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Table Of Contents
Multiple Spanning-Tree Regions
Operations Within an MST Region
Operations Between MST Regions
Interoperation Between Legacy and Standard Switches
Detecting Unidirectional Link Failure
Interoperability with IEEE 802.1D STP
Port Roles and the Active Topology
Bridge Protocol Data Unit Format and Processing
Processing Superior BPDU Information
Processing Inferior BPDU Information
Specifying the MST Region Configuration and Enabling MSTP
Configuring a Secondary Root Switch
Configuring the Switch Priority
MSTP and Ethernet Flow Points (EFPs)
Configuring the Forwarding-Delay Time
Configuring the Maximum-Aging Time
Configuring the Maximum-Hop Count
Specifying the Link Type to Ensure Rapid Transitions
Restarting the Protocol Migration Process
Displaying the MST Configuration and Status
Configuring MSTP
This chapter describes how to configure the Cisco implementation of the IEEE 802.1s Multiple STP (MSTP) on the Cisco ME 3800X and ME 3600X switch. STP is enabled by default on switch ports.
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The multiple spanning-tree (MST) implementation is a pre-standard implementation. It is based on the draft version of the IEEE standard.
The MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, thereby reducing the number of spanning-tree instances needed to support a large number of VLANs. The MSTP provides for multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the highly available network required in a service-provider environment.
When the switch is in the MST mode, the Rapid Spanning Tree Protocol (RSTP), which is based on IEEE 802.1w, is automatically enabled. The RSTP provides rapid convergence of the spanning tree through explicit handshaking that eliminates the IEEE 802.1D forwarding delay and quickly transitions root ports and designated ports to the forwarding state.
Both MSTP and RSTP improve the spanning-tree operation and maintain backward compatibility with equipment that is based on the (original) 802.1D spanning tree, with existing Cisco-proprietary Multiple Instance STP (MISTP), and with existing Cisco per-VLAN spanning-tree plus (PVST+) and rapid per-VLAN spanning-tree plus (rapid PVST+). For information about PVST+ and rapid PVST+, see Chapter 14 "Configuring STP." For information about other spanning-tree features such as Port Fast, root guard, and so forth, see Chapter 16 "Configuring Optional Spanning-Tree Features."
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Understanding MSTP
MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped 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 spanning-tree instances required to support a large number of VLANs.
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For configuration information, see the "Configuring MSTP Features" section.
Multiple Spanning-Tree Regions
For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the switches with the same MST configuration information. A collection of interconnected switches that have the same MST configuration comprises an MST region as shown in Figure 15-1.
The MST configuration controls to which MST region each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. You configure the switch for a region by using the spanning-tree mst configuration global configuration command, after which the switch enters the MST configuration mode. From this mode, you can map VLANs to an MST instance by using the instance MST configuration command, specify the region name by using the name MST configuration command, and set the revision number by using the revision MST configuration command.
A region can have one member or multiple members with the same MST configuration; each member must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but each region can support up to 65 spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.
IST, CIST, and CST
Unlike PVST+ and rapid PVST+ in which all the spanning-tree instances are independent, the MSTP establishes and maintains two types of spanning trees:
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Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a special instance for a region, known as the internal spanning tree (IST). All other MST instances are numbered from 1 to 4094.
The IST is the only spanning-tree instance that sends and receives BPDUs; all of the other spanning-tree instance information is contained in M-records, which are encapsulated within MSTP BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that need to be processed by a switch to support multiple spanning-tree instances is significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root switch ID, root path cost, and so forth. By default, all VLANs are assigned to the IST.
An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region B, even if regions A and B are interconnected.
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The spanning tree computed in a region appears as a subtree in the CST that encompasses the entire switched domain. The CIST is formed as a result of the spanning-tree algorithm running between switches that support the IEEE 802.1w, IEEE 802.1s, and IEEE 802.1D protocols. The CIST inside an MST region is the same as the CST outside a region.
For more information, see the "Operations Within an MST Region" section and the "Operations Between MST Regions" section.
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Operations Within an MST Region
The IST connects all the MSTP switches in a region. When the IST converges, the root of the IST becomes the IST master (shown in Figure 15-1), which is the switch within the region with the lowest bridge ID and path cost to the CST root. The IST master also is the CST root if there is only one region within the network. If the CST root is outside the region, one of the MSTP switches at the boundary of the region is selected as the IST master.
