Consolidated Platform Configuration Guide, Cisco IOS XE 3.7E and Later (Catalyst 3650 Switches)
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Restriction for Optional Spanning-Tree Features
PortFast minimizes the time that interfaces must wait for spanning tree to converge, so it is effective only when used on
interfaces connected to end stations. If you enable PortFast on an interface connecting to another switch, you risk creating
a spanning-tree loop.
You cannot have a switch stack containing a mix of Catalyst 3850 and Catalyst 3650 switches.
Information About Optional Spanning-Tree Features
PortFast
PortFast immediately brings an interface configured as an access or trunk port to the forwarding state from a blocking state,
bypassing the listening and learning states.
Interfaces connected to a single workstation or server should not receive bridge protocol data units (BPDUs). An interface
with PortFast enabled goes through the normal cycle of spanning-tree status changes when the switch is restarted.
You can enable this feature by enabling it on either the interface or on all nontrunking ports.
BPDU Guard
The Bridge Protocol Data Unit
(BPDU) guard feature can be globally enabled on the switch or can be enabled
per port, but the feature operates with some differences.
When you enable BPDU guard at the global level on
PortFast edge-enabled ports, spanning tree shuts down ports that are in a
PortFast edge-operational state if any BPDU is received on them. In a valid
configuration, PortFast edge-enabled ports do not receive BPDUs. Receiving a
BPDU on a Port Fast edge-enabled port means an invalid configuration, such as
the connection of an unauthorized device, and the BPDU guard feature puts the
port in the error-disabled state. When this happens, the switch shuts down the
entire port on which the violation occurred.
When you enable BPDU guard at
the interface level on any port without also enabling the PortFast edge
feature, and the port receives a BPDU, it is put in the error-disabled state.
The BPDU guard feature
provides a secure response to invalid configurations because you must manually
put the interface back in service. Use the BPDU guard feature in a
service-provider network to prevent an access port from participating in the
spanning tree.
BPDU Filtering
The BPDU filtering feature
can be globally enabled on the switch or can be enabled per interface, but the
feature operates with some differences.
Enabling BPDU filtering on
PortFast edge-enabled interfaces at the global level keeps those interfaces
that are in a PortFast edge-operational state from sending or receiving BPDUs.
The interfaces still send a few BPDUs at link-up before the switch begins to
filter outbound BPDUs. You should globally enable BPDU filtering on a switch so
that hosts connected to these interfaces do not receive BPDUs. If a BPDU is
received on a PortFast edge-enabled interface, the interface loses its PortFast
edge-operational status, and BPDU filtering is disabled.
Enabling BPDU filtering on an
interface without also enabling the PortFast edge feature keeps the interface
from sending or receiving BPDUs.
Caution
Enabling BPDU filtering on an
interface is the same as disabling spanning tree on it and can result in
spanning-tree loops.
You can enable the BPDU
filtering feature for the entire switch or for an interface.
UplinkFast
If a switch loses connectivity, it begins using the alternate paths as soon as the spanning tree selects a new root port.
You can accelerate the choice of a new root port when a link or switch fails or when the spanning tree reconfigures itself
by enabling UplinkFast. The root port transitions to the forwarding state immediately without going through the listening
and learning states, as it would with the normal spanning-tree procedures.
When the spanning tree reconfigures the new root port, other interfaces flood the network with multicast packets, one for
each address that was learned on the interface. You can limit these bursts of multicast traffic by reducing the max-update-rate
parameter (the default for this parameter is 150 packets per second). However, if you enter zero, station-learning frames
are not generated, so the spanning-tree topology converges more slowly after a loss of connectivity.
Note
UplinkFast is most useful in wiring-closet switches at the access or edge of the network. It is not appropriate for backbone
devices. This feature might not be useful for other types of applications.
UplinkFast provides fast convergence after a direct link failure and achieves load-balancing between redundant Layer 2 links
using uplink groups. An uplink group is a set of Layer 2 interfaces (per VLAN), only one of which is forwarding at any given
time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports, except for
self-looping ports. The uplink group provides an alternate path in case the currently forwarding link fails.
Cross-Stack UplinkFast
Cross-Stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1 second under normal
network conditions) across a switch stack. During the fast transition, an alternate redundant link on the switch stack is
placed in the forwarding state without causing temporary spanning-tree loops or loss of connectivity to the backbone. With
this feature, you can have a redundant and resilient network in some configurations. CSUF is automatically enabled when you
enable the UplinkFast feature.
