Cisco Industrial Ethernet 4000, 4010 and 5000 Switch Software. Configuration Guide (2022)

Configuring STP

Information About Configuring STP

Learning—The interface prepares to participate in frame forwarding.

Forwarding—The interface forwards frames.

Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no

spanning-tree instance running on the port.

An interface moves through these states:

From initialization to blocking

From blocking to listening or to disabled

From listening to learning or to disabled

From learning to forwarding or to disabled

From forwarding to disabled

Figure 36 on page 336 illustrates how an interface moves through the states.

Figure 36

Spanning-Tree Interface States

Power-on

initialization

Blocking

state

Listening

Disabled

state

Learning

state

Forwarding

state

When you power up the switch, spanning tree is enabled by default, and every interface in the switch, VLAN, or network

goes through the blocking state and the transitory states of listening and learning. Spanning tree stabilizes each interface

at the forwarding or blocking state.

When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs:

1. The interface is in the listening state while spanning tree waits for protocol information to move the interface to the

blocking state.

2. While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning state and resets

the forward-delay timer.

3. In the learning state, the interface continues to block frame forwarding as the switch learns end-station location

information for the forwarding database.

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4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state, where both

learning and frame forwarding are enabled.

Blocking State

A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to

each switch interface. A switch initially functions as the root until it exchanges BPDUs with other switches. This exchange

establishes which switch in the network is the root or root switch. If there is only one switch in the network, no exchange

occurs, the forward-delay timer expires, and the interface moves to the listening state. An interface always enters the

blocking state after switch initialization.

An interface in the blocking state performs these functions:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Does not learn addresses

Receives BPDUs

Listening State

The listening state is the first state a Layer 2 interface enters after the blocking state. The interface enters this state when

the spanning tree decides that the interface should participate in frame forwarding.

An interface in the listening state performs these functions:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Does not learn addresses

Receives BPDUs

Learning State

A Layer 2 interface in the learning state prepares to participate in frame forwarding. The interface enters the learning

state from the listening state.

An interface in the learning state performs these functions:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Learns addresses

Receives BPDUs

Forwarding State

A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state from the learning

state.

An interface in the forwarding state performs these functions:

Receives and forwards frames received on the interface

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Forwards frames switched from another interface

Learns addresses

Receives BPDUs

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree. An interface in

the disabled state is nonoperational.

A disabled interface performs these functions:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Does not learn addresses

Does not receive BPDUs

How a Switch or Port Becomes the Root Switch or Root Port

If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest MAC address

becomes the root switch. In Figure 37 on page 338, Switch A is elected as the root switch because the switch priority

of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic

patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root switch. By increasing the

priority (lowering the numerical value) of the ideal switch so that it becomes the root switch, you force a spanning-tree

recalculation to form a new topology with the ideal switch as the root.

Figure 37

Spanning-Tree Topology

RP = Root Port

DP = Designated Port

When the spanning-tree topology is calculated based on default parameters, the path between source and destination

end stations in a switched network might not be ideal. For instance, connecting higher-speed links to an interface that

has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100

link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree

port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet

port becomes the new root port.

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Spanning Tree and Redundant Connectivity

You can create a redundant backbone with spanning tree by connecting two switch interfaces to another device or to

two different devices, as shown in Figure 38 on page 339. Spanning tree automatically disables one interface but

enables it if the other one fails. If one link is high-speed and the other is low-speed, the low-speed link is always

disabled. If the speeds are the same, the port priority and port ID are added together, and spanning tree disables the link

with the lowest value.

Figure 38

Spanning Tree and Redundant Connectivity

Active link

Blocked link

Workstations

You can also create redundant links between switches by using EtherChannel groups. For more information, see

Configuring EtherChannels, page 1069

Spanning-Tree Address Management

IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to be used by

different bridge protocols. These addresses are static addresses that cannot be removed.

Regardless of the spanning-tree state, each switch receives but does not forward packets destined for addresses

between 0x0180C2000000 and 0x0180C200000F.

If spanning tree is enabled, the CPU on the switch receives packets destined for 0x0180C2000000 and

0x0180C2000010. If spanning tree is disabled, the switch forwards those packets as unknown multicast addresses.