When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CST and the IST master, with both of the path costs to the CST root and to the IST master set to zero. The switch also initializes all of its MST instances and claims to be the root for all of them. If the switch receives superior MST root information (lower bridge ID, lower path cost, and so forth) than currently stored for the port, it relinquishes its claim as the IST master.
During initialization, a region might have many subregions, each with its own IST master. As switches receive superior IST information, they leave their old subregions and join the new subregion that might contain the true IST master. Thus all subregions shrink, except for the one that contains the true IST master.
For correct operation, all switches in the MST region must agree on the same IST master. Therefore, any two switches in the region synchronize their port roles for an MST instance only if they converge to a common IST master.
Operations Between MST Regions
If there are multiple regions or legacy IEEE 802.1D switches within the network, MSTP establishes and maintains the CST, which includes all MST regions and all legacy STP switches in the network. The MST instances combine with the IST at the boundary of the region to become the CST.
The IST connects all the MSTP switches in the region and appears as a subtree in the CST that encompasses the entire switched domain, with the root of the subtree being the IST master. The MST region appears as a virtual switch to adjacent STP switches and MST regions.
Figure 15-1 shows a network with three MST regions and a legacy IEEE 802.1D switch (D). The IST master for region 1 (A) is also the CST root. The IST master for region 2 (B) and the IST master for region 3 (C) are the roots for their respective subtrees within the CST. The RSTP runs in all regions.
Figure 15-1 MST Regions, IST Masters, and the CST Root
Figure 15-1 does not show additional MST instances for each region. Note that the topology of MST instances can be different from that of the IST for the same region.
Only the CST instance sends and receives BPDUs, and MST instances add their spanning-tree information into the BPDUs to interact with neighboring switches and compute the final spanning-tree topology. Because of this, the spanning-tree parameters related to BPDU transmission (for example, hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MST instances. Parameters related to the spanning-tree topology (for example, switch priority, port VLAN cost, port VLAN priority) can be configured on both the CST instance and the MST instance.
MSTP switches use Version 3 RSTP BPDUs or 802.1D STP BPDUs to communicate with legacy IEEE 802.1D switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches.
IEEE 802.1s Terminology
Some MST naming conventions used in Cisco's prestandard implementation have been changed to identify some internal or regional parameters. These parameters are significant only within an MST region, as opposed to external parameters that are relevant to the whole network. Because the CIST is the only spanning-tree instance that spans the whole network, only the CIST parameters require the external rather than the internal or regional qualifiers.
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Table 15-1 compares the IEEE standard and the Cisco prestandard terminology.
Hop Count
The IST and MST instances do not use the message-age and maximum-age information in the configuration BPDU to compute the spanning-tree topology. Instead, they use 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 (trigger a reconfiguration). The root switch 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 switch receives this BPDU, it decrements the received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs it generates. When the count reaches zero, the switch discards the BPDU and ages the information held for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same throughout the region, and the same values are propagated by the region's designated ports at the boundary.
Boundary Ports
In the Cisco prestandard implementation, a boundary port connects an MST region to a single spanning-tree region running RSTP, to a single spanning-tree region running PVST+ or rapid PVST+, or to another MST region with a different MST configuration. A boundary port also connects to a LAN, the designated switch of which is either a single spanning-tree switch or a switch with a different MST configuration.
There is no definition of a boundary port in the IEEE 802.1s standard. The IEEE 802.1Q-2002 standard identifies two kinds of messages that a port can receive: internal (coming from the same region) and external. When a message is external, it is received only by the CIST. If the CIST role is root or alternate, or if the external BPDU is a topology change, it could have an impact on the MST instances. When a message is internal, the CIST part is received by the CIST, and each MST instance receives its respective M-record. The Cisco prestandard implementation treats a port that receives an external message as a boundary port. This means a port cannot receive a mix of internal and external messages.
An MST region includes both switches and LANs. A segment belongs to the region of its designated port. Therefore, a port in a different region than the designated port for a segment is a boundary port. This definition allows two ports internal to a region to share a segment with a port belonging to a different region, creating the possibility of receiving both internal and external messages on a port.
The primary change from the Cisco prestandard implementation is that a designated port is not defined as boundary, unless it is running in an STP-compatible mode.
