Table Of Contents
Spanning Tree Operation Within an MST Region
Spanning Tree Operations Between MST Regions
Detecting Unidirectional Link Failure
Interoperability with IEEE 802.1D
Licensing Requirements for MST
Entering MST Configuration Mode
Specifying the MST Configuration Revision Number
Specifying the Configuration on an MST Region
Mapping or Unmapping a VLAN to an MST Instance
Mapping Secondary VLANs to Same MSTI as Primary VLANs for Private VLANs
Configuring a Secondary Root Bridge
Configuring the Switch Priority
Configuring the Forwarding-Delay Time
Configuring the Maximum-Aging Time
Configuring the Maximum-Hop Count
Configuring an Interface to Proactively Send Prestandard MSTP Messages
Displaying and Clearing MST Statistics
Configuring MST
This chapter describes how to configure Multiple Spanning Tree (MST) on NX-OS devices.
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See the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 4.0de for information on creating Layer 2 interfaces.
Multiple Spanning Tree (MST), which is the IEEE 802.1s standard, allows you to assign two or more VLANs to a spanning tree instance. MST is not the default spanning tree mode; Rapid per VLAN Spanning Tree (Rapid PVST+) is the default mode. MST instances with the same name, revision number, and VLAN-to-instance mapping combine to form an MST region. The MST region appears as a single bridge to spanning tree configurations outside the region. MST forms a boundary to that interface when it receives an IEEE 802.1D Spanning Tree Protocol (STP) message from a neighboring device.
You cannot map VLANs 3968 to 4047 or 4094 to any MST instance. These VLANs are reserved for internal use by the device.
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This chapter includes the following section:
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Information About MST
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MST, which is the IEEE 802.1s standard, allows you to assign two or more VLANs to a spanning tree instance. MST is not the default spanning tree mode; Rapid per VLAN Spanning Tree (Rapid PVST+) is the default mode. MST instances with the same name, revision number, and VLAN-to-instance mapping combine to form an MST region. The MST region appears as a single bridge to spanning tree configurations outside the region. MST forms a boundary to that interface when it receives an IEEE 802.1D Spanning Tree Protocol (STP) message from a neighboring device.
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This section includes the following topics:
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MST Overview
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MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of STP instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
MST provides rapid convergence through explicit handshaking as each MST instance uses the IEEE 802.1w standard, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding state. (See Chapter 5, "Configuring Rapid PVST+" for complete information on the explicit handshake agreement.)
MAC address reduction is always enabled on the device (see Chapter 5, "Configuring Rapid PVST+" for complete information on MAC address reduction). You cannot disable this feature.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
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MST Regions
To allow devices to participate in MST instances, you must consistently configure the switches with the same MST configuration information (see the "MST Configuration Information" section).
A collection of interconnected devices that have the same MST configuration is an MST region. An MST region is a linked group of MST bridges with the same MST configuration.
The MST configuration controls the MST region to which each device belongs. The configuration includes the name of the region, the revision number, and the VLAN-to-MST instance assignment mapping.
A region can have one or multiple members with the same MST configuration. Each member must be capable of processing 802.1w bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network.
Each device can support up to 65 MST instances (MSTIs), including Instance 0, in a singe MST region. Instances are identified by any number in the range from 1 to 4094. The system reserves Instance 0 for a special instance, which is the IST. You can assign a VLAN to only one MST instance at a time. (See "IST, CIST, and CST" section for more information on the IST.)
The MST region appears as a single bridge to adjacent MST regions and to other Rapid PVST+ regions and 802.1D spanning tree protocols.
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MST BPDUs
Each device has only one MST BPDU per interface, and that BPDU carries an M-record for each MSTI on the device (see Figure 6-1). Only the IST sends BPDUs for the MST region; all M-records are encapsulated in that one BPDU that the IST sends (see the "IST, CIST, and CST Overview" section for more information on IST). Because the MST BPDU carries information for all instances, the number of BPDUs that need to be processed to support MST is significantly reduced compared with Rapid PVST+.
Figure 6-1 MST BPDU with M-Records for MSTIs
MST Configuration Information
The MST configuration that must be identical on all devices within a single MST region is configured by the user.