Accelerated Aging to Retain Connectivity

The default for aging dynamic addresses is 5 minutes, the default setting of the mac address-table aging-time global

configuration command. However, a spanning-tree reconfiguration can cause many station locations to change. Because

these stations could be unreachable for 5 minutes or more during a reconfiguration, the address-aging time is

accelerated so that station addresses can be dropped from the address table and then relearned. The accelerated aging

is the same as the forward-delay parameter value (spanning-tree vlan vlan-id forward-time seconds global

configuration command) when the spanning tree reconfigures.

Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN basis. A

spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that VLAN to be subject to

accelerated aging. Dynamic addresses on other VLANs can be unaffected and remain subject to the aging interval

entered for the switch.

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Spanning-Tree Modes and Protocols

The switch supports these spanning-tree modes and protocols:

PVST+—This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the

default spanning-tree mode used on all Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch

up to the maximum supported, ensuring that each has a loop-free path through the network.

The PVST+ provides Layer 2 load balancing for the VLAN on which it runs. You can create different logical topologies

by using the VLANs on your network to ensure that all of your links are used but that no one link is oversubscribed.

Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning-tree

information associated with that VLAN to all other switches in the network. Because each switch has the same

information about the network, this process ensures that the network topology is maintained.

Rapid PVST+—This spanning-tree mode is the same as PVST+ except that is uses a rapid convergence based on the

IEEE 802.1w standard. To provide rapid convergence, the rapid PVST+ immediately deletes dynamically learned

MAC address entries on a per-port basis upon receiving a topology change. By contrast, PVST+ uses a short aging

time for dynamically learned MAC address entries.

The rapid PVST+ uses the same configuration as PVST+ (except where noted), and the switch needs only minimal

extra configuration. The benefit of rapid PVST+ is that you can migrate a large PVST+ install base to rapid PVST+

without having to learn the complexities of the MSTP configuration and without having to reprovision your network.

In rapid-PVST+ mode, each VLAN runs its own spanning-tree instance up to the maximum supported.

MSTP—This spanning-tree mode is based on the IEEE 802.1s standard. You can map multiple VLANs to the same

spanning-tree instance, which reduces the number of spanning-tree instances required to support a large number

of VLANs. The MSTP runs on top of the RSTP (based on IEEE 802.1w), which provides for rapid convergence of the

spanning tree by eliminating the forward delay and by quickly transitioning root ports and designated ports to the

forwarding state. You cannot run MSTP without RSTP.

The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched

network. For more information, see Configuring MSTP, page 351

For information about the number of supported spanning-tree instances, see the next section.

Supported Spanning-Tree Instances

In PVST+ or rapid-PVST+ mode, the switch supports up to 128 spanning-tree instances.

In MSTP mode, the switch supports up to 65 MST instances. The number of VLANs that can be mapped to a particular

MST instance is unlimited.

For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see Changing the

Spanning-Tree Mode, page 345.

Spanning-Tree Interoperability and Backward Compatibility

Table 38Spanning-Tree Interoperability and Backward Compatibility, page 341 lists the interoperability and compatibility

among the supported spanning-tree modes in a network.

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Table 38

Spanning-Tree Interoperability and Backward Compatibility

PVST+

MSTP

Rapid PVST+

PVST+

Yes

Yes (with restrictions)

Yes (reverts to PVST+)

MSTP

Yes (with restrictions)

Yes

Yes (reverts to PVST+)

Rapid PVST+

Yes (reverts to PVST+)

Yes

In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a

PVST+ switch cannot connect to multiple MST regions.

When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the

rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances. In the rapid-PVST+

spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the PVST+ instances, the root switch must be

a PVST+ switch. The PVST+ switches should be at the edge of the network.

STP and IEEE 802.1Q Trunks

The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The

standard requires only one spanning-tree instance for all VLANs allowed on the trunks. However, in a network of Cisco

switches connected through IEEE 802.1Q trunks, the switches maintain one spanning-tree instance for each VLAN

allowed on the trunks.

When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco switch uses PVST+ to

provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it instead of PVST+. The switch

combines the spanning-tree instance of the IEEE 802.1Q VLAN of the trunk with the spanning-tree instance of the

non-Cisco IEEE 802.1Q switch.

However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud of non-Cisco IEEE

802.1Q switches. The non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link

between the switches.

PVST+ is automatically enabled on IEEE 802.1Q trunks, and no user configuration is required. The external spanning-tree

behavior on access ports is not affected by PVST+.

