Layer 2 Switching Configuration Guide, Cisco DCNM for LAN, Release 6.x
Configuring MST
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Configuring MST

Contents

Configuring MST

This chapter describes how to configure Multiple Spanning Tree (MST) on Cisco NX-OS devices using Cisco Data Center Network Manager (DCNM) for LAN.

For more information about the Cisco DCNM, see the .


Note


See the Interfaces Configuration Guide, Cisco DCNM for LAN, Release 5.x, 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.


Note


Spanning tree is used to refer to IEEE 802.1w and IEEE 802.1s. If the text is discussing the IEEE 802.1D Spanning Tree Protocol, 802.1D is stated specifically.


This chapter includes the following sections:

Information About MST


Note


System-message logging levels for the MST feature must meet or exceed Cisco DCNM requirements. During device discovery, Cisco DCNM detects inadequate logging levels and raises them to the minimum requirements. Cisco Nexus 7000 Series devices that run Cisco NX-OS Release 4.0 are an exception. For Cisco NX-OS Release 4.0, prior to device discovery use the command-line interface to configure logging levels to meet or exceed Cisco DCNM requirements. For more information, see the



Note


See the Interfaces Configuration Guide, Cisco DCNM for LAN, Release 5.x, for information on creating Layer 2 interfaces.


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.


Note


Spanning tree is used to refer to IEEE 802.1w and IEEE 802.1s. If the IEEE 802.1D Spanning Tree Protocol is discussed in this publication, 802.1D is stated specifically.


The Cisco NX-OS release that is running on a managed device might not support all the features or settings described in this chapter. For the latest feature information and caveats, see the documentation and release notes for your platform and software release.

MST Overview


Note


You must enable MST; Rapid PVST+ is the default spanning tree mode.


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 because 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.

MAC address reduction is always enabled on the device. You cannot disable this feature.

MST improves spanning tree operation and maintains backward compatibility with these STP versions:

  • Original 802.1D spanning tree
  • Rapid per-VLAN spanning tree (Rapid PVST+)

Note


  • IEEE 802.1 was defined in the Rapid Spanning Tree Protocol (RSTP) and was incorporated into IEEE 802.1D.
  • IEEE 802.1 was defined in MST and was incorporated into IEEE 802.1Q.

MST Regions

To allow devices to participate in MST instances, you must consistently configure the devices with the same MST configuration information.

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 single 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.

The MST region appears as a single bridge to adjacent MST regions and to other Rapid PVST+ regions and 802.1D spanning tree protocols.


Note


We do not recommend that you partition the network into a large number of regions.


MST BPDUs

Each device has only one MST BPDU per interface, and that BPDU carries an M-record for each MSTI on the device. Only the IST sends BPDUs for the MST region; all M-records are encapsulated in that one BPDU that the IST sends. 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 1. MST BPDU with M-Records for MSTIs. Only the IST sends BPDUs for the MST region; all M-records are encapsulated in that one BPDU that the IST sends.

Related Concepts

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 three parameters of the MST configuration as follows:

  • Name—32-character string, null padded and null terminated, identifying the MST region
  • Revision number—Unsigned 16-bit number that identifies the revision of the current MST configuration

Note


You must set the revision number when required as part of the MST configuration. The revision number is not incremented automatically each time that the MST configuration is committed.


  • VLAN-to-MST instance mapping—4096-element table that associates each of the potential 4094 VLANs supported in each virtual device context (VDC) to a given instance with the first (0) and last element (4095) set to 0. The value of element number X represents the instance to which VLAN X is mapped.

Note


When you change the VLAN-to-MSTI mapping, the system reconverges MST.


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

IST, CIST, and CST Overview

Unlike Rapid PVST+, in which all the STP instances are independent, MST establishes and maintains IST, CIST, and CST spanning trees, as follows:

  • An IST is the spanning tree that runs in an MST region. 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.
  • The CST interconnects the MST regions and any instance of 802.1D and 802.1w STP that may be running on the network. The CST is the one STP instance for the entire bridged network and encompasses all MST regions and 802.1w and 802.1D instances.
  • A CIST is a collection of the ISTs in each MST region. The CIST is the same as an IST inside an MST region, and the same as a CST outside an MST region.

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 devices 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.