CSUF might not provide a fast transition all the time; in these cases, the normal spanning-tree transition occurs, completing
in 30 to 40 seconds. For more information, see Related Topics.
How Cross-Stack UplinkFast Works
Cross-Stack UplinkFast (CSUF) ensures that one link in the stack is elected as the path to the root.
When certain link loss or spanning-tree events occur (described in the following topic), the Fast Uplink Transition Protocol
uses the neighbor list to send fast-transition requests to stack members.
The switch sending the fast-transition request needs to do a fast transition to the forwarding state of a port that it has
chosen as the root port, and it must obtain an acknowledgment from each stack switch before performing the fast transition.
Each switch in the stack decides if the sending switch is a better choice than itself to be the stack root of this spanning-tree
instance by comparing the root, cost, and bridge ID. If the sending switch is the best choice as the stack root, each switch
in the stack returns an acknowledgment; otherwise, it sends a fast-transition request. The sending switch then has not received
acknowledgments from all stack switches.
When acknowledgments are received from all stack switches, the Fast Uplink Transition Protocol on the sending switch immediately
transitions its alternate stack-root port to the forwarding state. If acknowledgments from all stack switches are not obtained
by the sending switch, the normal spanning-tree transitions (blocking, listening, learning, and forwarding) take place, and
the spanning-tree topology converges at its normal rate (2 * forward-delay time + max-age time).
The Fast Uplink Transition Protocol is implemented on a per-VLAN basis and affects only one spanning-tree instance at a time.
Events That Cause Fast Convergence
Depending on the network event or failure, the CSUF fast convergence might or might not occur.
Fast convergence (less than 1 second under normal network conditions) occurs under these circumstances:
The stack-root port link fails.
If two switches in the stack have alternate paths to the root, only one of the switches performs the fast transition.
The failed link, which connects the stack root to the spanning-tree root, recovers.
A network reconfiguration causes a new stack-root switch to be selected.
A network reconfiguration causes a new port on the current stack-root switch to be chosen as the stack-root port.
Note
The fast transition might not occur if multiple events occur simultaneously. For example, if a stack member is powered off,
and at the same time, the link connecting the stack root to the spanning-tree root comes back up, the normal spanning-tree
convergence occurs.
Normal spanning-tree convergence (30 to 40 seconds) occurs under these conditions:
The stack-root switch is powered off, or the software failed.
The stack-root switch, which was powered off or failed, is powered on.
A new switch, which might become the stack root, is added to the stack.
BackboneFast
BackboneFast detects indirect failures in the core of the backbone. BackboneFast is a complementary technology to the UplinkFast
feature, which responds to failures on links directly connected to access switches. BackboneFast optimizes the maximum-age
timer, which controls the amount of time the switch stores protocol information received on an interface. When a switch receives
an inferior BPDU from the designated port of another switch, the BPDU is a signal that the other switch might have lost its
path to the root, and BackboneFast tries to find an alternate path to the root.
BackboneFast starts when a root port or blocked interface on a switch receives inferior BPDUs from its designated switch.
An inferior BPDU identifies a switch that declares itself as both the root bridge and the designated switch. When a switch
receives an inferior BPDU, it means that a link to which the switch is not directly connected (an indirect link) has failed
(that is, the designated switch has lost its connection to the root switch). Under spanning-tree rules, the switch ignores
inferior BPDUs for the maximum aging time (default is 20 seconds).
The switch tries to find if it has an alternate path to the root switch. If the inferior BPDU arrives on a blocked interface,
the root port and other blocked interfaces on the switch become alternate paths to the root switch. (Self-looped ports are
not considered alternate paths to the root switch.) If the inferior BPDU arrives on the root port, all blocked interfaces
become alternate paths to the root switch. If the inferior BPDU arrives on the root port and there are no blocked interfaces,
the switch assumes that it has lost connectivity to the root switch, causes the maximum aging time on the root port to expire,
and becomes the root switch according to normal spanning-tree rules.
If the switch has alternate paths to the root switch, it uses these alternate paths to send a root link query (RLQ) request.
