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
Specifying the MST Name and Revision
Adding MST Instances and Mapping VLANs to the Instance
Mapping or Unmapping a VLAN to an MST Instance
Configuring the Primary and Secondary Roots and the Switch Priority
Configuring the Port Priority and Cost for MST Ports
Configuring the Default MST Values per Ports
Configuring the Hello Time, Forwarding Delay Time, Maximum Aging Time, and the Maximum Hop Count
Device View: Instance: Details: MST Setting Section
Device View: Instance: Details: Access Ports Section
Device View: Instance: Details: Trunk Ports Section
Network View: Device: Details: MST Setting Section
Network View: Device: Details: Access Ports Sections
Network View: Device: Details: Trunk Ports Section
Configuring MST
This chapter describes how to configure Multiple Spanning Tree (MST) on NX-OS devices.
For more information about the Data Center Network Manager features, see the Cisco DCNM Fundamentals Configuration Guide.
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Before using DCNM to configure any Spanning Tree Protocol parameters, you must set the logging level by entering the NX-OS global commands in the command line of your device:
--logging-level spanning-tree 6
--logging logfile messages 6
--logging event link-status default
See the Cisco NX-OS System Management Configuration Guide for information on logging levels.
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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 4, "Configuring Rapid PVST+" for complete information on the explicit handshake agreement.)
MAC address reduction is always enabled on the device (see Chapter 4, "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 5-1). 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 5-1 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 5-2. 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 5-2 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 5-2 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.
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 5-3).
Figure 5-3 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 5-4 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 5-4 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.
See the Cisco NX-OS Layer 2 Configuration Guide for complete information on configuring interfaces to proactively send prestandard MSTP 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 5-5).
Figure 5-5 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.
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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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Configuring MST
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This section includes the following topics:
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Enabling MST
You can enable MST; Rapid PVST+ is the default.
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You use the Spanning Tree pane to enable MST (see Figure 5-6).
Figure 5-6 Enabling MST
DETAILED STEPS
To enable MST on the device, follow these steps:
Step 1
Step 2
Rapid PVST+ is enabled on all VLANs by default.
Step 3
Step 4
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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 5-6).
DETAILED STEPS
To configure the MST name and revision number, follow these steps:
Step 1
Step 2
The system highlights the device in the Summary pane, and tabs appear in the Details pane
Step 3
Step 4
Step 5
The default is blank.
Step 6
The default is 0.
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Setting MST to Default Values
You use the Spanning Tree pane to set MST to the default values (see Figure 5-6).
DETAILED STEPS
To set MST to the default values, follow these steps:
Step 1
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This action removes the name and revision number and returns all VLANs to MST Instance 0.
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Adding MST Instances and Mapping VLANs to the Instance
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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You use the MST pane to add an MST and map VLANs to that instance (see Figure 5-7).
Figure 5-7 Configuring MST
DETAILED STEPS
To add an MST instance, follow these steps:
Step 1
Step 2
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The Summary pane expands to display MST instances currently configured on the device.
Step 4
The system adds an instance with the MST ID and VLAN fields blank.
Step 5
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Removing MST Instances
You can remove an MST instance using the MST pane (see Figure 5-8).
Figure 5-8 Removing MST Instances
DETAILED STEPS
To remove an MST instance, follow these steps:
Step 1
Step 2
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The Summary pane expands to display devices currently configured for that MST instance.
Step 4
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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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You use the MST pane to create the VLAN-to-MST instance mapping (see Figure 5-7).
DETAILED STEPS
To map and unmap VLANs to MST instances, follow these steps:
Step 1
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The Summary pane expands to display MST instances currently configured on the device.
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Configuring the Primary and Secondary Roots and the Switch Priority
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
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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 5-8),
DETAILED STEPS
To configure the primary and secondary roots or configure the switch priority, follow these steps:
Step 1
Step 2
The Summary pane expands to display the MST instances.
Step 3
Tabs appear in the Details pane.
Step 4
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The default value for the diameter is 7.
Step 10
The default value for the hello time is 2 seconds.
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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 5-8).
DETAILED STEPS
To configure the port priority and port cost, follow these steps:
Step 1
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The Summary pane expands to display the MST instances.
Step 3
Tabs appear in the Details pane.
Step 4
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The section expands to display the ports in that MST instance.
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The default value is 128.
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The default value is 128.
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The default port cost is determined by the port's bandwidth.
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Configuring the Default MST Values per Ports
You set the default values separately for MST access and trunk ports (see Figure 5-8).
DETAILED STEPS
To configure the default MST values for the specified port, follow these steps:
Step 1
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The Summary pane expands to display the MST instances.
Step 3
Tabs appear in the Details pane.
Step 4
Step 5
The section expands to display the ports in that MST instance.
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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 5-6).
DETAILED STEPS
To configure the hello time, forwarding delay time, maximum aging time, and maximum hop count for MST, follow these steps:
Step 1
Step 2
The system highlights the device in the Summary pane, and tabs appear in the Details pane.
Step 3
Step 4
Step 5
The default is 2 seconds.
Step 6
The default is 15 seconds.
Step 7
The default is 20 seconds.
Step 8
The default is 20.
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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.
You use the Spanning Tree pane to set the link type (see Figure 5-6).
DETAILED STEPS
To configure the link type, follow these steps:
Step 1
Step 2
Step 3
Step 4
The Port Setting section expands.
Step 5
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The default Link Type is Auto.
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Displaying Statistics
The following window appears in the Statistics tab:
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Field Descriptions for MST
These field descriptions are used for configuring MST. This section includes the following topics:
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Device View: Instance: Details: MST Setting Section
Device View: Instance: Details: Access Ports Section
Device View: Instance: Details: Trunk Ports Section
Network View: Device: Details: MST Setting Section
Network View: Device: Details: Access Ports Sections
Network View: Device: Details: Trunk Ports Section
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