You can configure the following three parameters of the MST configuration:
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MST BPDUs contain these three configuration parameters. An MST bridge accepts an MST BPDU into its own region only if these three configuration parameters match exactly. If one configuration attribute differs, the MST bridge considers the BPDU to be from another MST region.
IST, CIST, and CST
These sections describe internal spanning tree (IST), common and internal spanning tree (CIST), and common spanning tree (CST):
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IST, CIST, and CST Overview
Unlike Rapid PVST+ (see Chapter 5, "Configuring Rapid PVST+" for more information), in which all the STP instances are independent, MST establishes and maintains IST, CIST, and CST spanning trees, as follows:
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MST establishes and maintains additional spanning trees within each MST region; these spanning trees are called, multiple spanning tree instances (MSTIs).
Instance 0 is a special instance for a region, known as the IST. The IST always exists on all ports; you cannot delete the IST, or Instance 0. By default, all VLANs are assigned to the IST. All other MST instances are numbered from 1 to 4094.
The IST is the only STP instance that sends and receives BPDUs. All of the other MSTI information is contained in MST records (M-records), which are encapsulated within MST BPDUs.
All MSTIs within the same region share the same protocol timers, but each MSTI has its own topology parameters, such as the root bridge ID, the root path cost, and so forth.
An MSTI is local to the region; for example, MSTI 9 in region A is independent of MSTI 9 in region B, even if regions A and B are interconnected. Only CST information crosses region boundaries.
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The spanning tree computed in an MST region appears as a subtree in the CST that encompasses the entire switched domain. The CIST is formed by the spanning tree algorithm running among switches that support the 802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the CST outside a region.
For more information, see the "Spanning Tree Operation Within an MST Region" section and the "Spanning Tree Operations Between MST Regions" section.
Spanning Tree Operation Within an MST Region
The IST connects all the MST switches in a region. When the IST converges, the root of the IST becomes the CIST regional root as shown in Figure 6-2. The CIST regional root is also the CIST root if there is only one region in the network. If the CIST root is outside the region, the protocol selects one of the MST devices at the boundary of the region as the CIST regional root.
When an MST device initializes, it sends BPDUs that identify itself as the root of the CIST and the CIST regional root, with both the path costs to the CIST root and to the CIST regional root set to zero. The device also initializes all of its MSTIs and claims to be the root for all of them. If the device receives superior MSTI root information (lower switch ID, lower path cost, and so forth) than the information that is currently stored for the port, it relinquishes its claim as the CIST regional root.
During initialization, an MST region might have many subregions, each with its own CIST regional root. As switches receive superior IST information from a neighbor in the same region, they leave their old subregions and join the new subregion that contains the true CIST regional root. This action causes all subregions to shrink except for the subregion that contains the true CIST regional root.
All switches in the MST region must agree on the same CIST regional root. Any two switches in the region will only synchronize their port roles for an MSTI if they converge to a common CIST regional root.
Spanning Tree Operations Between MST Regions
If you have multiple regions or 802.1 w or 802.1D STP instances within a network, MST establishes and maintains the CST, which includes all MST regions and all 802.1w and 802.1D STP switches in the network. The MSTIs combine with the IST at the boundary of the region to become the CST.
The IST connects all the MST switches in the region and appears as a subtree in the CIST that encompasses the entire switched domain. The root of the subtree is the CIST regional root. The MST region appears as a virtual device to adjacent STP switches and MST regions.
Figure 6-2 shows a network with three MST regions and an 802.1D device (D). The CIST regional root for region 1 (A) is also the CIST root. The CIST regional root for region 2 (B) and the CIST regional root for region 3 (C) are the roots for their respective subtrees within the CIST.
Figure 6-2 MST Regions, CIST Regional Roots, and CST Root
Only the CST instance sends and receives BPDUs. MSTIs add their spanning tree information into the BPDUs (as M-records) to interact with neighboring switches within the same MST region and compute the final spanning tree topology. Because of this, the spanning tree parameters related to the BPDU transmission (for example, hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MSTIs. You can configure the parameters related to the spanning tree topology (for example, the switch priority, the port VLAN cost, and the port VLAN priority) on both the CST instance and the MSTI.
