Catalyst 5000 Family Software Configuration Guide (6.3 and 6.4) - Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast [Support]

Table Of Contents

Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast

Understanding How PortFast Works

Configuring PortFast

Enabling Spanning Tree PortFast

Disabling Spanning Tree PortFast

Understanding How PortFast BPDU Guard Works

Configuring PortFast BPDU Guard

Enabling PortFast BPDU Guard

Disabling PortFast BPDU Guard

Understanding How PortFast BPDU Filter Works

Configuring PortFast BPDU Filter

Enabling PortFast BPDU Filter

Disabling PortFast BPDU Filter

Understanding How UplinkFast Works

Configuring UplinkFast

Enabling UplinkFast

Disabling UplinkFast

Understanding How BackboneFast Works

Configuring BackboneFast

Enabling BackboneFast

Displaying BackboneFast Statistics

Disabling BackboneFast

Understanding How Loop Guard Works

Configuring Loop Guard

Enabling Loop Guard

Disabling Loop Guard

Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast

This chapter describes how to configure the PortFast, UplinkFast, and BackboneFast spanning tree features.

Note For information on configuring spanning tree, see Chapter 8, "Configuring Spanning Tree."

Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 5000 Family Command Reference.

This chapter consists of these sections:

•Understanding How PortFast Works

•Configuring PortFast

•Understanding How PortFast BPDU Guard Works

•Configuring PortFast BPDU Guard

•Understanding How PortFast BPDU Filter Works

•Configuring PortFast BPDU Filter

•Understanding How UplinkFast Works

•Configuring UplinkFast

•Understanding How BackboneFast Works

•Configuring BackboneFast

•Understanding How Loop Guard Works

•Configuring Loop Guard

Understanding How PortFast Works

The spanning tree PortFast feature causes a port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states. You can use PortFast on switch ports connected to a single workstation or server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state.

Caution PortFast should be used only when connecting a single end station to a switch port. If you enable PortFast on a port connected to another networking device, such as a switch, you can create network loops.

When the switch powers up, or when a device is connected to a port, the port normally enters the spanning tree listening state. When the forward delay timer expires, the port enters the learning state. When the forward delay timer expires a second time, the port is transitioned to the forwarding or blocking state.

When you enable PortFast on a port, the port is immediately and permanently transitioned to the spanning tree forwarding state.

Configuring PortFast

These sections describe how to configure spanning tree PortFast on the switch:

•Enabling Spanning Tree PortFast

•Disabling Spanning Tree PortFast

Enabling Spanning Tree PortFast

Caution PortFast should be used only when connecting a single end station to a switch port. If you enable PortFast on a port connected to another networking device, such as a switch, you can create network loops.

To enable PortFast on a switch port, perform this task in privileged mode:

Task

Command

Step 1

Enable PortFast on a switch port connected to a single workstation or server.

set spantree portfast mod/port enable

Step 2

Verify the PortFast setting.

show spantree mod/port

This example shows how to enable PortFast on a port and verify the configuration (the PortFast status is shown in the "Fast-Start" column):
Console> (enable) set spantree portfast 4/1 enable
Warning: Spantree port fast start should only be enabled on ports connected
to a single host.  Connecting hubs, concentrators, switches, bridges, etc. to
a fast start port can cause temporary spanning tree loops.  Use with caution.
Spantree port 4/1 fast start enabled.
Console> (enable) show spantree 4/1
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 4/1      1     blocking          19        20   enabled              
 4/1      100   forwarding        10        20   enabled              
 4/1      521   blocking          19        20   enabled              
 4/1      522   blocking          19        20   enabled              
 4/1      523   blocking          19        20   enabled              
 4/1      524   blocking          19        20   enabled              
 4/1      1003  not-connected     19        20   enabled              
 4/1      1005  not-connected     19         4   enabled              
Console> (enable)
Disabling Spanning Tree PortFast

To disable PortFast on a switch port, perform this task in privileged mode:

Task

Command

Step 1

Disable PortFast on a switch port.

set spantree portfast mod/port disable

Step 2

Verify the PortFast setting.

show spantree mod/port

This example shows how to disable PortFast on a port:
Console> (enable) set spantree portfast 4/1 disable
Spantree port 4/1 fast start disabled.
Console> (enable)
Understanding How PortFast BPDU Guard Works

To prevent loops from occuring in a network, the spanning tree PortFast mode should be configured only on nontrunking access ports because these ports usually do not transmit or receive BPDUs. The most secure implementation of PortFast is to enable it only on ports that connect end stations to switches.

