Multiple Spanning Tree (MST), which is the IEEE 802.1s standard,
allows you to assign two or more VLANs to a spanning tree instance. MST is not
the default spanning tree mode; Rapid per VLAN Spanning Tree (Rapid PVST+) is
the default mode. MST instances with the same name, revision number, and
VLAN-to-instance mapping combine to form an MST region. The MST region appears
as a single bridge to spanning tree configurations outside the region. MST
fails over to IEEE 802.1D Spanning Tree Protocol (STP) when it receives an
802.1D message from a neighboring switch.
Note
Spanning tree is used to refer to IEEE 802.1w and IEEE 802.1s. If
the text is discussing the IEEE 802.1D Spanning Tree Protocol, 802.1D is stated
specifically.
This chapter describes how to configure MST on Cisco Nexus 5000 Series switches. It contains the following sections:
You must enable MST; Rapid PVST+ is the default spanning tree mode.
MST maps multiple VLANs into a spanning tree instance, with each
instance having a spanning tree topology independent of other spanning tree
instances. This architecture provides multiple forwarding paths for data
traffic, enables load balancing, and reduces the number of STP instances
required to support a large number of VLANs. MST improves the fault tolerance
of the network because a failure in one instance (forwarding path) does not
affect other instances (forwarding paths).
MST provides rapid convergence through explicit handshaking as each
MST instance uses the IEEE 802.1w standard, which eliminates the 802.1D
forwarding delay and quickly transitions root bridge ports and designated ports
to the forwarding state.
MAC address reduction is always enabled while you are using MST. You
cannot disable this feature.
MST improves spanning tree operation and maintains backward
compatibility with these STP versions:
Original 802.1D spanning tree
Rapid per-VLAN spanning tree (Rapid PVST+)
IEEE 802.1w defined the Rapid Spanning Tree Protocol (RSTP) and
was incorporated into IEEE 802.1D.
IEEE 802.1s defined MST and was incorporated into IEEE 802.1Q.
MST Regions
To allow switches to participate in 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 is an MST region. An MST region is a linked group of MST bridges
with the same MST configuration.
The MST configuration controls the MST region to which each switch
belongs. The configuration includes the name of the region, the revision
number, and the MST VLAN-to-instance assignment map.
A region can have one or multiple members with the same MST
configuration. Each member must be capable of processing 802.1w bridge protocol
data units (BPDUs). There is no limit to the number of MST regions in a
network.
Each region can support up to 65 MST instances (MSTIs). Instances are
identified by any number in the range from 1 to 4094. The system reserves
Instance 0 for a special instance, which is the IST. You can assign a VLAN to
only one MST instance at a time.
The MST region appears as a single bridge to adjacent MST regions and
to other Rapid PVST+ regions and 802.1D spanning tree protocols.
Note
We recommend that you do not partition the network into a large
number of regions.
MST BPDUs
Each region has only one MST BPDU, and that BPDU carries an M-record
for each MSTI within the region (see the following figure). Only the IST sends
BPDUs for the MST region; all M-records are encapsulated in that one BPDU that
the IST sends. Because the MST BPDU carries information for all instances, the
number of BPDUs that need to be processed to support MSTIs is significantly
reduced.
Figure 1. MST BPDU with M-Records for MSTIs
MST Configuration Information
The MST configuration that must be identical on all switches within a single MST region is configured by the user.
You can configure the following three parameters of the MST configuration:
Name—32-character string, null padded and null terminated, identifying the MST region
Revision number—Unsigned 16-bit number that identifies the revision of the current MST configuration
Note
You must set the revision number when required as part of the MST configuration. The revision number is not incremented automatically each time that the MST configuration is committed.
MST configuration table—4096-element table that associates each of the potential 4094 VLANs supported to a given instance with the first (0) and last element (4095) set to 0. The value of element number X represents the instance to which VLAN X is mapped.
Caution
When you change the VLAN-to-MSTI mapping, the system restarts MST.
MST BPDUs contain these three configuration parameters. An MST bridge accepts an MST BPDU into its own region only if these three configuration parameters match exactly. If one configuration attribute differs, the MST bridge considers the BPDU to be from another MST region.
IST, CIST, and CST
IST, CIST, and CST Overview
Unlike Rapid PVST+, in which all the STP instances are independent,
MST establishes and maintains IST, CIST, and CST spanning trees, as follows:
An IST is the spanning tree that runs in an MST region.