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The other change from the prestandard implementation is that the CIST regional root switch ID field is now inserted where an RSTP or legacy IEEE 802.1Q switch has the sender switch ID. The whole region performs like a single virtual switch by sending a consistent sender switch ID to neighboring switches. In the example in Figure 15-1, switch C would receive a BPDU with the same consistent sender switch ID of root, whether or not A or B is designated for the segment.
IEEE 802.1s Implementation
The Cisco implementation of the IEEE MST standard includes features required to meet the standard, as well as some of the desirable prestandard functionality that is not yet incorporated into the published standard.
Port Role Naming Change
The boundary role is no longer in the final MST standard, but this boundary concept is maintained in Cisco's implementation. However, an MST instance port at a boundary of the region might not follow the state of the corresponding CIST port. Two cases exist now:
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Interoperation Between Legacy and Standard Switches
Because automatic detection of prestandard switches can fail, you can use an interface configuration command to identify prestandard ports. A region cannot be formed between a standard and a prestandard switch, but they can interoperate by using the CIST. Only the capability of load balancing over different instances is lost in that particular case. The CLI displays different flags depending on the port configuration when a port receives prestandard BPDUs. A syslog message also appears the first time a switch receives a prestandard BPDU on a port that has not been configured for prestandard BPDU transmission.
Figure 15-2 illustrates this scenario. Assume that A is a standard switch and B a prestandard switch, both configured to be in the same region. A is the root switch for the CIST, and thus B has a root port (BX) on segment X and an alternate port (BY) on segment Y. If segment Y flaps, and the port on BY becomes the alternate before sending out a single prestandard BPDU, AY cannot detect that a prestandard switch is connected to Y and continues to send standard BPDUs. The port BY is thus fixed in a boundary, and no load balancing is possible between A and B. The same problem exists on segment X, but B might transmit topology changes.
Figure 15-2 Standard and Prestandard Switch Interoperation
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Detecting Unidirectional Link Failure
This feature is not yet present in the IEEE MST standard, but it is included in this Cisco IOS release. 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.
When a designated port detects a conflict, it keeps its role, but reverts to discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.
Figure 15-3 illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root switch, and its BPDUs are lost on the link leading to switch B. RSTP 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 it sends and that switch B is the designated, not root switch. As a result, switch A blocks (or keeps blocking) its port, thus preventing the bridging loop.
Figure 15-3 Detecting Unidirectional Link Failure
Interoperability with IEEE 802.1D STP
A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy IEEE 802.1D switches. If this switch receives a legacy IEEE 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP switch also can detect that a port is at the boundary of a region when it receives a legacy BPDU, an MSTP BPDU (Version 3) associated with a different region, or an RSTP BPDU (Version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives IEEE 802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. Also, a switch might continue to assign a boundary role to a port when the switch to which this switch is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring switches), use the clear spanning-tree detected-protocols privileged EXEC command.
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are RSTP BPDUs. Therefore, MSTP switches send either a Version 0 configuration and TCN BPDUs or Version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch of which is either a single spanning-tree switch or a switch with a different MST configuration.
Understanding RSTP
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the IEEE 802.1D spanning tree), which is critical for networks carrying delay-sensitive traffic such as voice and video.
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For configuration information, see the "Configuring MSTP Features" section.
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root switch as described in the "Spanning-Tree Topology and BPDUs" section. Then the RSTP assigns one of these port roles to individual ports.
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A port with the root or a designated port role is included in the active topology. A port with the alternate or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in 802.1D). The port state controls the operation of the forwarding and learning processes. Table 15-2 provides a comparison of IEEE 802.1D and RSTP port states.
To be consistent with Cisco STP implementations, this guide documents the port state as blocking instead of discarding. Designated ports start in the listening state.
Rapid Convergence
The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch port, or a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through point-to-point links as follows:
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As shown in Figure 15-4, Switch A is connected to Switch B through a point-to-point link, and all of the ports are in the blocking state. Assume that the priority of Switch A is a smaller numerical value than the priority of Switch B. Switch A sends a proposal message (a configuration BPDU with the proposal flag set) to Switch B, proposing itself as the designated switch.