VLAN-Bridge Spanning Tree

Cisco VLAN-bridge spanning tree is used with the fallback bridging feature (bridge groups), which forwards non-IP

protocols such as DECnet between two or more VLAN bridge domains or routed ports. The VLAN-bridge spanning tree

allows the bridge groups to form a spanning tree on top of the individual VLAN spanning trees to prevent loops from

forming if there are multiple connections among VLANs. It also prevents the individual spanning trees from the VLANs

being bridged from collapsing into a single spanning tree.

To support VLAN-bridge spanning tree, some of the spanning-tree timers are increased.

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Default Spanning-Tree Settings

Table 39

Default Spanning-Tree Settings

Feature

Default Setting

Enable state

Enabled on VLAN 1.

Spanning-tree mode

PVST+. (Rapid PVST+ and MSTP are

disabled.)

Switch priority

32768.

Spanning-tree port priority (configurable on a per-interface basis)

128.

Spanning-tree port cost (configurable on a per-interface basis)

1000 Mb/s: 4.

100 Mb/s: 19.

10 Mb/s: 100.

Spanning-tree VLAN port priority (configurable on a per-VLAN basis)

128.

Spanning-tree VLAN port cost (configurable on a per-VLAN basis)

1000 Mb/s: 4.

100 Mb/s: 19.

10 Mb/s: 100.

Spanning-tree timers

Hello time: 2 seconds.

Forward-delay time: 15 seconds.

Maximum-aging time: 20 seconds.

Transmit hold count: 6 BPDUs

Disabling Spanning Tree

Spanning tree is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit specified

in the Supported Spanning-Tree Instances, page 340. Disable spanning tree only if you are sure there are no loops in the

network topology.

Caution: When spanning tree is disabled and loops are present in the topology, excessive traffic and indefinite

packet duplication can drastically reduce network performance.

Root Switch

The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting

of the switch priority and the switch MAC address, is associated with each instance. For each VLAN, the switch with the

lowest bridge ID becomes the root switch for that VLAN.

To configure a switch to become the root for the specified VLAN, use the spanning-tree vlan vlan-id root global

configuration command to modify the switch priority from the default value (32768) to a significantly lower value. When

you enter this command, the software checks the switch priority of the root switches for each VLAN. Because of the

extended system ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will cause this

switch to become the root for the specified VLAN.

If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the

specified VLAN 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 37 on page 335.)

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Note: The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the root

switch is less than 1.

Note: 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.

Note: 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 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.

Note: After configuring the switch as the root switch, we recommend that you avoid manually configuring the hello time,

forward-delay time, and maximum-age time through the spanning-tree vlan vlan-id hello-time, spanning-tree vlan

vlan-id forward-time, and the spanning-tree vlan vlan-id max-age global configuration commands.

Secondary Root Switch

When you configure a switch 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 VLAN 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 vlan vlan-id root primary global configuration command.

Port Priority

If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the forwarding state. You can

assign higher priority values (lower numerical values) to interfaces 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, spanning tree puts the

interface with the lowest interface number in the forwarding state and blocks the other interfaces.

Path Cost

The spanning-tree path cost default value is derived from the media speed of an interface. If a loop occurs, spanning

tree uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces

that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value,

spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

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Spanning-Tree Timers

Table 40

Spanning-Tree Timers

Variable

Description

Hello timer

Controls how often the switch broadcasts hello messages to other switches.

Forward-delay timer

Controls how long each of the listening and learning states last before the interface begins

forwarding.

Maximum-age timer

Controls the amount of time the switch stores protocol information received on an interface.

Transmit hold count

Controls the number of BPDUs that can be sent before pausing for 1 second.

Spanning-Tree Configuration Guidelines

If more VLANs are defined in the VTP than there are spanning-tree instances, you can enable PVST+ or rapid PVST+ on

only 128 VLANs on the switch. The remaining VLANs operate with spanning tree disabled. However, you can map

multiple VLANs to the same spanning-tree instances by using MSTP. For more information, see Configuring MSTP,

page 351

If 128 instances of spanning tree are already in use, you can disable spanning tree on one of the VLANs and then enable

it on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global configuration command to disable

spanning tree on a specific VLAN, and use the spanning-tree vlan vlan-id global configuration command to enable

spanning tree on the desired VLAN.