MST devices use Version 3 BPDUs. If the MST device falls back to 802.1D STP, the device uses only 802.1D BPDUs to communicate with 802.1D-only devices. MST devices use MST BPDUs to communicate with MST devices.
MST Terminology
MST naming conventions include identification of some internal or regional parameters. These parameters are used only within an MST region, compared to external parameters that are used throughout the whole network. Because the CIST is the only spanning tree instance that spans the whole network, only the CIST parameters require the external qualifiers and not the internal or regional qualifiers. The MST terminology is as follows:
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Hop Count
MST does not use the message-age and maximum-age information in the configuration BPDU to compute the STP topology inside the MST region. Instead, the protocol uses the path cost to the root and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the maximum hops inside the region and apply it to the IST and all MST instances in that region.
The hop count achieves the same result as the message-age information (triggers a reconfiguration). The root bridge of the instance always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the maximum value. When a device receives this BPDU, it decrements the received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs that it generates. When the count reaches zero, the device discards the BPDU and ages the information held for the port.
The message-age and maximum-age information in the 802,1w portion of the BPDU remain the same throughout the region (only on the IST), and the same values are propagated by the region-designated ports at the boundary.
You configure a maximum aging time as the number of seconds that a device waits without receiving spanning tree configuration messages before attempting a reconfiguration.
Boundary Ports
A boundary port is a port that connects to a LAN, the designated bridge of which is either a bridge with a different MST configuration (and so, a separate MST region) or a Rapid PVST+ or 802.1D STP bridge. A designated port knows that it is on the boundary if it detects an STP bridge or receives an agreement proposal from an MST bridge with a different configuration or a Rapid PVST+ bridge. This definition allows two ports that are internal to a region to share a segment with a port that belongs to a different region, creating the possibility of receiving both internal and external messages on a port (see Figure 6-3).
Figure 6-3 MST Boundary Ports
At the boundary, the roles of MST ports do not matter; the system forces their state to be the same as the IST port state. If the boundary flag is set for the port, the MST port-role selection process assigns a port role to the boundary and assigns the same state as the state of the IST port. The IST port at the boundary can take up any port role except a backup port role. (See the "Guidelines and Limitations" section for important information about boundary port states and load balancing.)
Detecting Unidirectional Link Failure
Currently, this feature is not present in the IEEE MST standard, but it is included in the standard-compliant implementation; it is based on the dispute mechanism. The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops.
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When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.
Figure 6-4 shows a unidirectional link failure that typically creates a bridging loop. Switch A is the root bridge, and its BPDUs are lost on the link leading to device B. Rapid PVST+ (802.1w) and MST BPDUs include the role and state of the sending port. With this information, device A can detect that device B does not react to the superior BPDUs that it sends and that device B is the designated, not root port. As a result, device A blocks (or keeps blocking) its port, which prevents the bridging loop.
Figure 6-4 Detecting a Unidirectional Link Failure
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Port Cost and Port Priority
Spanning tree uses port costs to break a tie for the designated port. Lower values indicate lower port costs, and spanning tree chooses the least costly path. Default port costs are taken from the bandwidth of the interface, as follows:
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You can configure the port costs in order to influence which port is chosen.
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The system uses port priorities to break ties among ports with the same cost. A lower number indicates a higher priority. The default port priority is 128. You can configure the priority to values between 0 and 224, in increments of 32.
Interoperability with IEEE 802.1D
A device that runs MST supports a built-in protocol migration feature that enables it to interoperate with 802.1D STP switches. If this device receives an 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. In addition, an MST device can detect that a port is at the boundary of a region when it receives an 802.1D BPDU, an MST BPDU (Version 3) associated with a different region, or an 802.1w BPDU (Version 2).
However, the device does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs because it cannot detect whether the 802.1D device has been removed from the link unless the 802.1D device is the designated device. A device might also continue to assign a boundary role to a port when the device to which this device is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches), enter the clear spanning-tree detected-protocols command.