The switch sends the RLQ request on all alternate paths to learn if any stack member has an alternate root to the root switch
and waits for an RLQ reply from other switches in the network and in the stack. The switch sends the RLQ request on all alternate
paths and waits for an RLQ reply from other switches in the network.
When a stack member receives an RLQ reply from a nonstack member on a blocked interface and the reply is destined for another
nonstacked switch, it forwards the reply packet, regardless of the spanning-tree interface state.
When a stack member receives an RLQ reply from a nonstack member and the response is destined for the stack, the stack member
forwards the reply so that all the other stack members receive it.
If the switch discovers that it still has an alternate path to the root, it expires the maximum aging time on the interface
that received the inferior BPDU. If all the alternate paths to the root switch indicate that the switch has lost connectivity
to the root switch, the switch expires the maximum aging time on the interface that received the RLQ reply. If one or more
alternate paths can still connect to the root switch, the switch makes all interfaces on which it received an inferior BPDU
its designated ports and moves them from the blocking state (if they were in the blocking state), through the listening and
learning states, and into the forwarding state.
EtherChannel Guard
You can use EtherChannel guard to detect an EtherChannel misconfiguration between the switch and a connected device. A misconfiguration
can occur if the switch interfaces are configured in an EtherChannel, but the interfaces on the other device are not. A misconfiguration
can also occur if the channel parameters are not the same at both ends of the EtherChannel.
If the switch detects a misconfiguration on the other device, EtherChannel guard places the switch interfaces in the error-disabled
state, and displays an error message.
Root Guard
If a switch outside the SP network becomes the root switch, the interface is blocked (root-inconsistent state), and spanning
tree selects a new root switch. The customer’s switch does not become the root switch and is not in the path to the root.
If the switch is operating in multiple spanning-tree (MST) mode, root guard forces the interface to be a designated port.
If a boundary port is blocked in an internal spanning-tree (IST) instance because of root guard, the interface also is blocked
in all MST instances. A boundary port is an interface that connects to a LAN, the designated switch of which is either an
IEEE 802.1D switch or a switch with a different MST region configuration.
Root guard enabled on an interface applies to all the VLANs to which the interface belongs. VLANs can be grouped and mapped
to an MST instance.
Caution
Misuse of the root guard feature can cause a loss of connectivity.
Loop Guard
You can use loop guard to prevent alternate or root ports from becoming designated ports because of a failure that leads to
a unidirectional link. This feature is most effective when it is enabled on the entire switched network. Loop guard prevents
alternate and root ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in PVST+ or rapid-PVST+ mode, loop guard prevents alternate and root ports from becoming designated
ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the interface is blocked by loop
guard in all MST instances. On a boundary port, loop guard blocks the interface in all MST instances.
How to Configure Optional Spanning-Tree Features
Enabling PortFast
(CLI)
An interface with the
PortFast feature enabled is moved directly to the spanning-tree forwarding
state without waiting for the standard forward-time delay.
If you enable the voice VLAN
feature, the PortFast feature is automatically enabled. When you disable voice
VLAN, the PortFast feature is not automatically disabled.
You can enable this feature
if your switch is running PVST+, Rapid PVST+, or MSTP.
Caution
Use PortFast only when
connecting a single end station to an access or trunk port. Enabling this
feature on an interface connected to a switch or hub could prevent spanning
tree from detecting and disabling loops in your network, which could cause
broadcast storms and address-learning problems.
This procedure is optional.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configureterminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/2
Specifies an
interface to configure, and enters interface configuration mode.
Step 4
spanning-tree portfast
[trunk]
Example:
Switch(config-if)# spanning-tree portfast trunk
Enables PortFast
on an access port connected to a single workstation or server. By specifying
the
trunk keyword, you can enable PortFast on a trunk
port.
Note
To enable PortFast
on trunk ports, you must use the
spanning-tree portfast trunk interface
configuration command. The
spanning-tree portfast command will not work on
trunk ports.
Make sure that
there are no loops in the network between the trunk port and the workstation or
server before you enable PortFast on a trunk port.
By default,
PortFast is disabled on all interfaces.
Step 5
end
Example:
Switch(config-if)# end
Returns to
privileged EXEC mode.
What to do next
You can use the
spanning-tree portfast default global
configuration command to globally enable the PortFast feature on all
nontrunking ports.