MST establishes and maintains additional spanning trees within each
MST region; these spanning trees are called, multiple spanning tree instances
(MSTIs).
Instance 0 is a special instance for a region, known as the IST. The
IST always exists on all ports; you cannot delete the IST, or Instance 0. By
default, all VLANs are assigned to the IST. All other MST instances are
numbered from 1 to 4094.
The IST is the only STP instance that sends and receives BPDUs. All of
the other MSTI information is contained in MST records (M-records), which are
encapsulated within MST BPDUs.
All MSTIs within the same region share the same protocol timers, but
each MSTI has its own topology parameters, such as the root bridge ID, the root
path cost, and so forth.
An MSTI is local to the region; for example, MSTI 9 in region A is
independent of MSTI 9 in region B, even if regions A and B are interconnected.
The CST interconnects the MST regions and any instance of 802.1D
and 802.1w STP that may be running on the network. The CST is the one STP
instance for the entire bridged network and encompasses all MST regions and
802.1w and 802.1D instances.
A CIST is a collection of the ISTs in each MST region. The CIST is
the same as an IST inside an MST region, and the same as a CST outside an MST
region.
The spanning tree computed in an MST region appears as a subtree in
the CST that encompasses the entire switched domain. The CIST is formed by the
spanning tree algorithm running among switches that support the 802.1w, 802.1s,
and 802.1D standards. The CIST inside an MST region is the same as the CST
outside a region.
Spanning Tree Operation Within an MST Region
The IST connects all the MST switches in a region. When the IST
converges, the root of the IST becomes the CIST regional root. The CIST
regional root is also the CIST root if there is only one region in the network.
If the CIST root is outside the region, the protocol selects one of the MST
switches at the boundary of the region as the CIST regional root.
When an MST switch initializes, it sends BPDUs that identify itself as
the root of the CIST and the CIST regional root, with both the path costs to
the CIST root and to the CIST regional root set to zero. The switch also
initializes all of its MSTIs 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 the information that is currently stored for the port,
it relinquishes its claim as the CIST regional root.
During initialization, an MST region might have many subregions, each
with its own CIST regional root. As switches receive superior IST information
from a neighbor in the same region, they leave their old subregions and join
the new subregion that contains the true CIST regional root. This action causes
all subregions to shrink except for the subregion that contains the true CIST
regional root.
All switches in the MST region must agree on the same CIST regional
root. Any two switches in the region will only synchronize their port roles for
an MSTI if they converge to a common CIST regional root.
Spanning Tree Operations Between MST Regions
If you have multiple regions or 802.1 w or 802.1D STP instances within
a network, MST establishes and maintains the CST, which includes all MST
regions and all 802.1w and 802.1D STP switches in the network. The MSTIs
combine with the IST at the boundary of the region to become the CST.
The IST connects all the MST switches in the region and appears as a
subtree in the CIST that encompasses the entire switched domain. The root of
the subtree is the CIST regional root. The MST region appears as a virtual
switch to adjacent STP switches and MST regions.
The following figure shows a network with three MST regions and an
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.
Figure 2. MST Regions, CIST Regional Roots, and CST Root
Only the CST instance sends and receives BPDUs. MSTIs add their
spanning tree information into the BPDUs (as M-records) to interact with
neighboring switches and compute the final spanning tree topology. Because of
this, the spanning tree parameters related to the BPDU transmission (for
example, hello time, forward time, max-age, and max-hops) are configured only
on the CST instance but affect all MSTIs. You can configure the parameters
related to the spanning tree topology (for example, the switch priority, the
port VLAN cost, and the port VLAN priority) on both the CST instance and the
MSTI.
MST switches use Version 3 BPDUs or 802.1D STP BPDUs to communicate
with 802.1D-only switches. MST switches use MST BPDUs to communicate with MST
switches.
MST Terminology
MST naming conventions include identification of some internal or
regional parameters. These parameters are used only within an MST region,
compared to external parameters that are used throughout the whole network.
Because the CIST is the only spanning tree instance that spans the whole
network, only the CIST parameters require the external qualifiers and not the
internal or regional qualifiers. The MST terminology is as follows:
The CIST root is the root bridge for the CIST, which is the unique
instance that spans the whole network.
The CIST external root path cost is the cost to the CIST root.
This cost is left unchanged within an MST region. An MST region looks like a
single switch to 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.