Spanning Tree Operation Within an MST Region

The IST connects all the MSTdevices in a region. When the IST converges, the root of the IST becomes the CIST regional root. 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 devices 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 devices in the MST region must agree on the same CIST regional root. Any two devices 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 devices 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 devices 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 devices and MST regions.

Figure 2. MST Regions, CIST Regional Roots, and CST Root. This figure 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.



Only the CST instance sends and receives BPDUs. MSTIs add their spanning tree information into the BPDUs (as M-records) to interact with neighboring devices within the same MST region and compute the final spanning tree topology. 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:

  • The CIST root is the root bridge for the CIST, which is the unique instance that spans the whole network.
  • The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region. An MST region looks like a single device to the CIST. The CIST external root path cost is the root path cost calculated between these virtual devices and devices that do not belong to any region.
  • If the CIST root is in the region, the CIST regional root is the CIST root. Otherwise, the CIST regional root is the closest device to the CIST root in the region. The CIST regional root acts as a root bridge for the IST.
  • The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant to the IST, instance 0.

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.

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.

Figure 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.

Detecting Unidirectional Link Failure:MST

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. This feature is based on the dispute mechanism.


Note


See the Interfaces Configuration Guide, Cisco DCNM for LAN, Release 5.x, for information on Unidirectional Link Detection (UDLD).


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 4. Detecting a Unidirectional Link Failure.

This figure 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 switch B. Rapid PVST+ (802.1w) and MST BPDUs include the role and state of the sending port. With this information, switch A can detect that switch B does not react to the superior BPDUs that it sends and that switch B is the designated, not root port. As a result, switch A blocks (or keeps blocking) its port, which prevents the bridging loop.



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:

  • 10 Mbps—2,000,000
  • 100 Mbps—200,000
  • 1 Gigabit Ethernet—20,000
  • 10 Gigabit Ethernet—2,000

You can configure the port costs in order to influence which port is chosen.


Note


MST always uses the long path-cost calculation method, so the range of valid values is between 1 and 200,000,000.


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 devices. If this devoce 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 devices), 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 devices 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.

You can also configure the interface to proactively send prestandard MSTP messages.

High Availability for MST

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.

The device supports full nondisruptive upgrades for MST. See the Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, for complete information on nondisruptive upgrades and high-availability features.

Virtualization Support for MST

The system provides support for virtual device contexts (VDCs), and each VDC runs a separate STP.

Figure 5. Separate STP in each VDC.

You can run Rapid PVST+ in one VDC and run MST in another VDC as shown in this figure. Each VDC will have its own MST. Ensure that you are in the correct VDC.




Note




Note


See the Virtual Device Context Configuration Guide, Cisco DCNM for LAN, Release 5.x, for complete information on VDCs and assigning resources.


Licensing Requirements for MST

The following table shows the licensing requirements for this feature:

Product

License Requirement

Cisco DCNM

MST requires no license. Any feature not included in a license package is bundled with the Cisco DCNM and is provided at no charge to you.

Cisco NX-OS

MST requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.

However, using VDCs requires an Advanced Services license.

Prerequisites for MST

MST has the following prerequisites:

  • You must be logged onto the device.
  • Before using Cisco DCNM to configure any Spanning Tree Protocol parameters, you must set the logging level by entering the NX-OS global command logging-level spanning-tree 6 in the command line of your device. See Cisco Nexus 7000 Series NX-OS System Management Configuration Guide, for information on logging levels

Guidelines and Limitations for Configuring MST


Note


When you change the VLAN-to-MSTI mapping, the system reconverges MST.


MST has the following configuration guidelines and limitations:

  • You must enable MST; Rapid PVST+ is the default spanning tree mode.
  • You can assign a VLAN to only one MST instance at a time.
  • You cannot map VLANs 3968 to 4047 or 4094 to an MST instance. These VLANs are reserved for internal use by the device.
  • You can have up to 65 MST instances on one device.
  • The maximum number of VLANs and ports is 75,000.
  • By default, all VLANs are mapped to MSTI 0 or the IST.
  • You can load balance only within the MST region.
  • Ensure that trunks carry all of the VLANs that are mapped to an MSTI or exclude all those VLANs that are mapped to an MSTI.
  • Always leave STP enabled.
  • Do not change timers because you can adversely affect your network stability.
  • Keep user traffic off the management VLAN; keep the management VLAN separate from user data.
  • Choose the distribution and core layers as the location of the primary and secondary root switches.
  • Port channeling—The port channel bundle is considered as a single port. The port cost is the aggregation of all the configured port costs assigned to that channel.
  • When you map a VLAN to an MSTI, the system automatically removes that VLAN from its previous MSTI.
  • You can map any number of VLANs to an MSTI.
  • All MST boundary ports must be forwarding for load balancing between Rapid PVST+ and an MST cloud or between a PVST+ and an MST cloud. The CIST regional root of the MST cloud must be the root of the CST. If the MST cloud consists of multiple MST regions, one of the MST regions must contain the CST root and all of the other MST regions must have a better path to the root contained within the MST cloud than a path through the Rapid PVST+ or PVST+ cloud.
  • Do not partition the network into a large number of regions. However, if this situation is unavoidable, we recommend that you partition the switched LAN into smaller LANs interconnected by non-Layer 2 devices.

Note


The software supports full nondisruptive upgrades for MST. See Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, for complete information about nondisruptive upgrades.


Platform Support for MST

The following platforms support this feature but may implement it differently. For platform-specific information, including guidelines and limitations, system defaults, and configuration limits, see the corresponding documentation.

Platform

Documentation

Cisco Nexus 7000 Series switches

Cisco Nexus 7000 Series switch documentation

Cisco Nexus 4000 Series switches

Cisco Nexus 4000 Series switch documentation

Cisco Nexus 5000 Series switches

Cisco Nexus 5000 Series switch documentation

Cisco Nexus 3000 Series switches

Cisco Nexus 3000 Series switch documentation

Cisco Catalyst 6500 Series switches

Cisco Catalyst 6500 Series switch documentation

Configuring MST


Note


You can pre-provision MST while the device is running Rapid PVST+. From the main menu, choose Tools > Global Preferences > Pre Provisioning to access or hide the screen that allows and displays this functionality. See the , for information on pre-provisioning.



Note


See the , for information on using the Topology feature with MSTP.


Enabling MST

You can enable MST; Rapid PVST+ is the default.

Figure 6. Enabling MST. You use the Spanning Tree pane in this figure to enable MST.


Note


When you change the spanning tree mode, traffic is disrupted because all spanning tree instances are stopped for the previous mode and started for the new mode.


Procedure
    Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
    Step 2   In the Summary pane, click the device on which you want to enable MST. Rapid PVST+ is enabled on all VLANs by default.
    Step 3   In the Protocol field, click the drop-down list and choose MST.
    Step 4   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

    Specifying the MST Name and Revision

    You can configure the name and revision number on the bridge. If two or more bridges are to be in the same MST region, they must have the identical MST name, VLAN-to-instance mapping, and MST revision number.

    You use the Spanning Tree pane to configure the MST name and revision number (see Figure 1).

    Procedure
      Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
      Step 2   In the Summary pane, click the device on which you want to specify the MST name and revision number. The system highlights the device in the Summary pane, and tabs appear in the Details pane.
      Step 3   In the Details pane, click the Configuration tab.
      Step 4   Click the Global Setting section.
      Step 5   In the MST Setting area, in the Name field, enter the MST name for the bridge. The default is blank.
      Step 6   In the MST Setting area, in the Revision Number field, enter the MST revision number. The default is 0.
      Step 7   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

      Setting MST to Default Values

      You use the Spanning Tree pane to set MST to the default values (see Figure 1).

      Procedure
        Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
        Step 2   Choose the device that you want to set to default values for MST.
        Step 3   From the menu bar, choose Actions > Set to default. This action removes the name and revision number and returns all VLANs to MST Instance 0.
        Step 4   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

        Adding MST Instances and Mapping VLANs to the Instance

        Figure 7. Configuring MST.

        You use the MST pane shown in this figure to add an MST and map VLANs to that instance.



        Procedure
          Step 1   From the Feature Selector pane, choose Switching > Spanning Tree > MST to open the MST pane.
          Step 2   In the Summary pane, click the Device View tab.
          Step 3   In the Summary pane, click the device to which you want to add or remove an MST instance. The Summary pane expands to display MST instances currently configured on the device.
          Step 4   From the menu bar, choose Actions > Add MST Instance. The system adds an instance with the MST ID and VLAN fields blank.
          Step 5   In the MST ID field, enter the number for the new MST.
          Step 6   In the VLANs field, enter the VLANs that you are mapping to the new MST.
          Note   

          You must add at least one VLAN to a newly created MST instance.