Caution: Switches that are not running spanning tree still forward BPDUs that they receive so that the other

switches on the VLAN that have a running spanning-tree instance can break loops. Therefore, spanning tree must

be running on enough switches to break all the loops in the network; for example, at least one switch on each loop

in the VLAN must be running spanning tree. It is not absolutely necessary to run spanning tree on all switches in the

VLAN. However, if you are running spanning tree only on a minimal set of switches, an incautious change to the

network that introduces another loop into the VLAN can result in a broadcast storm.

Note: If you have already used all available spanning-tree instances on your switch, adding another VLAN anywhere in

the VTP domain creates a VLAN that is not running spanning tree on that switch. If you have the default allowed list on

the trunk ports of that switch, the new VLAN is carried on all trunk ports. Depending on the topology of the network, this

could create a loop in the new VLAN that will not be broken, particularly if there are several adjacent switches that have

all run out of spanning-tree instances. You can prevent this possibility by setting up allowed lists on the trunk ports of

switches that have used up their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many

cases and can make it more labor-intensive to add another VLAN to the network.

Spanning-tree commands control the configuration of VLAN spanning-tree instances. You create a spanning-tree

instance when you assign an interface to a VLAN. The spanning-tree instance is removed when the last interface is

moved to another VLAN. You can configure switch and port parameters before a spanning-tree instance is created; these

parameters are applied when the spanning-tree instance is created.

The switch supports PVST+, rapid PVST+, and MSTP, but only one version can be active at any time. (For example, all

VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For information about the different spanning-tree

modes and how they interoperate, see Spanning-Tree Interoperability and Backward Compatibility, page 340.

For configuration information about UplinkFast and BackboneFast, see Information About Configuring the Optional

Spanning-Tree Features, page 371.

Caution: Loop guard works only on point-to-point links. We recommend that each end of the link has a directly

connected device that is running STP.

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Changing the Spanning-Tree Mode

Command

Purpose

configure terminal

Enters global configuration mode.

spanning-tree mode {pvst | mst |

Configures a spanning-tree mode.

rapid-pvst}

pvst—Enables PVST+ (the default setting).

mst—Enables MSTP (and RSTP). For more configuration steps, see

Configuring MSTP, page 351

rapid-pvst—Enables rapid PVST+.

interface interface-id

(Recommended for rapid-PVST+ mode only) Specifies an interface to

configure, and enters interface configuration mode. Valid interfaces

include physical ports, VLANs, and port channels.

spanning-tree link-type point-to-point

(Recommended for rapid-PVST+ mode only) Specifies that the link type

for this port is point-to-point.

If you connect this port (local port) to a remote port through a

point-to-point link and the local port becomes a designated port, the

switch negotiates with the remote port and rapidly changes the local

port to the forwarding state.

end

Returns to privileged EXEC mode.

clear spanning-tree detected-protocols

(Recommended for rapid-PVST+ mode only) Restarts the protocol

migration process on the entire switch if any port on the switch is

connected to a port on a legacy IEEE 802.1D switch,

This step is optional if the designated switch detects that this switch is

running rapid PVST+.

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Configuring the Root Switch

Command

Purpose

configure terminal

Enters global configuration mode.

spanning-tree vlan vlan-id root primary

Configures a switch to become the root for the specified

[diameter net-diameter [hello-time seconds]]

VLAN.

vlan-id—Specifies a single VLAN identified by VLAN ID

number, a range of VLANs separated by a hyphen, or a

series of VLANs separated by a comma.

(Optional) diameter net-diameter—Specifies the

maximum number of switches between any two end

stations.

(Optional) hello-time seconds—Specifies the interval in

seconds between the generation of configuration

messages by the root switch.

end

Returns to privileged EXEC mode.

Configuring a Secondary Root Switch

Command

Purpose

configure terminal

Enters global configuration mode.

spanning-tree vlan vlan-id root secondary

Configures a switch to become the secondary root for the specified

[diameter net-diameter [hello-time

VLAN.

seconds]]

vlan-id—Specifies a single VLAN identified by VLAN ID number,

a range of VLANs separated by a hyphen, or a series of VLANs

separated by a comma. The range is 1 to 4096.

(Optional) diameter net-diameter—Specifies the maximum

number of switches between any two end stations. The range is

2 to 7.

(Optional) hello-time seconds—Specifies the interval in seconds

between the generation of configuration messages by the root

switch. The range is 1 to 10; the default is 2.