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 bridge for the IST.
The CIST internal root path cost is the cost to the CIST regional
root in a region. This cost is only relevant to the IST, instance 0.
Hop Count
MST does not use the message-age and maximum-age information in the
configuration BPDU to compute the STP topology inside the MST region. Instead,
the protocol uses the path cost to the root and a hop-count mechanism similar
to the IP time-to-live (TTL) mechanism.
By using the
spanning-tree mst max-hops global configuration
command, you can configure the maximum hops inside the region and apply it to
the IST and all MST instances in that region.
The hop count achieves the same result as the message-age information
(triggers a reconfiguration). The root bridge of the instance always sends a
BPDU (or M-record) with a cost of 0 and the hop count set to the maximum value.
When a 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
that 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 802.1w portion of
the BPDU remain the same throughout the region (only on the IST), and the same
values are propagated by the region-designated ports at the boundary.
You configure a maximum aging time as the number of seconds that a
switch waits without receiving spanning tree configuration messages before
attempting a reconfiguration.
Boundary Ports
A boundary port is a port that connects to a LAN, the designated
bridge of which is either a bridge with a different MST configuration (and so,
a separate MST region) or a Rapid PVST+ or 802.1D STP bridge. A designated port
knows that it is on the boundary if it detects an STP bridge or receives an
agreement proposal from an MST bridge with a different configuration or a Rapid
PVST+ bridge. This definition allows two ports that are internal to a region to
share a segment with a port that belongs to a different region, creating the
possibility of receiving both internal and external messages on a port (see the
following figure).
Figure 3. MST Boundary Ports
At the boundary, the roles of MST ports do not matter; the system
forces their state to be the same as the IST port state. If the boundary flag
is set for the port, the MST port-role selection process assigns a port role to
the boundary and assigns the same state as the state of the IST port. The IST
port at the boundary can take up any port role except a backup port role.
Detecting Unidirectional Link Failure
Currently, this feature is not present in the IEEE MST standard, but
it is included in the standard-compliant implementation. 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 a discarding state because disrupting connectivity in case of
inconsistency is preferable to opening a bridging loop.
The following figure shows a unidirectional link failure that
typically creates a bridging loop. Switch A is the root bridge, and its BPDUs
are lost on the link leading to switch B. Rapid PVST+ (802.1w) and MST BPDUs
include the role and state of the sending port. With this information, switch A
can detect that switch B does not react to the superior BPDUs that it sends and
that switch B is the designated, not root port. As a result, switch A blocks
(or keeps blocking) its port, which prevents the bridging loop. The block is
shown as an STP dispute.
Figure 4. Detecting a Unidirectional Link Failure
Port Cost and Port Priority
Spanning tree uses port costs to break a tie for the designated port. Lower values indicate lower port costs, and spanning tree chooses the least costly path. Default port costs are taken from the bandwidth of the interface, as follows:
10 Mbps—2,000,000
100 Mbps—200,000
1 Gigabit Ethernet—20,000
10 Gigabit Ethernet—2,000
You can configure the port costs in order to influence which port is chosen.
Note
MST always uses the long path cost calculation method, so the range of valid values is between 1 and 200,000,000.
The system uses port priorities to break ties among ports with the same cost. A lower number indicates a higher priority. The default port priority is 128. You can configure the priority to values between 0 and 224, in increments of 32.
Interoperability with IEEE 802.1D
A switch that runs MST supports a built-in protocol migration feature
that enables it to interoperate with 802.1D STP switches. If this switch
receives an 802.1D configuration BPDU (a BPDU with the protocol version set to
0), it sends only 802.1D BPDUs on that port. In addition, an MST switch can
detect that a port is at the boundary of a region when it receives an 802.1D
BPDU, an MST BPDU (Version 3) associated with a different region, or an 802.1w
BPDU (Version 2).
However, the switch does not automatically revert to the MST mode if
it no longer receives 802.1D BPDUs because it cannot detect whether the 802.1D
switch has been removed from the link unless the 802.1D 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), enter the
clear spanning-tree detected-protocols command.
All Rapid PVST+ switches (and all 8021.D STP switches) on the link
can process MST BPDUs as if they are 802.1w BPDUs. MST switches can send either
Version 0 configuration and topology change notification (TCN) BPDUs or
Version 3 MST BPDUs on a boundary port. A boundary port connects to a LAN, the
designated switch of which is either a single spanning tree switch or a switch
with a different MST configuration.