All Rapid PVST+ switches (and all 8021.D STP switches) on the link can process MST BPDUs as if they are 802.1w BPDUs. MST switches can send either Version 0 configuration and topology change notification (TCN) BPDUs or Version 3 MST BPDUs on a boundary port. A boundary port connects to a LAN, the designated device of which is either a single spanning tree device or a device with a different MST configuration.
MST interoperates with the Cisco prestandard MSTP whenever it receives prestandard MSTP on an MST port; no explicit configuration is necessary. In Cisco NX-OS Release 4.0(2) and later releases, you can configure specified interfaces to send prestandard MSTP messages all the time; it does not have to wait to receive a prestandard MST message to begin sending prestandard MST messages.
High Availability
The software supports high availability for MST. However, the statistics and timers are not restored when MST restarts. The timers start again and the statistics begin from 0.
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Virtualization Support
The system provides support for virtual device contexts (VDCs), and each VDC runs a separate STP (see Figure 6-5).
Figure 6-5 Separate STP in each VDC
You can run Rapid PVST+ in one VDC and run MST in another VDC. Each VDC will have its own MST, Ensure that you are in the correct VDC.
For example, VDC1 can run MST, VDC2 can run Rapid PVST+, and VDC 3 can run MST.
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Licensing Requirements for MST
The following table shows the licensing requirements for this feature:
However, using VDCs requires an Advanced Services license.
Prerequisites for MST
MST has the following prerequisites:
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Guidelines and Limitations
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Follow these guidelines when using MST:
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See the "Mapping Secondary VLANs to Same MSTI as Primary VLANs for Private VLANs" section for information on synchronizing the secondary and primary VLANs into the same MSTI.
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Configuring MST
This section includes the following topics:
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Enabling MST
You can enable MST; Rapid PVST+ is the default.
You cannot simultaneously run MST and Rapid PVST+ on the same VDC.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to enable MST on the device:
switch# config tswitch(config)# spanning-tree mode mstswitch(config)# exitswitch#
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Entering MST Configuration Mode
You enter MST configuration mode to configure the MST name, VLAN-to-instance mapping, and MST revision number on the device.
If two or more devices are in the same MST region, they must have the identical MST name, VLAN-to-instance mapping, and MST revision number.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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or
abort4.
DETAILED STEPS
This example shows how to enter MST configuration submode on the device:
switch# config tswitch(config)# spanning-tree mst configurationswitch(config-mst)#This example shows how to commit the changes and leave MST configuration submode on the device:
sswitch(config-mst)# exitswitch(config)#This example shows how to leave MST-submode configuration on the device without committing the changes:
sswitch(config-mst)# abortswitch(config)#Specifying the MST Name
You can configure a region name on the bridge. If two or more bridges are in the same MST region, they must have the identical MST name, VLAN-to-instance mapping, and MST revision number.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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abort5.
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DETAILED STEPS
This example shows how to set the name of the MST region:
switch# config tswitch(config)# spanning-tree mst configurationswitch(config-mst)# name accountingswitch(config-mst)#Specifying the MST Configuration Revision Number
You configure the revision number on the bridge. If two or more bridges are in the same MST region, they must have the identical MST name, VLAN-to-instance mapping, and MST revision number.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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abort5.
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DETAILED STEPS
This example shows how to configure the revision number of the MSTI region for 5:
switch# config tswitch(config)# spanning-tree mst configurationswitch(config-mst)# revision 5switch(config-mst)#Specifying the Configuration on an MST Region
If two or more devices are to be in the same MST region, they must have the same VLAN-to-instance mapping, the same configuration revision number, and the same MST name.
A region can have one member or multiple members with the same MST configuration; each member must be capable of processing IEEE 802.1w RSTP BPDUs. There is no limit to the number of MST regions in a network, but each region can support only up to 65 MST instances. You can assign a VLAN to only one MST instance at a time.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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abort7.