Use the same network diameter and hello-time values that you used

when configuring the primary root switch. See Configuring the Root

Switch, page 346.

end

Returns to privileged EXEC mode.

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Configuring Port Priority

Command

Purpose

configure terminal

Enters global configuration mode.

interface interface-id

Specifies an interface to configure, and enters interface

configuration mode.

Valid interfaces include physical ports and port-channel

logical interfaces (port-channel port-channel-number).

spanning-tree port-priority priority

Configures the port priority for an interface.

spanning-tree vlan vlan-id port-priority priority

Configures the port priority for a VLAN.

end

Returns to privileged EXEC mode.

Configuring Path Cost

Command

Purpose

configure terminal

Enters global configuration mode.

interface interface-id

Specifies an interface to configure, and enters interface

configuration mode. Valid interfaces include physical ports and

port-channel logical interfaces (port-channel

port-channel-number).

spanning-tree cost cost

Configures the cost for an interface.

spanning-tree vlan vlan-id cost cost

Configures the cost for a VLAN.

If a loop occurs, spanning tree uses the path cost when selecting

an interface to place into the forwarding state. A lower path cost

represents higher-speed transmission.

end

Returns to privileged EXEC mode.

Configuring Optional STP Parameters

Before You Begin

Exercise care when configuring the priority, and hello time for STP.

For most situations, we recommend that you use the spanning-tree vlan vlan-id root primary and the spanning-tree

vlan vlan-id root secondary global configuration commands to modify the switch priority.

Command

Purpose

configure terminal

Enters global configuration mode.

spanning-tree vlan vlan-id priority priority

Configures the switch priority of a VLAN.

spanning-tree vlan vlan-id hello-time seconds

Configures the hello time of a VLAN.

spanning-tree vlan vlan-id max-age seconds

Configures the maximum-aging time of a VLAN.

spanning-tree vlan vlan-id forward-time seconds

Configures the forward time of a VLAN.

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Command

Purpose

spanning-tree vlan vlan-id max-age seconds

Configures the maximum-aging time of a VLAN.

spanning-tree transmit hold-count value

Configures the number of BPDUs that can be sent before

pausing for 1 second.

Note: Changing this parameter to a higher value can have a

significant impact on CPU utilization, especially in

Rapid-PVST mode. Lowering this value can slow down

convergence in certain scenarios. We recommend that you

maintain the default setting.

end

Returns to privileged EXEC mode.

Monitoring and Maintaining STP

Command

Purpose

show spanning-tree active

Displays spanning-tree information on active interfaces only.

show spanning-tree detail

Displays a detailed summary of interface information.

show spanning-tree interface interface-id

Displays spanning-tree information for the specified

interface.

show spanning-tree summary

Displays a summary of interface states.

show spanning-tree vlan vlan-id

Displays spanning-tree VLAN entries.

copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Additional References

The following sections provide references related to switch administration:

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Additional References

Related Topic

Document Title

Cisco IOS basic commands

Cisco IOS Configuration Fundamentals Command Reference

VLAN configuration

Configuring VLANs, page 289

Multiple Spanning Tree Protocol configuration

Configuring MSTP, page 351

Optional Spanning-Tree configuration

Configuring Optional Spanning-Tree Features, page 371

Standards

Title

No new or modified standards are supported by this

—

feature, and support for existing standards has not

been modified by this feature.

MIBs

MIBs Link

—

To locate and download MIBs using Cisco IOS XR software, use the

Cisco MIB Locator found at the following URL and choose a platform

under the Cisco Access Products menu:

http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

Title

No new or modified RFCs are supported by this

—

feature, and support for existing RFCs has not been

modified by this feature.

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Information About Configuring MSTP

This chapter describes how to configure the Cisco implementation of the IEEE 802.1s Multiple STP (MSTP) on the switch.

Note: The multiple spanning-tree (MST) implementation is based on the IEEE 802.1s standard.

The MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, 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) IEEE 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+).

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.

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 1 on page 3.

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 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. Instances can be identified by any number in the range from

0 to 4096. You can assign a VLAN to only one spanning-tree instance at a time.

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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:

An internal spanning tree (IST), which is the spanning tree that runs in an MST region.

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 4096.

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 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.

A common and internal spanning tree (CIST), which is a collection of the ISTs in each MST region, and the common

spanning tree (CST) that interconnects the MST regions and single spanning trees.