The PortFast BPDU guard feature prevents loops by moving a nontrunking port into ErrDisable state when a BPDU is received on that port. When the BPDU guard feature is enabled on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces should not receive BPDUs. If a PortFast-configured interface receives a BPDU, an invalid configuration exists, such as connection of an unauthorized device. The BPDU guard feature provides a secure response to invalid configurations because the administrator must manually put the interface back in service and correct the invalid configuration.

Note When enabled on the switch, spanning tree applies the BPDU guard feature to all PortFast-configured interfaces.

Configuring PortFast BPDU Guard

These sections describe how to configure spanning tree PortFast BPDU guard on the switch:

•Enabling PortFast BPDU Guard

•Disabling PortFast BPDU Guard

Enabling PortFast BPDU Guard

Note The PortFast feature is configured on an individual port and the PortFast BPDU guard option is configured globally. When you disable the PortFast option on a port, PortFast BPDU guard becomes inactive.

To enable PortFast BPDU guard on a nontrunking switch port, perform this task in privileged mode:

Task

Command

Step 1

Enable PortFast BPDU guard on the switch.

set spantree portfast bpdu-guard enable

Step 2

Verify the PortFast BPDU guard setting.

show spantree summary

This example shows how to enable PortFast BPDU guard and verify the configuration:
console > (enable) set spantree portfast bpdu-guard enable
Spantree portfast bpdu-guard enabled on this switch.
Console> (enable) show spantree summary
Root switch for vlans: none.
Portfast bpdu-guard enabled for bridge.
Uplinkfast disabled for bridge.
Backbonefast disabled for bridge.
Vlan  Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
   1         0         0        0          4          4
   2         0         0        0          4          4
   3         0         0        0          4          4
   4         0         0        0          4          4
   5         0         0        0          4          4
   6         0         0        0          4          4
  10         0         0        0          4          4
  20         0         0        0          4          4
  50         0         0        0          4          4
 100         0         0        0          4          4
 152         0         0        0          4          4
 200         0         0        0          5          5
 300         0         0        0          4          4
 400         0         0        0          4          4
 500         0         0        0          4          4
 521         0         0        0          4          4
 524         0         0        0          4          4
 570         0         0        0          4          4
 801         0         0        0          0          0
 802         0         0        0          0          0
 850         0         0        0          4          4
 917         0         0        0          4          4
 999         0         0        0          4          4
1003         0         0        0          0          0
1005         0         0        0          0          0
      Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total        0         0        0         85         85
Console> (enable)
Disabling PortFast BPDU Guard

To disable PortFast BPDU guard, perform this task in privileged mode:

Task

Command

Step 1

Disable PortFast BPDU guard.

set spantree portfast bpdu-guard disable

Step 2

Verify the PortFast BPDU guard setting.

show spantree summary

This example shows how to disable spanning tree PortFast BPDU guard on the switch and verify the configuration:
console > (enable) set spantree portfast bpdu-guard disable
Spantree portfast bpdu-guard disabled on this switch.
Console> (enable) show spantree summary
Summary of connected spanning tree ports by vlan
Portfast bpdu-guard disabled for bridge.
Uplinkfast disabled for bridge.
Backbonefast disabled for bridge.
Vlan  Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
   1         0         0        0          4          4
   2         0         0        0          4          4
   3         0         0        0          4          4
   4         0         0        0          4          4
   5         0         0        0          4          4
   6         0         0        0          4          4
  10         0         0        0          4          4
  20         0         0        0          4          4
  50         0         0        0          4          4
 100         0         0        0          4          4
 152         0         0        0          4          4
 200         0         0        0          5          5
 300         0         0        0          4          4
 400         0         0        0          4          4
 500         0         0        0          4          4
 521         0         0        0          4          4
 524         0         0        0          4          4
 570         0         0        0          4          4
 801         0         0        0          0          0
 802         0         0        0          0          0
 850         0         0        0          4          4
 917         0         0        0          4          4
 999         0         0        0          4          4
1003         0         0        0          0          0
1005         0         0        0          0          0
      Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total        0         0        0         85         85
Console> (enable) 
Understanding How PortFast BPDU Filter Works

BPDU filtering provides a method for you to avoid transmitting BPDUs on a PortFast-enabled port; connected to an end system, which helps save CPU time. This feature is on a per-switch basis. After BPDU filtering is enabled it applies to all PortFast-enabled ports.