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To return to defaults, do the following:
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This example shows how to enter MST configuration mode, map VLANs 10 to 20 to MST instance 1, name the region region1, set the configuration revision to 1, display the pending configuration, apply the changes, and return to global configuration mode:
switch(config)# spanning-tree mst configurationswitch(config-mst)# instance 1 vlan 10-20switch(config-mst)# name region1switch(config-mst)# revision 1switch(config-mst)# show pendingPending MST configurationName [region1]Revision 1Instances configured 2Instance Vlans Mapped-------- ---------------------0 1-9,21-40941 10-20-------------------------------switch(config-mst) exitswitch(config)#Mapping or Unmapping a VLAN to an MST Instance
If two or more bridges are to be in thesame MST region, they must have the identical MST name, VLAN-to-instance mapping, and MST revision number.
You cannot map VLANs 3968 to 4047 or 4094 to any MST instance. These VLANs are reserved for internal use by the device.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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abort5.
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DETAILED STEPS
This example shows how to map VLAN 200 to MSTI 3:
switch# config tswitch(config)# spanning-tree mst configurationswitch(config-mst)# instance 3 vlan 200switch(config-mst)#Mapping Secondary VLANs to Same MSTI as Primary VLANs for Private VLANs
When you are working with private VLANs on the system, all secondary VLANs must be in the same MSTI and their associated primary VLAN. Enter the command to accomplish this synchronization automatically.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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abort5.
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DETAILED STEPS
This example shows how to automatically map all the secondary VLANs to the same MSTI as their associated primary VLANs in all private VLANs:
switch# config tswitch(config)# spanning-tree mst configurationswitch(config-mst)# private-vlan synchronizeswitch(config-mst)#Configuring the Root Bridge
You can configure the device to become the root bridge.
The spanning-tree vlan vlan_ID primary root command fails if the value required to be the root bridge is less than 4096. If the software cannot lower the bridge priority any lower, the device returns the following message:
Error: Failed to set root bridge for VLAN 1
It may be possible to make the bridge root by setting the priority
for some (or all) of these instances to zero.
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Enter the diameter keyword, which is available only for MSTI 0 (or the IST), to specify the Layer 2 network diameter (that is, the maximum number of Layer 2 hops between any two end stations in the Layer 2 network). When you specify the network diameter, the device automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can enter the hello keyword to override the automatically calculated hello time.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the device as the root switch for MSTI 5:
switch# config tswitch(config)# spanning-tree mst 5 root primaryswitch(config)#Configuring a Secondary Root Bridge
You use this command on more than one device to configure multiple backup root bridges. Enter the same network diameter and hello-time values that you used when you configured the primary root bridge with the spanning-tree mst root primary global configuration command.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the device as the secondary root switch for MSTI 5:
switch# config tswitch(config)# spanning-tree mst 5 root secondaryswitch(config)#Configuring the Switch Priority
You can configure the switch priority for an MST instance so that it is more likely that the specified device is chosen as the root bridge.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the priority of the bridge to 4096 for MSTI 5:
switch# config tswitch(config)# spanning-tree mst 5 priority 4096switch(config)#Configuring the Port Priority
If a loop occurs, MST uses the port priority when selecting an interface to put into the forwarding state. You can assign lower priority values to interfaces that you want selected first and higher priority values to the interface that you want selected last. If all interfaces have the same priority value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to set the MST interface port priority for MSTI 3 on Ethernet port 3/1 to 64:
switch# config tswitch(config)# interface ethernet 3/1switch(config-if)# spanning-tree mst 3 port-priority 64switch(config-if)# exitswitch(config)Configuring the Port Cost
The MST port cost default value is derived from the media speed of an interface. If a loop occurs, MST uses the cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces that you want selected first and higher cost to interfaces values that you want selected last. If all interfaces have the same cost value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to set the MST interface port cost on Ethernet 3/1 for MSTI 4:
switch# config tswitch(config)# interface ethernet 3/1switch(config-if)# spanning-tree mst 4 cost 17031970switch(config-if)# exitswitch(config)Configuring the Hello Time
You can configure the interval between the generation of configuration messages by the root bridge for all instances on the device by changing the hello time.