The spanning tree computed in a 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 IEEE 802.1w,

IEEE 802.1s, and IEEE 802.1D standards. The CIST inside an MST region is the same as the CST outside a region.

For more information, see Operations Within an MST Region, page 2 and the Operations Between MST Regions, page 3.

Note: The implementation of the IEEE 802.1s standard, changes some of the terminology associated with MST

implementations.

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 CIST

regional root (called the IST master before the implementation of the IEEE 802.1s standard) as shown in Figure 1 on

page 3. It is the switch within the region with the lowest switch ID and path cost to the CIST 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, one of the MSTP

switches at the boundary of the region is selected as the CIST regional root.

When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CIST and the CIST regional root, with

both of the path costs to the CIST root and to the CIST regional root 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 switch ID,

lower path cost, and so forth) than currently stored for the port, it relinquishes its claim as the CIST regional root.

During initialization, a region might have many subregions, each with its own CIST regional root. As switches receive

superior IST information, they leave their old subregions and join the new subregion that contains the true CIST regional

root. All subregions shrink, except for the one that contains the true CIST regional root.

For correct operation, all switches in the MST region must agree on the same CIST regional root. Therefore, any two

switches in the region only synchronize their port roles for an MST instance if they converge to a common CIST regional

root.

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.

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The IST connects all the MSTP 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 switch to adjacent

STP switches and MST regions.

Figure 1 on page 3 shows a network with three MST regions and a legacy IEEE 802.1D switch (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. The RSTP runs in all regions.

Figure 39

MST Regions, CIST Masters, and CST Root

IST master

and CST root

Legacy IEEE 802.1D

MST Region 1

IST master

MST Region 2

MST Region 3

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, and port VLAN priority) can be configured on both the CST instance and

the MST instance.

MSTP switches use Version 3 RSTP BPDUs or IEEE 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.

The CIST root is the root switch for the unique instance that spans the whole network, the CIST.

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The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region.

Remember that an MST region looks like a single switch for the CIST. The CIST external root path cost is the root

path cost calculated between these virtual switches and switches that do not belong to any region.

The CIST regional root was called the IST master in the prestandard implementation. If the CIST root is in the region,

the CIST regional root is the CIST root. Otherwise, the CIST regional root is the closest switch to the CIST root in the

region. The CIST regional root acts as a root switch 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.

Table 41 on page 354 compares the IEEE standard and the Cisco prestandard terminology.

Table 41

IEEE standard and the Cisco Prestandard Terminology

IEEE Standard

Cisco Prestandard

Cisco Standard

CIST regional root

IST master

CIST regional root

CIST internal root path cost

IST master path cost

CIST internal path cost

CIST external root path cost

Root path cost

MSTI regional root

Instance root

MSTI internal root path cost

Root path cost

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 (triggers 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 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.

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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.

Note: If there is a legacy STP switch on the segment, messages are always considered external.

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 this example, 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:

The boundary port is the root port of the CIST regional root—When the CIST instance port is proposed and is in sync,

it can send back an agreement and move to the forwarding state only after all the corresponding MSTI ports are in

sync (and forwarding). The MSTI ports now have a special master role.

The boundary port is not the root port of the CIST regional root—The MSTI ports follow the state and role of the CIST

port. The standard provides less information, and it might be difficult to understand why an MSTI port can be

alternately blocking when it receives no BPDUs (MRecords). In this case, although the boundary role no longer exists,

the show commands identify a port as boundary in the type column of the output.

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 2 on page 6 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 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 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.

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Figure 40

Standard and Prestandard Switch Interoperation

Segment X

MST

Region

Switch A

Switch B

Segment Y

Note: We recommend that you minimize the interaction between standard and prestandard MST implementations.

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 3 on page 6 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, preventing the

bridging loop.

Figure 41

Detecting Unidirectional Link Failure

Superior

Switch

BPDU

Inferior BPDU,

Designated + Learning bit set

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. A switch might also 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.

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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.

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).

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 Configuring STP, page 1. Then the RSTP assigns one of these port roles to

individual ports:

Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.

Designated port—Connects to the designated switch, which incurs the lowest path cost when forwarding packets

from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the

designated port.

Alternate port—Offers an alternate path toward the root switch to that provided by the current root port.

Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the spanning tree. A

backup port can exist only when two ports are connected in a loopback by a point-to-point link or when a switch

has two or more connections to a shared LAN segment.

Disabled port—Has no role within the operation of the spanning tree.