The PortFast BPDU filter allows access ports to move directly to the forwarding state as soon as end hosts are connected. Spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled or not. PortFast BPDU filtering is enabled globally but applies to PortFast-enabled ports only.

Configuring PortFast BPDU Filter

These sections describe how to configure the spanning tree PortFast BPDU filter option on the switch:

•Enabling PortFast BPDU Filter

•Disabling PortFast BPDU Filter

Enabling PortFast BPDU Filter

Note The PortFast feature is configured on an individual port, and the PortFast BPDU filter option is configured globally. When you disable the PortFast option on a port, PortFast BPDU filter becomes inactive.

To enable the PortFast BPDU filter option on a nontrunking switch port, perform this task in privileged mode:

Task

Command

Step 1

Enable the PortFast BPDU filter option on the switch.

set spantree portfast bpdu-filter enable

Step 2

Verify the PortFast BPDU filter setting.

show spantree summary

show spantree portfast

This example shows how to enable the PortFast BPDU filter option on the switch and verify the configuration in PVST+ mode:

Note For additional PVST+ information, see Chapter 8, "Configuring Spanning Tree."
Console> (enable) set spantree portfast bpdu-filter enable
Usage: set spantree portfast <mod/port> <enable|disable>
       set spantree portfast bpdu-guard <enable|disable>
       set spantree portfast bpdu-filter <enable|disable>
Spantree portfast bpdu-filter enabled on this switch.
Console> (enable) show spantree portfast
Portfast BPDU guard is disabled.
Portfast BPDU filter is disabled.
Console> (enable) show spantree summary
Root switch for vlans: none.
Portfast bpdu-filter enabled for bridge.
Uplinkfast disabled for bridge.
Backbonefast disabled for bridge.
Vlan  Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
   1         0         0        0          4          4
   2         0         0        0          4          4
   3         0         0        0          4          4
   4         0         0        0          4          4
   5         0         0        0          4          4
   6         0         0        0          4          4
  10         0         0        0          4          4
  20         0         0        0          4          4
  50         0         0        0          4          4
 100         0         0        0          4          4
 152         0         0        0          4          4
 200         0         0        0          5          5
 300         0         0        0          4          4
 400         0         0        0          4          4
 500         0         0        0          4          4
 521         0         0        0          4          4
 524         0         0        0          4          4
 570         0         0        0          4          4
 801         0         0        0          0          0
 802         0         0        0          0          0
 850         0         0        0          4          4
 917         0         0        0          4          4
 999         0         0        0          4          4
1003         0         0        0          0          0
1005         0         0        0          0          0
      Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total        0         0        0         85         85
Console> (enable)
Disabling PortFast BPDU Filter

To disable the PortFast BPDU filter option on the switch, perform this task in privileged mode:

Task

Command

Step 1

Disable the PortFast BPDU filter option on the switch.

set spantree portfast bpdu-filter disable

Step 2

Verify the PortFast BPDU filter setting.