After receiving the proposal message, Switch B selects as its new root port the port from which the proposal message was received, forces all nonedge ports to the blocking state, and sends an agreement message (a BPDU with the agreement flag set) through its new root port.
After receiving Switch B's agreement message, Switch A also immediately transitions its designated port to the forwarding state. No loops in the network are formed because Switch B blocked all of its nonedge ports and because there is a point-to-point link between Switches A and B.
When Switch C is connected to Switch B, a similar set of handshaking messages are exchanged. Switch C selects the port connected to Switch B as its root port, and both ends immediately transition to the forwarding state. With each iteration of this handshaking process, one more switch joins the active topology. As the network converges, this proposal-agreement handshaking progresses from the root toward the leaves of the spanning tree.
The switch learns the link type from the port duplex mode: a full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection. You can override the default setting that is controlled by the duplex setting by using the spanning-tree link-type interface configuration command.
Figure 15-4 Proposal and Agreement Handshaking for Rapid Convergence
Synchronization of Port Roles
When the switch receives a proposal message on one of its ports and that port is selected as the new root port, the RSTP forces all other ports to synchronize with the new root information.
The switch is synchronized with superior root information received on the root port if all other ports are synchronized. An individual port on the switch is synchronized if
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If a designated STP port is in the forwarding state and is not configured as an edge port, it transitions to the blocking state when the RSTP forces it to synchronize with new root information. In general, when the RSTP forces a port to synchronize with root information and the port does not satisfy any of the above conditions, its port state is set to blocking.
After ensuring all of the ports are synchronized, the switch sends an agreement message to the designated switch corresponding to its root port. When the switches connected by a point-to-point link are in agreement about their port roles, the RSTP immediately transitions the port states to forwarding. The sequence of events is shown in Figure 15-5.
Figure 15-5 Sequence of Events During Rapid Convergence
Bridge Protocol Data Unit Format and Processing
The RSTP BPDU format is the same as the IEEE 802.1D BPDU format except that the protocol version is set to 2. A new one-byte Version 1 Length field is set to zero, which means that no version 1 protocol information is present. Table 15-3 shows the RSTP flag fields.
The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch on that LAN. The port role in the proposal message is always set to the designated port.
The sending switch sets the agreement flag in the RSTP BPDU to accept the previous proposal. The port role in the agreement message is always set to the root port.
The RSTP does not have a separate topology change notification (TCN) BPDU. It uses the topology change (TC) flag to show the topology changes. However, for interoperability with 802.1D switches, the RSTP switch processes and generates TCN BPDUs.
The learning and forwarding flags are set according to the state of the sending port.
Processing Superior BPDU Information
If a port receives superior root information (lower bridge ID, lower path cost, and so forth) than currently stored for the port, the RSTP triggers a reconfiguration. If the port is proposed and is selected as the new root port, RSTP forces all the other ports to synchronize.
If the BPDU received is an RSTP BPDU with the proposal flag set, the switch sends an agreement message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the switch does not set the proposal flag and starts the forward-delay timer for the port. The new root port requires twice the forward-delay time to transition to the forwarding state.
If the superior information received on the port causes the port to become a backup or alternate port, RSTP sets the port to the blocking state but does not send the agreement message. The designated port continues sending BPDUs with the proposal flag set until the forward-delay timer expires, at which time the port transitions to the forwarding state.
Processing Inferior BPDU Information
If a designated port receives an inferior BPDU (higher bridge ID, higher path cost, and so forth than currently stored for the port) with a designated port role, it immediately replies with its own information.
Topology Changes
This section describes the differences between the RSTP and the IEEE 802.1D in handling spanning-tree topology changes.
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This behavior is only required to support IEEE 802.1D switches. The RSTP BPDUs never have the TCA bit set.
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When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port and ignores the protocol type.
If the switch receives an IEEE 802.1D BPDU after the port's migration-delay timer has expired, it assumes that it is connected to an IEEE 802.1D switch and starts using only IEEE 802.1D BPDUs. However, if the RSTP switch is using IEEE 802.1D BPDUs on a port and receives an RSTP BPDU after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port.
Configuring MSTP Features
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Default MSTP Configuration
Table 15-4 shows the default MSTP configuration.
For information about the supported number of spanning-tree instances, see the "Supported Spanning-Tree Instances" section.