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 IEEE 802.1D). The port state controls the operation of the forwarding and

learning processes. Table 42 on page 357 provides a comparison of IEEE 802.1D and RSTP port states.

Table 42

EEE 802.1D and RSTP Port States

Operational Status

STP Port State

RSTP Port State

Is Port Included in the

(IEEE 802.1D)

Active Topology?

Enabled

Blocking

Discarding

Enabled

Listening

Discarding

Enabled

Learning

Yes

Enabled

Forwarding

Yes

Disabled

Discarding

To be consistent with Cisco STP implementations, this guide defines the port state as blocking instead of discarding.

Designated ports start in the listening state.

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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:

Edge ports—If you configure a port as an edge port on an RSTP switch by using the spanning-tree portfast interface

configuration command, the edge port immediately transitions to the forwarding state. An edge port is the same as

a Port Fast-enabled port, and you should enable it only on ports that connect to a single end station.

Root ports—If the RSTP selects a new root port, it blocks the old root port and immediately transitions the new root

port to the forwarding state.

Point-to-point links—If you connect a port to another port through a point-to-point link and the local port becomes

a designated port, it negotiates a rapid transition with the other port by using the proposal-agreement handshake to

ensure a loop-free topology.

As shown in Figure 4 on page 9, 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.

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Figure 42

Proposal and Agreement Handshaking for Rapid Convergence

Switch A

Switch B

Proposal

Designated

Root

Agreement

switch

Designated

Root

switch

Switch C

Proposal

Designated

Root

switch

Agreement

DP = designated port

RP = root port

F = forwarding

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

That port is in the blocking state.

It is an edge port (a port configured to be at the edge of the network).

If a designated 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 that 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 5 on

page 10.

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Figure 43

Sequence of Events During Rapid Convergence

Agreement

1. Proposal

5. Forward

Edge port

2. Block

3. Block

9. Forward

11. Forward

8. Agreement

6. Proposal

7. Proposal

10. Agreement

Root port

Designated port

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

1-byte Version 1 Length field is set to zero, which means that no version 1 protocol information is present. Table 3 shows

the RSTP flag fields.

Table 43

RSTP Flag Fields

Bit

Function

Topology change (TC)

Proposal

2-3:

Port role:

Unknown

Alternate port

Root port

Designated port

Learning

Forwarding

Agreement

Topology change acknowledgement (TCA)

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.

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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 IEEE 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 switch 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 IEEE 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 switch 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.

Detection—Unlike IEEE 802.1D in which any transition between the blocking and the forwarding state causes a

topology change, only transitions from the blocking to the forwarding state cause a topology change with RSTP (only

an increase in connectivity is considered a topology change). State changes on an edge port do not cause a topology

change. When an RSTP switch detects a topology change, it deletes the learned information on all of its nonedge

ports except on those from which it received the TC notification.

Notification—Unlike IEEE 802.1D, which uses TCN BPDUs, the RSTP does not use them. However, for IEEE 802.1D

interoperability, an RSTP switch processes and generates TCN BPDUs.

Acknowledgement—When an RSTP switch receives a TCN message on a designated port from an IEEE 802.1D

switch, it replies with an IEEE 802.1D configuration BPDU with the TCA bit set. However, if the TC-while timer (the

same as the topology-change timer in IEEE 802.1D) is active on a root port connected to an IEEE 802.1D switch and

a configuration BPDU with the TCA bit set is received, the TC-while timer is reset.

This behavior is only required to support IEEE 802.1D switches. The RSTP BPDUs never have the TCA bit set.

Propagation—When an RSTP switch receives a TC message from another switch through a designated or root port,

it propagates the change to all of its nonedge, designated ports and to the root port (excluding the port on which it

is received). The switch starts the TC-while timer for all such ports and flushes the information learned on them.

Protocol migration—For backward compatibility with IEEE 802.1D switches, RSTP selectively sends IEEE 802.1D

configuration BPDUs and TCN BPDUs on a per-port basis.

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.

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If the switch receives an IEEE 802.1D BPDU after the port 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.