Note
MST interoperates with the Cisco prestandard MSTP whenever it
receives prestandard MSTP on an MST port; no explicit configuration is
necessary.
Interoperability with Rapid PVST+: Understanding PVST
Simulation
MST interoperates with Rapid PVST+ with no need for user
configuration. The PVST simulation feature enables this seamless
interoperability.
Note
PVST simulation is enabled by default. That is, by default, all
interfaces on the switch interoperate between MST and Rapid PVST+.
However, you may want to control the connection between MST and Rapid
PVST+ to protect against accidentally connecting an MST-enabled port to a Rapid
PVST+-enabled port. Because Rapid PVST+ is the default STP mode, you may
encounter many Rapid PVST+-enabled connections.
Disabling Rapid PVST+ simulation, which can be done per port or
globally for the entire switch, moves the MST-enabled port to the blocking
state once it detects it is connected to a Rapid PVST+-enabled port. This port
remains in the inconsistent state until the port stops receiving Rapid
PVST+/SSTP BPDUs, and then the port resumes the normal STP transition process.
Configuring MST
MST Configuration Guidelines
When configuring MST, follow these guidelines:
When you work with private VLANs, enter the
private-vlan synchronize command to map the
secondary VLANs to the same MST instance as the primary VLAN.
When you are in the MST configuration mode, the following
guidelines apply:
Each command reference line creates its pending regional
configuration.
The pending region configuration starts with the current
region configuration.
To leave the MST configuration mode without committing any
changes, enter the
abort command.
To leave the MST configuration mode and commit all the changes
that you made before you left the mode, enter the
exit command.
Enabling MST
You must enable MST; Rapid PVST+ is the default.
Caution
Changing the spanning tree mode disrupts traffic because all
spanning tree instances are stopped for the previous mode and started for the
new mode.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mode mst
Enables MST on the switch.
Step 3
switch(config)#
no spanning-tree mode mst
(Optional)
Disables MST on the switch and returns you to Rapid PVST+.
This example shows how to enable MST on the switch:
switch# configure terminal
switch(config)# spanning-tree mode mst
Note
Because STP is enabled by default, entering a
show running-config command to view the resulting
configuration does not display the command that you entered to enable STP.
Entering MST Configuration Mode
You enter MST configuration mode to configure the MST name,
VLAN-to-instance mapping, and MST revision number on the switch.
For two or more switches to be in the same MST region, they must have
the identical MST name, VLAN-to-instance mapping, and MST revision number.
Note
Each command reference line creates its pending regional
configuration in MST configuration mode. In addition, the pending region
configuration starts with the current region configuration.
When you are working in MST configuration mode, note the difference
between the
exit and
abort commands.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mst configuration
Enters MST configuration mode on the system. You must be in the
MST configuration mode to assign the MST configuration parameters, as follows:
MST name
Instance-to-VLAN mapping
MST revision number
Synchronize primary and secondary VLANs in private VLANs
Step 3
switch(config-mst)#
exit or
switch(config-mst)#
abort
The first form commits all the changes and exits MST
configuration mode.
The second form exits the MST configuration mode without
committing any of the changes.
Step 4
switch(config)#
no spanning-tree mst configuration
(Optional)
Returns the MST region configuration to the following default
values:
The region name is an empty string.
No VLANs are mapped to any MST instance (all VLANs are mapped
to the CIST instance).
The revision number is 0.
Specifying the MST Name
You configure a region name on the bridge. For two or more bridges to
be in the same MST region, they must have the identical MST name,
VLAN-to-instance mapping, and MST revision number.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mst configuration
Enters MST configuration submode.
Step 3
switch(config-mst)#
namename
Specifies the name for MST region. The
name string has a maximum length of 32
characters and is case-sensitive. The default is an empty string.
This example shows how to set the name of the MST region:
switch# configure terminal
switch(config)# spanning-tree mst configuration
switch(config-mst)# name accounting
Specifying the MST Configuration Revision Number
You configure the revision number on the bridge. For two or more
bridges to be in the same MST region, they must have the identical MST name,
VLAN-to-instance mapping, and MST revision number.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mst configuration
Enters MST configuration submode.
Step 3
switch(config-mst)#
revisionversion
Specifies the revision number for the MST region. The range is
from 0 to 65535, and the default value is 0.