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BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the hello time of the device to 1 second:
switch# config tswitch(config)# spanning-tree mst hello-time 1switch(config)#Configuring the Forwarding-Delay Time
You can set the forward delay timer for all MST instances on the device with one command.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the forward-delay time of the device to 10 seconds:
switch# config tswitch(config)# spanning-tree mst forward-time 10switch(config)#Configuring the Maximum-Aging Time
You can set the maximum-aging timer for all MST instances on the device with one command (the maximum age time only applies to the IST).
The maximum-aging timer is the number of seconds that a device waits without receiving spanning tree configuration messages before attempting a reconfiguration.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the maximum-aging timer of the device to 40 seconds:
switch# config tswitch(config)# spanning-tree mst max-age 40switch(config)#Configuring the Maximum-Hop Count
You can configure the maximum hops inside the region and apply it to the IST and all MST instances in that region. MST uses the path cost to the IST regional root and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism. The hop count achieves the same result as the message-age information (triggers a reconfiguration).
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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2.
3.
4.
5.
DETAILED STEPS
This example shows how to set the maximum hops to 40:
switch# config tswitch(config)# spanning-tree mst max-hops 40switch(config)#Configuring an Interface to Proactively Send Prestandard MSTP Messages
By default, interfaces on a device running MST send prestandard, rather than standard, MSTP messages after they receive a prestandard MSTP message from another interface. In Cisco Release 4.0(2) and later releases, you can configure the interface to proactively send prestandard MSTP messages. That is, the specified interface would not have to wait to receive a prestandard MSTP message; the interface with this configuration always sends prestandard MSTP messages.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to set the MST interface so that it always sends MSTP messages in the prestandard format:
switch# config tswitch(config)# interface ethernet 3/1switch(config-if)# spanning-tree mst pre-standardswitch(config-if)# exitswitch(config)Specifying the Link Type
Rapid connectivity (802.1w standard) is established only on point-to-point links. By default, the link type is controlled from the duplex mode of the interface. A full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection.
If you have a half-duplex link physically connected point to point to a single port on a remote device, you can override the default setting on the link type and enable rapid transitions.
If you set the link to shared, STP falls back to 802.1D.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
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DETAILED STEPS
This example shows how to configure the link type as point to point:
switch# config tswitch (config)# interface ethernet 1/4switch(config-if)# spanning-tree link-type point-to-pointswitch(config-if)# exitswitch(config)#Reinitializing the Protocol
An MST bridge can detect that a port is at the boundary of a region when it receives a legacy BPDU or an MST BPDU that is associated with a different region. However, the STP protocol migration cannot determine whether the legacy device, which is a device that runs only IEEE 802.1D, has been removed from the link unless the legacy device is the designated switch. Enter this command to reinitialize the protocol negotiation (force the renegotiation with neighboring devices) on the entire device or on specified interfaces.
BEFORE YOU BEGIN
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
1.
DETAILED STEPS
This example shows how to reinitialize MST on the Ethernet interface on slot 2, port 8:
switch# clear spanning-tree detected-protocol interface ethernet 2/8
Verifying MST Configurations
To display MST configuration information, perform one of the following tasks:
For detailed information about the fields in the output from these commands, see the Cisco NX-OS Layer 2 Switching Command Reference.