          Step 7   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

          Removing MST Instances

          You can remove an MST instance.

          Figure 8. Removing MST Instances. You can remove an MST instance using the MST pane in this figure.

          Procedure
            Step 1   From the Feature Selector pane, choose Switching > Spanning Tree > MST to open the MST pane.
            Step 2   In the Summary pane, click the Network View tab.
            Step 3   In the Summary pane, click the device from which you want to remove an MST instance. The Summary pane expands to display MST instances currently configured on the device.
            Step 4   From the menu bar, choose Actions > Delete MST Instance.

            The system asks for confirmation that you want to delete this MST instance.

            Step 5   Click Yes.
            Step 6   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

            Mapping or Unmapping a VLAN to an MST Instance

            If two or more bridges are to be in the same 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.


            Note


            When you change the VLAN-to-MSTI mapping, the system reconverges MST.



            Note


            You cannot disable an MSTI.


            You use the MST pane to create the VLAN-to-MST instance mapping.

            Procedure
              Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the MST pane.
              Step 2   In the Summary pane, click the Device View tab.
              Step 3   In the Summary pane, click the device to which you want to add or remove an MST instance. The Summary pane expands to display MST instances currently configured on the device.
              Step 4   Click the MST to which you want to add or remove a VLAN.
              Step 5   From the menu bar, choose Actions > Add VLAN to MST Instance.
              Step 6   Add the VLANs that you want in the new MST.
              Step 7   (Optional)From the menu bar, choose Actions > Remove VLAN to remove and to unmap VLANs from the MST instance,
              Step 8   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

              Configuring the Primary and Secondary Roots and the Switch Priority for MST

              You can configure the primary and secondary roots, or 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. You can also choose the diameter and hello time for this configuration


              Note


              We recommend that you designate the primary and secondary roots, rather than choosing a specific value for the switch priority.


              To configure multiple backup root bridges, you should designate more than one device as the secondary root. For the secondary root, use the same network diameter and hello-time values that you used when you configured the primary root bridge.

              You use the MST pane to configure the device as the primary or secondary root and the switch priority (see Figure 1).

              Procedure
                Step 1   From the Feature Selector pane, choose Switching > MST to open the MST pane.
                Step 2   In the Summary pane, click the device that you are working with. The Summary pane expands to display the MST instances.
                Step 3   In the Summary pane, click the MST instance for which you want to designate the primary and secondary roots or the switch priority. Tabs appear in the Details pane.
                Step 4   In the Details pane, click the Details tab.
                Step 5   In the Details tab, click the MST Setting section.
                Step 6   (Optional)In the Switch Priority area, in the Root field, click the drop-down list and click primary to set the device as the primary root.
                Step 7   (Optional)In the Switch Priority area, in the Root field, click the drop-down list and click secondary to set the device as a secondary root.
                Step 8   (Optional)In the Switch Priority area, in the Switch Priority field, click the drop-down list and click the value that you want for the switch priority to set the switch priority to a specific value.
                Step 9   In the Diameter field, click the drop-down list and click the value that you want for the diameter. The default value for the diameter is 7.
                Step 10   In the Hello Time field, click the drop-down list and click the value that you want for the number of seconds between hello time messages. The default value for the hello time is 2 seconds.
                Step 11   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                Configuring the Port Priority and Cost for MST Ports

                If a loop occurs, MST uses the port priority and cost when selecting an interface to put into the forwarding state. The MST path-cost default value is derived from the media speed of an interface. You can assign lower priority values and cost values to interfaces that you want selected first and higher values to interfaces that you want selected last. If all interfaces have the same priority and cost value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

                You use the MST pane to configure the port priority and port cost values separately for MST access and trunk ports (see Figure 1).