MSTP Configuration Guidelines
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Specifying the MST Region Configuration and Enabling MSTP
For two or more switches to be in the same MST region, they must have the same VLAN-to-instance mapping, the same configuration revision number, and the same name.
A region can have one member or multiple members with the same MST configuration; each member must be capable of processing RSTP BPDUs. There is no limit to the number of MST regions in a network, but each region can support up to 65 spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.
Beginning in privileged EXEC mode, follow these steps to specify the MST region configuration and enable MSTP. This procedure is required.
To return to the default MST region configuration, use the no spanning-tree mst configuration global configuration command. To return to the default VLAN-to-instance map, use the no instance instance-id [vlan vlan-range] MST configuration command. To return to the default name, use the no name MST configuration command. To return to the default revision number, use the no revision MST configuration command. To re-enable rapid PVST+, use the no spanning-tree mode or the spanning-tree mode pvst global configuration command.
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 1Instance Vlans Mapped-------- ---------------------0 1-9,21-40941 10-20-------------------------------Switch(config-mst)# exitSwitch(config)#Configuring the Root Switch
The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A bridge ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For a group of VLANs, the switch with the lowest bridge ID becomes the root switch.
To configure a switch to become the root, use the spanning-tree mst instance-id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root switch for the specified spanning-tree instance. When you enter this command, the switch checks the switch priorities of the root switches. Because of the extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switch to become the root for the specified spanning-tree instance.
If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own priority to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value as shown in Table 14-1.)
If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.
The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning-tree primary root.
Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch 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 use the hello keyword to override the automatically calculated hello time.
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Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst instance-id root global configuration command.
Configuring a Secondary Root Switch
When you configure a switch with the extended system ID support as the secondary root, the switch priority is modified from the default value (32768) to 28672. The switch is then likely to become the root switch for the specified instance if the primary root switch fails. This is assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use the same network diameter and hello-time values that you used when you configured the primary root switch with the spanning-tree mst instance-id root primary global configuration command.
Beginning in privileged EXEC mode, follow these steps to configure a switch as the secondary root switch. This procedure is optional.
Step 1
configure terminal
Enter global configuration mode.
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spanning-tree mst instance-id root secondary [diameter net-diameter [hello-time seconds]]
Configure a switch as the secondary root switch.
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Use the same network diameter and hello-time values that you used when configuring the primary root switch. See the "Configuring the Root Switch" section.
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end
Return to privileged EXEC mode.
Step 4
show spanning-tree mst instance-id
Verify your entries.
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copy running-config startup-config
(Optional) Save your entries in the configuration file.
To return the switch to its default setting, use the no spanning-tree mst instance-id root global configuration command.
Configuring Port Priority
If a loop occurs, the MSTP uses the port priority when selecting an STP port to put into the forwarding state. You can assign higher priority values (lower numerical values) to STP ports that you want selected first and lower priority values (higher numerical values) that you want selected last. If all interfaces have the same priority value, the MSTP puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the MSTP port priority of an interface. This procedure is optional.
Step 1
configure terminal
Enter global configuration mode.
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interface interface-id
Specify an interface to configure, and enter interface configuration mode.
Valid interfaces include physical NNIs or ENIs with spanning tree enabled, VLANs, and NNI or ENI port channels. The VLAN ID range is 1 to 4094. The port-channel range is 1 to 48.
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If the interface is a VLAN, only ports with spanning tree enabled in the VLAN will run spanning tree.
If the interface is a port channel, all members of the port channel must be NNIs or ENIs with spanning tree enabled.Step 3
spanning-tree mst instance-id port-priority priority
Configure the port priority.
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The priority values are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, and 240. All other values are rejected.
Step 4
end
Return to privileged EXEC mode.
Step 5
show spanning-tree mst interface interface-id
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show spanning-tree mst instance-id
Verify your entries.
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copy running-config startup-config
(Optional) Save your entries in the configuration file.
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To return the interface to its default setting, use the no spanning-tree mst instance-id port-priority interface configuration command.
Configuring Path Cost
The MSTP path cost default value is derived from the media speed of an STP port. If a loop occurs, the MSTP uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to STP ports that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value, the MSTP puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the MSTP cost of an interface. This procedure is optional.
Step 1
configure terminal
Enter global configuration mode.