This example shows how to configure the revision number of the MSTI
region for 5:
switch# configure terminal
switch(config)# spanning-tree mst configuration
switch(config-mst)# revision 5
Specifying the Configuration on an MST Region
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 MST name.
A region can have one member or multiple members with the same MST
configuration; each member must be capable of processing IEEE 802.1w RSTP
BPDUs. There is no limit to the number of MST regions in a network, but each
region can support only up to 65 MST instances. You can assign a VLAN to only
one MST instance at a time.
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, enter a hyphen; for example, enter the
instance 1 vlan 1-63 command to map VLANs
1 through 63 to MST instance 1.
To specify a VLAN series, enter a comma; for example, enter the
instance 1 vlan 10, 20, 30 command to map
VLANs 10, 20, and 30 to MST instance 1.
Step 4
switch(config-mst)#
name
name
Specifies the instance name. The
name string has a maximum length of 32
characters and is case sensitive.
Step 5
switch(config-mst)#
revision
version
Specifies the configuration revision number. The range is from 0
to 65535.
To return to defaults, do the following:
To return to the default MST region configuration settings, enter
the
no spanning-tree mst configuration
configuration command.
To return to the default VLAN-to-instance map, enter the
no instanceinstance-idvlanvlan-range MST configuration command.
To return to the default name, enter the
no name MST configuration command.
To return to the default revision number, enter the
no revision MST configuration command.
To reenable Rapid PVST+, enter the
no spanning-tree mode or the
spanning-tree mode rapid-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 configuration
switch(config-mst)# instance 1 vlan 10-20
switch(config-mst)# name region1
switch(config-mst)# revision 1
switch(config-mst)# show pending
Pending MST configuration
Name [region1]
Revision 1
Instances configured 2
Instance Vlans Mapped
-------- ---------------------
0 1-9,21-4094
1 10-20
-------------------------------
Mapping and Unmapping VLANs to MST Instances
Caution
When you change the VLAN-to-MSTI mapping, the system restarts MST.
Note
You cannot disable an MSTI.
For two or more bridges to be in the same MST region, they must have
the identical MST name, VLAN-to-instance mapping, and MST revision number.
Instance 0 is reserved for the IST for each MST region.
For
vlan-range the range is from 1 to
4094.
When you map VLANs to an MSTI, the mapping is incremental, and
the VLANs specified in the command are added to or removed from the VLANs that
were previously mapped.
Step 4
switch(config-mst)#
no instanceinstance-idvlanvlan-range
Deletes the specified instance and returns the VLANs to the
default MSTI, which is the CIST.
This example shows how to map VLAN 200 to MSTI 3:
switch# configure terminal
switch(config)# spanning-tree mst configuration
switch(config-mst)# instance 3 vlan 200
Mapping Secondary VLANs to Same MSTI as Primary VLANs for Private
VLANs
When you are working with private VLANs on the system, all secondary
VLANs must be in the same MSTI and their associated primary VLAN.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mst configuration
Enters MST configuration submode.
Step 3
switch(config-mst)#
private-vlan synchronize
Automatically maps all secondary VLANs to the same MSTI and their
associated primary VLAN for all private VLANs.
This example shows how to automatically map all the secondary VLANs to
the same MSTI as their associated primary VLANs in all private VLANs:
switch# configure terminal
switch(config)# spanning-tree mst configuration
switch(config-mst)# private-vlan synchronize
Configuring the Root Bridge
You can configure the switch to become the root bridge.
Note
The root bridge for each MSTI should be a backbone or distribution
switch. Do not configure an access switch as the spanning tree primary root
bridge.
Enter the
diameter keyword, which is available only for MSTI 0
(or the IST), to specify the network diameter (that is, the maximum number of
hops between any two end stations in the 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 enter the
hello keyword to override the automatically
calculated hello time.
Note
With the switch configured as the root bridge, do not manually
configure the hello time, forward-delay time, and maximum-age time using the
spanning-tree mst hello-time,
spanning-tree mst forward-time, and
spanning-tree mst max-age global configuration
commands.
Configures a switch as the root bridge as follows:
For
instance-id, you can specify a single
instance, a range of instances separated by a hyphen, or a series of instances
separated by a comma. The range is from 1 to 4094.
For
diameternet-diameter, specify the maximum number of
hops between any two end stations. The default is 7. This keyword is available
only for MST instance 0.