                Procedure
                  Step 1   From the Feature Selector pane, choose Switching > MST to open the MST pane.
                  Step 2   In the Summary pane, click the device that you are working with. The Summary pane expands to display the MST instances.
                  Step 3   In the Summary pane, click the MST Instance for which you want to configure the port priority and cost. Tabs appear in the Details pane.
                  Step 4   In the Details pane, click the Details tab.
                  Step 5   In the Details tab, click the Access Ports section to configure the access ports or click the Trunk Ports section to configure trunk ports. The section expands to display the ports in that MST instance.
                  Step 6   Click the port for which you want to set the priority and the cost.
                  Step 7   In the Priority field, click the drop-down list and choose the value that you want for the MST priority for that port. The default value is 128.
                  Step 8   In the Cost field, click auto to disable the cost for that port. The default value is 128.
                  Step 9   Enter a value to set the cost for the port. The default port cost is determined by the port’s bandwidth.
                  Step 10   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                  Configuring the Default MST Values per Ports

                  You set the default values separately for MST access and trunk ports (see Figure 1).

                  Procedure
                    Step 1   From the Feature Selector pane, choose Switching > MST to open the MST pane.
                    Step 2   In the Summary pane, click the device that you are working with. The Summary pane expands to display the MST instances.
                    Step 3   In the Summary pane, click the MST instance for which you want to configure the port priority and cost. Tabs appear in the Details pane.
                    Step 4   In the Details pane, click the Details tab.
                    Step 5   In the Details tab, click the Access Ports section to configure the access ports or click the Trunk Ports section to configure trunk ports. The section expands to display the ports in that MST instance.
                    Step 6   Click the port for which you want to set the MST default values.
                    Step 7   From the menu bar, choose Actions > Set to default for Access Port to set the default for an access port or choose Set to default for Trunk Port to set the default for a trunk port.
                    Step 8   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                    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 NX-OS 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.

                    You use the Spanning Tree pane to the interface to proactively send prestandard MST messages (see Figure 1).

                    Procedure
                      Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
                      Step 2   In the Summary pane, click the device on which you want to configure the interface to proactively send prestandard MST messages. The system highlights the device in the Summary pane, and tabs appear in the Details pane.
                      Step 3   In the Details pane, click the Configuration tab.
                      Step 4   Click the Port Setting section.
                      Step 5   Click the port that you want to configure.
                      Step 6   In the MST PreStd field, check the box. The default is blank.
                      Step 7   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                      Configuring the Hello Time, Forwarding Delay Time, Maximum Aging Time, and the Maximum Hop Count

                      The interval between the generation of configuration messages by the root bridge for all instances on the device is the hello time. The maximum-aging timer is the number of seconds that a device waits without receiving spanning tree configuration messages before attempting a reconfiguration. You can configure the maximum hops inside the region and apply it to all MST instances in that region. The hop count achieves the same result as the message-age information (triggers a reconfiguration).

                      You use the Spanning Tree pane to configure the hello time, forwarding delay time, maximum aging time, and maximum hop count for MST (see Figure 1).

                      When you configure virtual port channels (vPCs) on the switch, we recommend that you set the STP hello time to 4 seconds on both the primary and secondary vPC peer devices.

                      See the Interfaces Configuration Guide, Cisco DCNM for LAN, Release 5.x, for more information on vPCs.

                      Procedure
                        Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
                        Step 2   In the Summary pane, click the device on which you want to specify the MST name and revision number. The system highlights the device in the Summary pane, and tabs appear in the Details pane.
                        Step 3   In the Details pane, click the Configuration tab.
                        Step 4   Click the Global Setting section.
                        Step 5   In the MST Setting area, in the Hello Time field, click the drop-down list and choose the value that you want for the hello time. The default is 2 seconds.
                        Step 6   In the MST Setting area, in the Forwarding Delay Time field, enter the value that you want for the forwarding delay time. The default is 15 seconds.
                        Step 7   In the MST Setting area, in the Max Age Time field, enter the value that you want for the maximum aging time. The default is 20 seconds.
                        Step 8   In the MST Setting area, in the Max Hop Count field, enter the value that you want for the maximum hop count. The default is 20.
                        Step 9   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                        Specifying the Link Type for MST

                        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.

                        You use the Spanning Tree pane to set the link type (see Figure 1).

                        Procedure
                          Step 1   From the Feature Selector pane, choose Switching > Spanning Tree to open the Spanning Tree pane.
                          Step 2   In the Summary pane, click the device.
                          Step 3   In the Details pane, click the Configuration tab.
                          Step 4   In the Details pane, click the Port Setting section. The Port Setting section expands.
                          Step 5   In the Port Setting section, click the interface that you want to configure.
                          Step 6   In the Link Type column, click the drop-down list and choose the link type. The default Link Type is Auto.
                          Step 7   (Optional) From the menu bar, choose File > Deploy to apply your changes to the device.