Default MSTP Settings

Table 44

Default MSTP Settings

Feature

Default Setting

Spanning-tree mode

PVST+ (Rapid PVST+ and MSTP are disabled)

Switch priority (configurable on a per-CIST port basis)

32768

Spanning-tree port priority (configurable on a per-CIST port basis)

128

Spanning-tree port cost (configurable on a per-CIST port basis)

1000 Mbps: 4

100 Mbps: 19

10 Mbps: 100

Hello time

2 seconds

Forward-delay time

15 seconds

Maximum-aging time

20 seconds

Maximum hop count

20 hops

MSTP Configuration Guidelines

These are the configuration guidelines for MSTP:

When you enable MST by using the spanning-tree mode mst global configuration command, RSTP is automatically

enabled.

For two or more switches to be in the same MST region, they must have the same VLAN-to-instance map, the same

configuration revision number, and the same name.

The switch supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance

is unlimited.

PVST+, rapid PVST+, and MSTP are supported, but only one version can be active at any time. (For example, all

VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For more information, see “Spanning-Tree

Interoperability and Backward Compatibility” section on page 10.

VTP propagation of the MST configuration is not supported. However, you can manually configure the MST

configuration (region name, revision number, and VLAN-to-instance mapping) on each switch within the MST region

by using the command-line interface (CLI) or through the SNMP support.

For load balancing across redundant paths in the network to work, all VLAN-to-instance mapping assignments must

match; otherwise, all traffic flows on a single link.

All MST boundary ports must be forwarding for load balancing between a PVST+ and an MST cloud or between a

rapid-PVST+ and an MST cloud. For this to occur, the IST master of the MST cloud should also 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

PVST+ or rapid-PVST+ cloud. You might have to manually configure the switches in the clouds.

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Information About Configuring MSTP

Partitioning the network into a large number of regions is not recommended. However, if this situation is unavoidable,

we recommend that you partition the switched LAN into smaller LANs interconnected by routers or non-Layer 2

devices.

For configuration information about UplinkFast and BackboneFast, see “Information About Configuring the Optional

Spanning-Tree Features” section on page 1.

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 only support

up to 65 spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.

Root Switch

The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A switch 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

switch 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 1 on page 4.)

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.

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.

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Configuring MSTP

How to Configure MSTP

Port Priority

If a loop occurs, the MSTP uses the port priority when selecting an interface to put into the forwarding state. You can

assign higher priority values (lower numerical values) to interfaces 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.

Path Cost

The MSTP path cost default value is derived from the media speed of an interface. 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 interfaces 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.

Link Type to Ensure Rapid Transitions

If you connect a port to another 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, page 8.

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.

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.

Restarting the Protocol Migration Process

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 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 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. 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.

How to Configure MSTP

Specifying the MST Region Configuration and Enabling MSTP

This task is required.

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Configuring MSTP

How to Configure MSTP

Command

Purpose

configure terminal

Enters global configuration mode.

spanning-tree mst configuration

Enters MST configuration mode.

instance instance-id vlan vlan-range

Maps VLANs to an MST instance.

instance-id—range is 0 to 4096.

vlan vlan-range—range is 1 to 4096.

When you map VLANs to an MST instance, the mapping is

incremental, and the VLANs specified in the command are added to

or removed from the VLANs that were previously mapped.

To specify a VLAN range, use a hyphen; for example, instance 1 vlan

1-63 maps VLANs 1 through 63 to MST instance 1.

To specify a VLAN series, use a comma; for example, instance 1 vlan 10,

20, 30 maps VLANs 10, 20, and 30 to MST instance 1.

name name

Specifies the configuration name. The name string has a maximum length

of 32 characters and is case sensitive.

revision version

Specifies the configuration revision number. The range is 0 to 65535.

show pending

Verifies your configuration by displaying the pending configuration.

exit

Applies all changes, and returns to global configuration mode.

spanning-tree mode mst

Enables MSTP. RSTP is also enabled.

Caution: Changing spanning-tree modes can disrupt traffic because

all spanning-tree instances are stopped for the previous mode and

restarted in the new mode.

You cannot run both MSTP and PVST+ or both MSTP and rapid PVST+ at

the same time.

end

Returns to privileged EXEC mode.

Configuring the Root Switch

Before You Begin

After configuring the switch as the root switch, we recommend that you avoid manually configuring the hello time,

forward-delay time, and maximum-age time through the spanning-tree mst hello-time, spanning-tree mst

forward-time, and the spanning-tree mst max-age global configuration commands.

This task is optional.

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Cisco Industrial Ethernet 4000, 4010 and 5000 Switch Software. Configuration Guide (2022) - page 14