Displaying and Clearing MST Statistics
To display MST configuration information, perform one of the following tasks:
MST Example Configuration
The following example shows how to configure MST:
switch# configure terminalswitch(config)# spanning-tree mode mstswitch(config)# spanning-tree port type edge bpduguard defaultswitch(config)# spanning-tree port type edge bpdufilter defaultswitch(config)# spanning-tree port type network defaultswitch(config)# spanning-tree mst 0-64 priority 24576switch(config)# spanning-tree mst configurationswitch(config-mst)# name cisco_region_1switch(config-mst)# revision 2switch(config-mst)# instance 1 vlan 1-21switch(config-mst)# instance 2 vlan 22-42switch(config-mst)# instance 3 vlan 43-63switch(config-mst)# instance 4 vlan 64-84switch(config-mst)# instance 5 vlan 85-105switch(config-mst)# instance 6 vlan 106-126switch(config-mst)# instance 7 vlan 127-147switch(config-mst)# instance 8 vlan 148-168switch(config-mst)# instance 9 vlan 169-189switch(config-mst)# instance 10 vlan 190-210switch(config-mst)# instance 11 vlan 211-231switch(config-mst)# instance 12 vlan 232-252switch(config-mst)# instance 13 vlan 253-273switch(config-mst)# instance 14 vlan 274-294switch(config-mst)# instance 15 vlan 295-315switch(config-mst)# instance 16 vlan 316-336switch(config-mst)# instance 17 vlan 337-357switch(config-mst)# instance 18 vlan 358-378switch(config-mst)# instance 19 vlan 379-399switch(config-mst)# instance 20 vlan 400-420switch(config-mst)# instance 21 vlan 421-441switch(config-mst)# instance 22 vlan 442-462switch(config-mst)# instance 23 vlan 463-483switch(config-mst)# instance 24 vlan 484-504switch(config-mst)# instance 25 vlan 505-525switch(config-mst)# instance 26 vlan 526-546switch(config-mst)# instance 27 vlan 547-567switch(config-mst)# instance 28 vlan 568-588switch(config-mst)# instance 29 vlan 589-609switch(config-mst)# instance 30 vlan 610-630switch(config-mst)# instance 31 vlan 631-651switch(config-mst)# instance 32 vlan 652-672switch(config-mst)# instance 33 vlan 673-693switch(config-mst)# instance 34 vlan 694-714switch(config-mst)# instance 35 vlan 715-735switch(config-mst)# instance 36 vlan 736-756switch(config-mst)# instance 37 vlan 757-777switch(config-mst)# instance 38 vlan 778-798switch(config-mst)# instance 39 vlan 799-819switch(config-mst)# instance 40 vlan 820-840switch(config-mst)# instance 41 vlan 841-861switch(config-mst)# instance 42 vlan 862-882switch(config-mst)# instance 43 vlan 883-903switch(config-mst)# instance 44 vlan 904-924switch(config-mst)# instance 45 vlan 925-945switch(config-mst)# instance 46 vlan 946-966switch(config-mst)# instance 47 vlan 967-987switch(config-mst)# instance 48 vlan 988-1008switch(config-mst)# instance 49 vlan 1009-1029switch(config-mst)# instance 50 vlan 1030-1050switch(config-mst)# instance 51 vlan 1051-1071switch(config-mst)# instance 52 vlan 1072-1092switch(config-mst)# instance 53 vlan 1093-1113switch(config-mst)# instance 54 vlan 1114-1134switch(config-mst)# instance 55 vlan 1135-1155switch(config-mst)# instance 56 vlan 1156-1176switch(config-mst)# instance 57 vlan 1177-1197switch(config-mst)# instance 58 vlan 1198-1218switch(config-mst)# instance 59 vlan 1219-1239switch(config-mst)# instance 60 vlan 1240-1260switch(config-mst)# instance 61 vlan 1261-1281switch(config-mst)# instance 62 vlan 1282-1302switch(config-mst)# instance 63 vlan 1303-1323switch(config-mst)# instance 64 vlan 1324-1344switch(config-mst)# exitswitch(config)# interface Ethernet3/1switch(config-if)# switchportswitch(config-if)# no shutdownswitch(config-if)# spanning-tree port type edgeswitch(config-if)# exitswitch(config)# interface Ethernet3/2switch(config-if)# switchportswitch(config-if)# switchport mode trunkswitch(config-if)# no shutdownswitch(config-if)# spanning-tree guard rootswitch(config-if)# exitswitch(config)#See Chapter 7, "Configuring STP Extensions" for information on STP port types, and BPDU Guard, BPDU Filter, and Root Guard settings.
Default Settings
Table 6-1 lists the default settings for MST parameters.
Additional References
For additional information related to implementing MST, see the following sections:
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Related Documents
Standards
IEEE 802.1Q-2006 (formerly known as IEEE 802.1s), IEEE 802.1D-2004 (formerly known as IEEE 802.1w), IEEE 802.1D, IEEE 802.1t
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MIBs
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To locate and download MIBs, go to the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