                          Displaying MST Statistics

                          The following window appears in the Statistics tab:

                          • Spanning Tree Statistics—Displays information on Rapid PVST+, including BPDUs sent and received.

                          Field Descriptions for MST

                          These field descriptions are used for configuring MST.

                          Device View: Instance: Details: MST Setting Section

                          Table 1 Device View: Instance: Details: MST Setting Section

                          Field

                          Description

                          MST Instance

                          Display only. Number that uniquely identifies an MST instance.

                          Device

                          Display only. Name of the device in which the VLAN is configured.

                          VLANs

                          VLANs in an MST instance. The default is VLAN1.

                          Note   

                          When working in Instance 0, you cannot change this field.

                          Switch Priority

                          Switch Priority

                          Priority of the bridge. The default value is 32768.

                          Root

                          Primary or secondary root for Rapid PVST+. The default is blank.

                          Diameter

                          Maximum allowed hops between any two end stations in the Layer 2 network. The default is blank.

                          Hello Time

                          Period between broadcasting hello messages. The default is blank.

                          Port Status

                          Total Ports

                          Display only. Total number of ports in that MST instance.

                          Blocking Ports

                          Display only. Number of ports in an MST instance that are in the STP blocking state.

                          Forwarding Ports

                          Display only. Number of ports in an MST instance that are in the STP forwarding state.

                          Device View: Instance: Details: Access Ports Section

                          Table 2  Device View: Instance: Details: Access Ports Section

                          Field

                          Description

                          Name

                          Display only. Name of the port.

                          Mode

                          Display only. Port mode.

                          Configuration

                          Priority

                          MST port priority. The default value is 128.

                          Cost

                          • Auto—STP port cost derived from media speed of interface.
                          • Value—Manually configured port cost. Valid values are 1 to 200000000.

                          Status

                          Role

                          Display only. MST port role. Valid values are as follows:

                          • Root port
                          • Designated port
                          • Alternate port
                          • Backup port
                          • Disabled

                          State

                          Display only. MST port state. Valid values are as follows:

                          • Blocking
                          • Learning
                          • Forwarding
                          • Disabled

                          Priority

                          Display only. MST port priority. The default value is 128.

                          Cost

                          Display only. MST port cost. The value is derived from the media speed of the interface; valid values are 1 to 200000000.

                          Link Type

                          Display only. Type of link. The default is auto.

                          Boundary Port

                          Display only. Displays boundary ports between MST regions.

                          PreStd Port

                          Display only. Displays ports between MST regions and prestandard ports.

                          Device View: Instance: Details: Trunk Ports Section

                          Table 3  Device View: Instance: Details: Trunk Ports Section

                          Field

                          Description

                          Name

                          Display only. Name of the port.

                          Configuration

                          Priority

                          MST port priority. The default value is 128.

                          Cost

                          • Auto—STP port cost derived from media speed of interface.
                          • Value—Manually configured port cost. Valid values are 1 to 200000000.

                          Status

                          Role

                          Display only. MST port role. Valid values are as follows:

                          • Root port
                          • Designated port
                          • Alternate port
                          • Backup port
                          • Disabled

                          State

                          Display only. MST port state. Valid values are as follows:

                          • Blocking
                          • Learning
                          • Forwarding
                          • Disabled

                          Priority

                          Display only. MST port priority. The default value is 128.

                          Cost

                          Display only. MST port cost. The value is derived from the media speed of the interface; valid values are 1 to 200000000.

                          Link Type

                          Display only. Type of link. The default is auto.

                          Boundary Port

                          Display only. Displays boundary ports between MST regions.

                          PreStd Port

                          Display only. Displays ports between MST regions and pre-standard ports.

                          Network View: Device: Details: MST Setting Section

                          Table 4 Network View: Device: Details: MST Setting Section

                          Field

                          Description

                          MST Instance

                          Display only. Number that uniquely identifies an MST instance.

                          Device

                          Display only. Name of the device in which the VLAN is configured.

                          VLANs

                          VLANs in an MST instance. The default is VLAN1.

