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 surethat you understand the potential impact of any command.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Theory

The configurations in this document apply to Catalyst 2926G, 2948G, 2980G, 4500/4000, 5500/5000, and6500/6000 Switches that run Catalyst OS (CatOS). Refer to these documents for information on theconfiguration of STP on other switch platforms:

• Configuring STP and IEEE 802.1s MST (Catalyst 6500/6000 Switches that run Cisco IOS®Software)

• Understanding and Configuring STP (Catalyst 4500/4000 Switches that run Cisco IOS Software)• Configuring STP section of Configuring the System (Catalyst 2900XL/3500XL Switches)• Configuring STP (Catalyst 3550 Switches)• Configuring STP (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 802.1Dis the most popular and widely implemented. You implement STP on bridges and switches in order to preventloops in the network. Use STP in situations where you want redundant links, but not loops. Redundant linksare as important as backups in the case of a failover in a network. A failure of your primary activates thebackup links so that users can continue to use the network. Without STP on the bridges and switches, such afailure can result in a loop. If two connected switches run different flavors of STP, they require differenttimings to converge. When different flavors are used in the switches, it creates timing issues between

Blocking and Forwarding states. Therefore, it is recommended to use the same flavors of STP. Consider thisnetwork:

In this network, a redundant link is planned between Switch A and Switch B. However, this setup creates thepossibility of a bridging loop. For example, a broadcast or multicast packet that transmits from Station M andis 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 thatspans 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 appropriatestandby 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 inthe network. All other decisions in the network, such as which port to block and which port to put inforwarding mode, are made from the perspective of this root bridge. A switched environment, which isdifferent from a bridge environment, most likely deals with multiple VLANs. When you implement a rootbridge 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 VLANscan 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 rootswitch, 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 ofthe network. Bridge protocol data units (BPDUs) carry this information. Each switch compares the parametersin the BPDU that the switch sends to a neighbor with the parameters in the BPDU that the switch receivesfrom the neighbor.

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

Refer to Configuring Optional STP Features for more information 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 bethe most powerful switch, but choose the most centralized switch on the network. All data flow across thenetwork is from the perspective of this switch. Also, choose the least disturbed switch in the network. Thebackbone switches often serve as the spanning tree root because these switches typically do not connect to endstations. 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 thanall 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 portfastvariable on a port, the port immediately switches from blocking mode to forwarding mode. Enablement ofportfast helps to prevent timeouts on clients who use Novell Netware or use DHCP in order to obtain an IPaddress. However, do not use this command when you have switch−to−switch connection. In this case, thecommand can result in a loop. The 30− to 60−second delay that occurs during the transition from blocking toforwarding 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 switchselection 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 theBPDU that the switch receives from the neighbors. The switches then agree on which switch is the rootswitch. 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, theswitches adhere to these rules:

• STP Rule 1All 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 acomparison of the information in all the BPDUs that the switches receive on all ports. The switch usesthe port with the least amount of information in the BPDU in order to get to the root switch; the portwith the least amount of information in the BPDU is the root port. After a switch determines the rootport, the switch proceeds to rule 2.

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

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

• STP Rule 3In a single LAN segment, the port of the designated switch that connects to that LANsegment must be placed in forwarding mode.

• STP Rule 4All the other ports in all the switches (VLAN−specific) must be placed in blockingmode. The rule only applies to ports that connect to other bridges or switches. STP does not affectports that connect to workstations or PCs. These ports remain forwarded.

Note: The addition or removal of VLANs when STP runs in per−VLAN spanning tree (PVST /

PVST+) mode triggers spanning tree recalculation for that VLAN instance and the traffic is disruptedonly for that VLAN. The other VLAN parts of a trunk link can forward traffic normally. The additionor removal of VLANs for a Multiple Spanning Tree (MST) instance that exists triggers spanning treerecalculation for that instance and traffic is disrupted for all the VLAN parts of that MST instance.

Note: By default, spanning tree runs on every port. The spanning tree feature cannot be turned off in switcheson a per−port basis. Although it is not recommended, you can turn off STP on a per−VLAN basis, or globallyon the switch. Extreme care should be taken whenever you disable spanning tree because this creates Layer 2loops within the network.

Step−by−Step Instructions

Complete these steps:

1. Issue the show version command in order to display the software version that the switch runs.Note: All switches run the same software version.

Switch−15> (enable)show version

System Bootstrap Version: 5.1(2)

Hardware Version: 1.0 Model: WS−C5505 Serial #: 066509927

Mod Port Model Serial # Versions

−−− −−−− −−−−−−−−−− −−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−1 0 WS−X5530 008676033 Hw : 2.3Fw : 5.1(2)Fw1: 4.4(1)Sw : 4.2(1)

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

2. Issue the set spantree root vlan_id command in order to set the priority of the switch to 8192 for theVLAN or VLANs that the vlan_id specifies.Note: The default priority for switches is 32768. When you set the priority with this command, youforce the selection of Switch 15 as the root switch because Switch 15 has the lowest priority.

Switch−15> (enable)set spantree root 1VLAN 1 bridge priority set to 8192.

VLAN 1 bridge max aging time set to 20.VLAN 1 bridge hello time set to 2.

VLAN 1 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 1.Switch−15> (enable)

Switch−15> (enable)set spantree root 200VLAN 200 bridge priority set to 8192.

VLAN 200 bridge max aging time set to 20.VLAN 200 bridge hello time set to 2.

VLAN 200 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 200.Switch−15> (enable)

Switch−15> (enable)set spantree root 201VLAN 201 bridge priority set to 8192.

VLAN 201 bridge max aging time set to 20.VLAN 201 bridge hello time set to 2.