For
hello-timeseconds, specify the interval in seconds
between the generation of configuration messages by the root bridge. The range
is from 1 to 10 seconds; the default is 2 seconds.
Step 3
switch(config)#
no spanning-tree mstinstance-idroot
(Optional)
Returns the switch priority, diameter, and hello time to default
values.
This example shows how to configure the switch as the root switch for
MSTI 5:
switch# configure terminal
switch(config)# spanning-tree mst 5 root primary
Configuring a Secondary Root Bridge
You can execute this command on more than one switch to configure
multiple backup root bridges. Enter the same network diameter and hello-time
values that you used when you configured the primary root bridge with the
spanning-tree mst root primary configuration
command.
Configures a switch as the secondary root bridge as follows:
For
instance-id, you can specify a single
instance, a range of instances separated by a hyphen, or a series of instances
separated by a comma. The range is from 1 to 4094.
For
diameternet-diameter, specify the maximum number of
hops between any two end stations. The default is 7. This keyword is available
only for MST instance 0.
For
hello-timeseconds, specify the interval in seconds
between the generation of configuration messages by the root bridge. The range
is from 1 to 10 seconds; the default is 2 seconds.
Step 3
switch(config)#
no spanning-tree mstinstance-idroot
(Optional)
Returns the switch priority, diameter, and hello-time to default
values.
This example shows how to configure the switch as the secondary root
switch for MSTI 5:
If a loop occurs, MST uses the port priority when selecting an
interface to put into the forwarding state. You can assign lower priority
values to interfaces that you want selected first and higher priority values to
the interface that you want selected last. If all interfaces have the same
priority value, MST puts the interface with the lowest interface number in the
forwarding state and blocks the other interfaces.
For
instance-id, you can specify a single MSTI, a range of
MSTIs separated by a hyphen, or a series of MSTIs separated by a comma. The
range is from 1 to 4094.
For
priority, the range is 0 to 224 in increments of
32. The default is 128. A lower number indicates a higher priority.
The priority values are 0, 32, 64, 96, 128, 160, 192, and 224. The
system rejects all other values.
This example shows how to set the MST interface port priority for MSTI
3 on Ethernet port 3/1 to 64:
You can only apply this command to a physical Ethernet interface.
Configuring the Port Cost
The MST path cost default value is derived from the media speed of an
interface. If a loop occurs, MST uses the cost when selecting an interface to
put in the forwarding state. You can assign lower cost values to interfaces
that you want selected first and higher cost to interfaces values that you want
selected last. If all interfaces have the same cost value, MST puts the
interface with the lowest interface number in the forwarding state and blocks
the other interfaces.
If a loop occurs, MST uses the path cost when selecting an
interface to place into the forwarding state. A lower path cost represents
higher-speed transmission as follows:
For
instance-id, you can specify a single
instance, a range of instances separated by a hyphen, or a series of instances
separated by a comma. The range is from 1 to 4094.
For
cost, the range is from 1 to 200000000. The
default value is
auto, which is derived from the media speed
of the interface.
This example shows how to set the MST interface port cost on Ethernet
3/1 for MSTI 4:
You can configure the switch priority for an MST instance so that it
is more likely that the specified switch is chosen as the root bridge.
Note
Exercise care when using this command. For most situations, we
recommend that you enter the
spanning-tree mst root primary and the
spanning-tree mst root secondary global
configuration commands to modify the switch priority.
For
instance-id, you can specify a single
instance, a range of instances separated by a hyphen, or a series of instances
separated by a comma. The range is from 1 to 4094.
For
priority, the range is from 0 to 61440 in
increments of 4096; the default is 32768. A lower number indicates that the
switch will most likely be chosen as the root bridge.
Priority values are 0, 4096, 8192, 12288, 16384, 20480, 24576,
28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. The system
rejects all other values.
This example shows how to configure the priority of the bridge to 4096
for MSTI 5:
switch# configure terminal
switch(config)# spanning-tree mst 5 priority 4096
Configuring the Hello Time
You can configure the interval between the generation of configuration
messages by the root bridge for all instances on the switch by changing the
hello time.
Note
Exercise care when using this command. For most situations, we
recommend that you enter the
spanning-tree mstinstance-idroot primary and the
spanning-tree mstinstance-idroot secondary configuration commands to modify
the hello time.
Configures the hello time for all MST instances. The hello time is
the interval between the generation of configuration messages by the root
bridge. These messages mean that the switch is alive. For
seconds, the range is from 1 to 10, and the
default is 2 seconds.