Enabling BPDU Guard
(CLI)
You can enable the BPDU guard
feature if your switch is running PVST+, Rapid PVST+, or MSTP.
Caution
Configure PortFast edge only
on ports that connect to end stations; otherwise, an accidental topology loop
could cause a data packet loop and disrupt switch and network operation.
This procedure is optional.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Specifies the
interface connected to an end station, and enters interface configuration mode.
Step 5
spanning-tree
portfast edge
Example:
Switch(config-if)# spanning-tree portfast edge
Enables the
PortFast edge feature.
Step 6
end
Example:
Switch(config-if)# end
Returns to
privileged EXEC mode.
What to do next
To prevent the port from
shutting down, you can use the
errdisable detect cause bpduguard shutdown vlan
global configuration command to shut down just the offending VLAN on the port
where the violation occurred.
You also can use the
spanning-tree bpduguard enable interface
configuration command to enable BPDU guard on any port without also enabling
the PortFast edge feature. When the port receives a BPDU, it is put it in the
error-disabled state.
Enabling BPDU Filtering
(CLI)
You can also use the
spanning-tree bpdufilter enable interface
configuration command to enable BPDU filtering on any interface without also
enabling the PortFast edge feature. This command prevents the interface from
sending or receiving BPDUs.
Caution
Enabling BPDU filtering on an
interface is the same as disabling spanning tree on it and can result in
spanning-tree loops.
You can enable the BPDU
filtering feature if your switch is running PVST+, Rapid PVST+, or MSTP.
Caution
Configure PortFast edge only
on interfaces that connect to end stations; otherwise, an accidental topology
loop could cause a data packet loop and disrupt switch and network operation.
This procedure is optional.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Specifies the
interface connected to an end station, and enters interface configuration mode.
Step 5
spanning-tree
portfast edge
Example:
Switch(config-if)# spanning-tree portfast edge
Enables the
PortFast edge feature on the specified interface.
Step 6
end
Example:
Switch(config-if)# end
Returns to
privileged EXEC mode.
Enabling UplinkFast for Use with Redundant Links
(CLI)
Note
When you enable UplinkFast,
it affects all VLANs on the switch or switch stack. You cannot configure
UplinkFast on an individual VLAN.
You can configure the
UplinkFast or the Cross-Stack UplinkFast (CSUF) feature for Rapid PVST+ or for
the MSTP, but the feature remains disabled (inactive) until you change the
spanning-tree mode to PVST+.
This procedure is optional.
Follow these steps to enable UplinkFast and CSUF.
Before you begin
UplinkFast cannot be enabled
on VLANs that have been configured with a switch priority. To enable UplinkFast
on a VLAN with switch priority configured, first restore the switch priority on
the VLAN to the default value using the
no spanning-tree vlanvlan-idpriority global configuration command.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
(Optional) For
pkts-per-second, the range is 0 to 32000 packets
per second; the default is 150.
If you set the
rate to 0, station-learning frames are not generated, and the spanning-tree
topology converges more slowly after a loss of connectivity.
When you enter
this command, CSUF also is enabled on all nonstack port interfaces.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
When UplinkFast is enabled,
the switch priority of all VLANs is set to 49152. If you change the path cost
to a value less than 3000 and you enable UplinkFast or UplinkFast is already
enabled, the path cost of all interfaces and VLAN trunks is increased by 3000
(if you change the path cost to 3000 or above, the path cost is not altered).
The changes to the switch priority and the path cost reduce the chance that a
switch will become the root switch.
When UplinkFast is disabled,
the switch priorities of all VLANs and path costs of all interfaces are set to
default values if you did not modify them from their defaults.
When you enable the
UplinkFast feature using these instructions, CSUF is automatically globally
enabled on nonstack port interfaces.
Disabling UplinkFast
(CLI)
This procedure is
optional.
Follow these steps
to disable UplinkFast and Cross-Stack UplinkFast (CSUF).
Before you begin
UplinkFast must be
enabled.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configureterminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
no spanning-tree
uplinkfast
Example:
Switch(config)# no spanning-tree uplinkfast
Disables
UplinkFast and CSUF on the switch and all of its VLANs.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
When UplinkFast is disabled,
the switch priorities of all VLANs and path costs of all interfaces are set to
default values if you did not modify them from their defaults.