VLAN 201 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 201.Switch−15> (enable)

Switch−15> (enable)set spantree root 202VLAN 202 bridge priority set to 8192.

VLAN 202 bridge max aging time set to 20.VLAN 202 bridge hello time set to 2.

VLAN 202 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 202.Switch−15>

Switch−15> (enable)set spantree root 203VLAN 203 bridge priority set to 8192.

VLAN 203 bridge max aging time set to 20.VLAN 203 bridge hello time set to 2.

VLAN 203 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 203.Switch−15>

Switch−15> (enable)set spantree root 204VLAN 204 bridge priority set to 8192.

VLAN 204 bridge max aging time set to 20.VLAN 204 bridge hello time set to 2.

VLAN 204 bridge forward delay set to 15.

Switch is now the root switch for active VLAN 204.Switch−15> (enable)

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

Switch−15> (enable)set spantree root 1,200−204VLANs 1,200−204 bridge priority set to 8189.

VLANs 1,200−204 bridge max aging time set to 20.VLANs 1,200−204 bridge hello time set to 2.

VLANs 1,200−204 bridge forward delay set to 15.

Switch is now the root switch for active VLANs 1,200−204.Switch−15> (enable)

The set spantree priority command provides a third method to specify the root switch:

Switch−15> (enable)set spantree priority 8192 1Spantree 1 bridge priority set to 8192.Switch−15> (enable)

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

3. Issue the set spantree portfast mod_num/port_num enable command in order to configure thePortFast setting on Switches 12, 13, 14, 16, and 17.Note: Only configure this setting on ports that connect to workstations or PCs. Do not enable PortFaston any port that connects to another switch.

This example only configures Switch 12. You can configure other switches in the same way. Switch12 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.

♦ Ports 3/1 through 3/24 connect to PCs.

♦ Ports 4/1 through 4/24 connect to UNIX workstations.

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

Switch−12> (enable)set spantree portfast 3/1−24 enable

Warning: Spantree port fast start should only be enabled on ports connectedto a single host. Connecting hubs, concentrators, switches, bridges, etc. toa fast start port can cause temporary spanning−tree loops. Use with caution.Spantree ports 3/1−24 fast start enabled.Switch−12> (enable)

Switch−12> (enable)set spantree portfast 4/1−24 enable

4. Issue the show spantree vlan_id command in order to verify that Switch 15 is the root of all theappropriate VLANs.

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

Switch−15> (enable)show spantree 1VLAN 1

spanning−tree enabled

spanning−tree type ieee

Designated Root 00−10−0d−b1−78−00

!−−− This is the MAC address of the root switch for VLAN 1.Designated Root Priority 8192Designated Root Cost 0Designated Root Port 1/0

Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 secBridge ID MAC ADDR 00−10−0d−b1−78−00Bridge ID Priority 8192

Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec

This output shows that Switch 15 is the designated root on the spanning tree for VLAN 1. The MACaddress of the designated root switch, 00−10−0d−b1−78−00, is the same as the bridge ID MACaddress of Switch 15, 00−10−0d−b1−78−00. Another indicator that this switch is the designatedroot 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:

Switch−12> (enable)show spantree 1VLAN 1

spanning−tree enabled

spanning−tree type IEEEDesignated Root 00−10−0d−b1−78−00!−−− This is the MAC address of the root switch for VLAN 1.Designated Root Priority 8192Designated Root Cost 19Designated Root Port 2/3

Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 secBridge ID MAC ADDR 00−10−0d−b2−8c−00Bridge ID Priority 32768

Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec

Note: The output of the show spantree vlan_id command for the other switches and VLANs can alsoindicate 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.

The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OITto view an analysis of show command output.

• show spantree vlan_id 󰀀Shows the current state of the spanning tree for this VLAN ID, from theperspective 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.

STP Path Cost Automatically Changes When a Port Speed/Duplex IsChanged

STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the portcost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port withthe lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A change inthe path cost can change the spanning tree topology.

Refer to the Calculating and Assigning Port Costs section of Configuring Spanning Tree for more informationon how to calculate the port cost.

Troubleshoot Commands

The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OITto view an analysis of show command output.

Note: Refer to Important Information on Debug Commands before you use debug commands.

• show spantree vlan_id 󰀀Shows the current state of the spanning tree for this VLAN ID, from theperspective 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 Convergencefeature is enabled.

• show spantree blockedportsDisplays only the blocked ports.

• show spantree portstateDetermines the current spanning tree state of a Token Ring port within aspanning tree.

• show spantree portvlancostShows the path cost for the VLANs on a port.• show spantree uplinkfastShows the UplinkFast settings.

Command Summary

Syntax:As used in thisdocument:Syntax:As used in thisdocument:Syntax:As used in thisdocument:Syntax:

show versionshow versionset spantree root [vlan_id]set spantree root 1set spantree root 1,200−204set spantree priority [vlan_id]set spantree priority 8192 1set spantree portfast mod_num/port_num{enable | disable}

As used in thisdocument:Syntax:As used in thisdocument:

set spantree portfast 3/1−24 enableshow spantree [vlan_id]show spantree 1Related Information

• Spanning Tree Protocol Problems and Related Design Considerations• Understanding Spanning−Tree Protocol Topology Changes• Configuring Spanning Tree• Configuring Spanning Tree• Configuring Spanning Tree• LAN Product Support

• LAN Switching Technology Support

• Technical Support & Documentation − Cisco Systems

Contacts & Feedback | Help | Site Map

Updated: Aug 17, 2006Document ID: 5234

因篇幅问题不能全部显示，请点此查看更多更全内容

查看全文