This example shows how to configure the hello time of the switch to 1
second:
switch# configure terminal
switch(config)# spanning-tree mst hello-time 1
Configuring the Forwarding-Delay Time
You can set the forward delay timer for all MST instances on the
switch with one command.
Configures the forward time for all MST instances. The forward
delay is the number of seconds that a port waits before changing from its
spanning tree blocking and learning states to the forwarding state. For
seconds, the range is from 4 to 30, and the
default is 15 seconds.
This example shows how to configure the forward-delay time of the
switch to 10 seconds:
switch# configure terminal
switch(config)# spanning-tree mst forward-time 10
Configuring the Maximum-Aging Time
The maximum-aging timer is the number of seconds that a switch waits
without receiving spanning tree configuration messages before attempting a
reconfiguration.
You set the maximum-aging timer for all MST instances on the switch
with one command (the maximum age time only applies to the IST).
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
spanning-tree mst max-ageseconds
Configures the maximum-aging time for all MST instances. The
maximum-aging time is the number of seconds that a switch waits without
receiving spanning tree configuration messages before attempting a
reconfiguration. For
seconds, the range is from 6 to 40, and the
default is 20 seconds.
This example shows how to configure the maximum-aging timer of the
switch to 40 seconds:
switch# configure terminal
switch(config)# spanning-tree mst max-age 40
Configuring the Maximum-Hop Count
MST uses the path cost to the IST regional root and a hop-count
mechanism similar to the IP time-to-live (TTL) mechanism. You 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).
Specifies the number of hops in a region before the BPDU is
discarded, and the information held for a port is aged. For
hop-count, the range is from 1 to 255, and the
default value is 20 hops.
This example shows how to set the maximum hops to 40:
switch# configure terminal
switch(config)# spanning-tree mst max-hops 40
Configuring PVST Simulation Globally
You can block this automatic feature either globally or per port. You
can enter the global command, and change the PVST simulation setting for the
entire switch while you are in interface command mode.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
no spanning-tree mst simulate pvst global
Disables all interfaces on the switch from automatically
interoperating with connected switch that is running in Rapid PVST+ mode. The
default for this is enabled; that is, by default, all interfaces on the switch
operate seamlessly between Rapid PVST+ and MST.
This example shows how to prevent the switch from automatically
interoperating with a connecting switch that is running Rapid PVST+:
switch# configure terminal
switch(config)# no spanning-tree mst simulate pvst global
Configuring PVST Simulation Per Port
MST interoperates seamlessly with Rapid PVST+. However, to prevent an
accidental connection to a switch that does not run MST as the default STP
mode, you may want to disable this automatic feature. If you disable PVST
simulation, the MST-enabled port moves to the blocking state once it detects it
is connected to a Rapid PVST+-enabled port. This port remains in the
inconsistent state until the port stops receiving BPDUs, and then the port
resumes the normal STP transition process.
You can block this automatic feature either globally or per port.
Rapid connectivity (802.1w standard) is established only on
point-to-point links. By default, the link type is controlled from the duplex
mode of the interface. A full-duplex port is considered to have a
point-to-point connection; a half-duplex port is considered to have a shared
connection.
If you have a half-duplex link physically connected point-to-point to
a single port on a remote switch, you can override the default setting on the
link type and enable rapid transitions.
If you set the link to shared, STP reverts to 802.1D.
Procedure
Command or Action
Purpose
Step 1
switch#
configure terminal
Enters configuration mode.
Step 2
switch(config)#
interfacetypeslot/port
Specifies the interface to configure, and enters interface
configuration mode.
Configures the link type to be either point to point or shared.
The system reads the default value from the switch connection. Half-duplex
links are shared and full-duplex links are point to point. If the link type is
shared, the STP reverts to 802.1D. The default is auto, which sets the link
type based on the duplex setting of the interface.
This example shows how to configure the link type as point to point:
An MST bridge can detect that a port is at the boundary of a region
when it receives a legacy BPDU or an MST BPDU that is associated with a
different region. However, the STP protocol migration cannot determine whether
the legacy switch, which is a switch that runs only IEEE 802.1D, has been
removed from the link unless the legacy switch is the designated switch. Enter
this command to restart the protocol negotiation (force the renegotiation with
neighboring switches) on the entire switch or on specified interfaces.