When you disable the
UplinkFast feature using these instructions, CSUF is automatically globally
disabled on nonstack port interfaces.
Enabling BackboneFast
(CLI)
You can enable BackboneFast
to detect indirect link failures and to start the spanning-tree reconfiguration
sooner.
You can configure the
BackboneFast feature for Rapid PVST+ or for the MSTP, but the feature remains
disabled (inactive) until you change the spanning-tree mode to PVST+.
This procedure is optional.
Follow these steps to enable BackboneFast on the switch.
Before you begin
If you use BackboneFast, you
must enable it on all switches in the network. BackboneFast is not supported on
Token Ring VLANs. This feature is supported for use with third-party switches.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configureterminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
spanning-tree
backbonefast
Example:
Switch(config)# spanning-tree backbonefast
Enables
BackboneFast.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Enabling EtherChannel Guard
(CLI)
You can enable
EtherChannel guard to detect an EtherChannel misconfiguration if your
switch is running PVST+, Rapid PVST+, or
MSTP.
This procedure is optional.
Follow these steps to enable EtherChannel Guard on the
switch.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
You can use the
show interfaces status err-disabled privileged
EXEC command to show which
switch ports are disabled because of an
EtherChannel misconfiguration. On the remote device, you can enter the
show etherchannel summary privileged EXEC command
to verify the EtherChannel configuration.
After the configuration is
corrected, enter the
shutdown and
no shutdown interface configuration commands on
the port-channel interfaces that were misconfigured.
Enabling Root Guard
(CLI)
Root guard enabled on an
interface applies to all the VLANs to which the interface belongs. Do not
enable the root guard on interfaces to be used by the UplinkFast feature. With
UplinkFast, the backup interfaces (in the blocked state) replace the root port
in the case of a failure. However, if root guard is also enabled, all the
backup interfaces used by the UplinkFast feature are placed in the
root-inconsistent state (blocked) and are prevented from reaching the
forwarding state.
Note
You cannot enable both root
guard and loop guard at the same time.
You can enable this feature
if your switch is running PVST+, Rapid PVST+, or MSTP.
This procedure is optional.
Follow these steps to enable
root guard on the switch.
Procedure
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables
privileged EXEC mode. Enter your password if prompted.
Step 2
configureterminal
Example:
Switch# configure terminal
Enters the global
configuration mode.
Step 3
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/2
Specifies an
interface to configure, and enters interface configuration mode.
Step 4
spanning-tree
guard root
Example:
Switch(config-if)# spanning-tree guard root
Enables root guard
on the interface.
By default, root
guard is disabled on all interfaces.
Step 5
end
Example:
Switch(config-if)# end
Returns to
privileged EXEC mode.
Enabling Loop Guard
(CLI)
You can use loop guard to
prevent alternate or root ports from becoming designated ports because of a
failure that leads to a unidirectional link. This feature is most effective
when it is configured on the entire switched network. Loop guard operates only
on interfaces that are considered point-to-point by the spanning tree.
Note
You cannot enable both loop
guard and root guard at the same time.
You can enable this feature
if your
switch is running PVST+, Rapid PVST+, or
MSTP.
This procedure is optional.
Follow these steps to enable loop guard on the
switch.
Procedure
Command or Action
Purpose
Step 1
Enter one of the following
commands:
show spanning-tree
active
show spanning-tree
mst
Example:
Switch# show spanning-tree active
or
Switch# show spanning-tree mst
Verifies which
interfaces are alternate or root ports.
Step 2
configure
terminal
Example:
Switch# configure terminal
Enters global
configuration mode.
Step 3
spanning-tree
loopguard default
Example:
Switch(config)# spanning-tree loopguard default
Enables loop
guard.
By default, loop
guard is disabled.
Step 4
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Monitoring the Spanning-Tree Status
Table 1. Commands for Monitoring the
Spanning-Tree Status
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
interfaceinterface-id
Displays spanning-tree
information for the specified interface.
show spanning-tree
mst interfaceinterface-id
Displays MST information for
the specified interface.
show spanning-tree
summary [totals]
Displays a summary of
interface states or displays the total lines of the spanning-tree state
section.
show spanning-tree mst interfaceinterface-idportfast edge
Displays spanning-tree portfast information for the specified
interface.
Additional
References for Optional Spanning Tree Features
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