                          Note   

                          When working in Instance 0, you cannot change this field.

                          Switch Priority

                          Switch Priority

                          Priority of the bridge. The default value is 32768.

                          Root

                          Primary or secondary root for Rapid PVST+. The default is blank.

                          Diameter

                          Maximum allowed hops between any two end stations in the Layer 2 network. The default is blank.

                          Hello Time

                          Period between broadcasting hello messages. The default is blank.

                          Port Status

                          Total Ports

                          Display only. Total number of ports in that MST instance.

                          Blocking Ports

                          Display only. Number of ports in an MST instance that are in the STP blocking state.

                          Forwarding Ports

                          Display only. Number of ports in an MST instance that are in the STP forwarding state.

                          Network View: Device: Details: Access Ports Sections

                          Table 5  Network View: Device: Details: Access Ports Section

                          Field

                          Description

                          Name

                          Display only. Name of the port.

                          Mode

                          Display only. Port mode.

                          Configuration

                          Priority

                          MST port priority. The default value is 128.

                          Cost

                          • Auto—STP port cost derived from media speed of interface.
                          • Value—Manually configured port cost. Valid values are 1 to 200000000.

                          Status

                          Role

                          Display only. MST port role. Valid values are as follows:

                          • Root port
                          • Designated port
                          • Alternate port
                          • Backup port
                          • Disabled

                          State

                          Display only. MST port state. Valid values are as follows:

                          • Blocking
                          • Learning
                          • Forwarding
                          • Disabled

                          Priority

                          Display only. MST port priority. The default value is 128.

                          Cost

                          Display only. MST port cost. The value is derived from the media speed of the interface; valid values are 1 to 200000000.

                          Link Type

                          Display only. Type of link. The default is auto.

                          Boundary Port

                          Display only. Displays boundary ports between MST regions.

                          PreStd Port

                          Display only. Displays ports between MST regions and pre-standard ports.

                          Network View: Device: Details: Trunk Ports Section

                          Table 6  Network View: Device: Details: Trunk Ports Section

                          Field

                          Description

                          Name

                          Display only. Name of the port.

                          Configuration

                          Priority

                          MST port priority. The default value is 128.

                          Cost

                          • Auto—STP port cost derived from media speed of interface.
                          • Value—Manually configured port cost. Valid values are 1 to 200000000.

                          Status

                          Role

                          Display only. MST port role. Valid values are as follows:

                          • Root port
                          • Designated port
                          • Alternate port
                          • Backup port
                          • Disabled

                          State

                          Display only. MST port state. Valid values are as follows:

                          • Blocking
                          • Learning
                          • Forwarding
                          • Disabled

                          Priority

                          Display only. MST port priority. The default value is 128.

                          Cost

                          Display only. MST port cost. The value is derived from the media speed of the interface; valid values are 1 to 200000000.

                          Link Type

                          Display only. Type of link. The default is auto.

                          Boundary Port

                          Display only. Displays boundary ports between MST regions.

                          PreStd Port

                          Display only. Displays ports between MST regions and pre-standard ports.

                          Additional References for MST -- DCNM Version

                          Related Documents

                          Related Topic

                          Document Title

                          Layer 2 interfaces

                          Interfaces Configuration Guide, Cisco DCNM for LAN, Release 5.x

                          Cisco DCNM fundamentals

                          VDCs

                          Virtual Device Context Configuration Guide, Cisco DCNM for LAN, Release 5.x

                          Licensing

                          Cisco DCNM Installation and Licensing Guide, Release 5.x

                          Release notes

                          Cisco DCNM Release Notes, Release 5.x

                          Standards

                          Standards

                          Title

                          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

                          MIBs

                          MIBs

                          MIBs Link

                          • CISCO-STP-EXTENSION-MIB
                          • BRIDGE-MIB

                          To locate and download MIBs, go to the following URL: http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

                          Feature History for Configuring MST

                          This table lists the release history for this feature.

                          Table 7 Feature History for Configuring MSTs

                          Feature Name

                          Releases

                          Feature Information

                          Proactively send prestandard MST messages on selected interfaces.

                          4.1(2)

                          Rather than wait for an interface to receive a prestandard MST message before it replies with a prestandard MST message, you can configure the interface to send prestandard MST messages always.