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interface interface-id
Specify an interface to configure, and enter interface configuration mode. Valid interfaces include physical NNIs or ENIs with spanning tree enabled, VLANs, and NNI or ENI port channels. The VLAN ID range is 1 to 4094. The port-channel range is 1 to 48.
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If the interface is a VLAN, only ports with spanning tree enabled in the VLAN will run spanning tree.
If the interface is a port channel, all members of the port channel must be NNIs or ENIs with spanning tree enabled.Step 3
spanning-tree mst instance-id cost cost
Configure the cost.
If a loop occurs, the MSTP uses the path cost when selecting an interface to place into the forwarding state. A lower path cost represents higher-speed transmission.
•
•
Step 4
end
Return to privileged EXEC mode.
Step 5
show spanning-tree mst interface interface-id
or
show spanning-tree mst instance-id
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Note
To return the interface to its default setting, use the no spanning-tree mst instance-id cost interface configuration command.
Configuring the Switch Priority
You can configure the switch priority and make it more likely that the switch will be chosen as the root switch.
Note
Beginning in privileged EXEC mode, follow these steps to configure the switch priority. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst instance-id priority global configuration command.
MSTP and Ethernet Flow Points (EFPs)
For MSTP to peer with a neighbor on a port that has an Ethernet Virtual Connection (EVC) EFP service instance configured, you need to enter the l2 protocol peer stp service-instance configuration command on the service instance. See the "Configuring Ethernet Virtual Connections (EVCs)" chapter for more information on EFPs.
This example shows how to configure Layer 2 protocol peer on a service instance:
Switch (config)#interface gigabitethernet0/1Switch (config-if)#switchport mode trunkSwitch (config-if)#switchport trunk allowed vlan noneSwitch (config-if)#service instance 1 EthernetSwitch (config-if-srv)#encapsulation untaggedSwitch (config-if-srv)#l2protocol peer stpSwitch (config-if-srv)#bridge-domain 10Switch (config-if-srv)#endConfiguring the Hello Time
You can configure the interval between the generation of configuration messages by the root switch by changing the hello time.
Note
Beginning in privileged EXEC mode, follow these steps to configure the hello time for all MST instances. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst hello-time global configuration command.
Configuring the Forwarding-Delay Time
Beginning in privileged EXEC mode, follow these steps to configure the forwarding-delay time for all MST instances. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst forward-time global configuration command.
Configuring the Maximum-Aging Time
Beginning in privileged EXEC mode, follow these steps to configure the maximum-aging time for all MST instances. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst max-age global configuration command.
Configuring the Maximum-Hop Count
Beginning in privileged EXEC mode, follow these steps to configure the maximum-hop count for all MST instances. This procedure is optional.
To return the switch to its default setting, use the no spanning-tree mst max-hops global configuration command.
Specifying the Link Type to Ensure Rapid Transitions
If you connect an STP port to another STP port through a point-to-point link and the local port becomes a designated port, the RSTP negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology as described in the "Rapid Convergence" section.
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 switch running MSTP, you can override the default setting of the link type and enable rapid transitions to the forwarding state.
Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This procedure is optional.
To return the port to its default setting, use the no spanning-tree link-type interface configuration command.
Designating the Neighbor Type
A topology could contain both prestandard and IEEE 802.1s standard compliant devices. By default, ports can automatically detect prestandard devices, but they can still receive both standard and prestandard BPDUs. When there is a mismatch between a device and its neighbor, only the CIST runs on the interface.
You can choose to set a port to send only prestandard BPDUs. The prestandard flag appears in all the show commands, even if the port is in STP compatibility mode.
Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This procedure is optional.
To return the port to its default setting, use the no spanning-tree mst prestandard interface configuration command.
Restarting the Protocol Migration Process
A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch also can detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (Version 3) associated with a different region, or an RST BPDU (Version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. A switch also might continue to assign a boundary role to a port when the switch to which it is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches) on the switch, use the clear spanning-tree detected-protocols privileged EXEC command.
To restart the protocol migration process on a specific interface, use the clear spanning-tree detected-protocols interface interface-id privileged EXEC command.
Displaying the MST Configuration and Status
To display the spanning-tree status, use one or more of the privileged EXEC commands in Table 15-5:
For information about other keywords for the show spanning-tree privileged EXEC command, see the command reference for this release.