show spantree summary

show spantree portfast

This example shows how to disable spanning tree PortFast BPDU filter on the switch and verify the configuration:
Console> (enable) set spantree portfast bpdu-filter disable
Spantree portfast bpdu-filter disabled on this switch.
Console> (enable) show spantree summary
Summary of connected spanning tree ports by vlan
Portfast bpdu-filter disabled for bridge.
Uplinkfast disabled for bridge.
Backbonefast disabled for bridge.
Vlan  Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
   1         0         0        0          4          4
   2         0         0        0          4          4
   3         0         0        0          4          4
   4         0         0        0          4          4
   5         0         0        0          4          4
   6         0         0        0          4          4
  10         0         0        0          4          4
  20         0         0        0          4          4
  50         0         0        0          4          4
 100         0         0        0          4          4
 152         0         0        0          4          4
 200         0         0        0          5          5
 300         0         0        0          4          4
 400         0         0        0          4          4
 500         0         0        0          4          4
 521         0         0        0          4          4
 524         0         0        0          4          4
 570         0         0        0          4          4
 801         0         0        0          0          0
 802         0         0        0          0          0
 850         0         0        0          4          4
 917         0         0        0          4          4
 999         0         0        0          4          4
1003         0         0        0          0          0
1005         0         0        0          0          0
      Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total        0         0        0         85         85
Console> (enable) 
Understanding How UplinkFast Works

The UplinkFast feature provides fast convergence in the network access layer after a spanning tree topology change by using uplink groups. An uplink group is a set of ports (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 (not including self-looped ports). The uplink group provides an alternate path in case the link that is currently forwarding fails.

Note UplinkFast is most beneficial in wiring-closet switches with a limited number of active VLANs. This feature might not be useful for other types of applications and should not be enabled on backbone or distribution layer switches.

Figure 9-1 shows an example UplinkFast network topology. Switch A, the root switch, is connected directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected to Switch B over link L3 is in blocking state.

Figure 9-1 UplinkFast Example Before Direct Link Failure

If Switch C detects a link failure on the currently active link L2 (a direct link failure), UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state immediately, without transitioning the port through the listening and learning states (as shown in Figure 9-2). This switchover takes approximately one to five seconds.

Figure 9-2 UplinkFast Example After Direct Link Failure

As soon as the switch transitions the alternate port to the forwarding state, the switch begins transmitting dummy multicast frames on that port, one for each entry in the local EARL table (except those entries associated with the failed root port). By default, approximately 15 dummy multicast frames are transmitted per 100 milliseconds.

Each dummy multicast frame uses the station address in the EARL table entry as its source MAC address and a dummy multicast address (01-00-0C-CD-CD-CD) as the destination MAC address.

Switches receiving these dummy multicast frames immediately update their EARL table entries for each source MAC address to use the new port, allowing the switches to begin using the new path almost immediately.

If connectivity on the original root port is restored, the switch waits for a period equal to twice the forward delay time plus 5 seconds before transitioning the port to the forwarding state. This wait allows the neighbor port time to transition through the listening and learning states to the forwarding state.

Configuring UplinkFast

These sections describe how to configure the UplinkFast feature on the switch:

•Enabling UplinkFast

•Disabling UplinkFast

Enabling UplinkFast

When you enable spanning tree UplinkFast on the switch, UplinkFast processing is enabled and the spanning tree bridge priority for all VLANs is set to 49152, making it unlikely that the switch will become the root switch. The spanning tree port cost and port-VLAN cost of all ports on the switch is increased by 3000.

The station_update_rate value in the UplinkFast command represents the number of dummy multicast packets transmitted per 100 milliseconds (the default is 15 packets per 100 milliseconds) in the event of a direct link failure.

Use the all-protocols on keywords on switches that have UplinkFast enabled but do not have protocol filtering enabled, and that are connected to upstream switches in the network that have protocol filtering enabled. The all-protocols on keywords cause the switch to generate multicasts for each protocol-filtering group.

On switches with both UplinkFast and protocol filtering enabled, or if no other switches have protocol filtering enabled, you do not need to use the all-protocols on keywords.

Note When you enable spanning tree UplinkFast, it affects all VLANs on the switch. You cannot configure UplinkFast on a per-VLAN basis.

To enable UplinkFast, perform this task in privileged mode:

Task

Command

Step 1

Enable UplinkFast on the switch.

set spantree uplinkfast enable [rate station_update_rate] [all-protocols {off | on}]

Step 2

Verify that UplinkFast is enabled.

show spantree uplinkfast

This example shows how to enable UplinkFast with a station-update rate of 40 packets per 100 milliseconds and how to verify that UplinkFast is enabled:
Console> (enable) set spantree uplinkfast enable
VLANs 1-1005 bridge priority set to 49152.
The port cost and portvlancost of all ports set to above 3000.
Station update rate set to 40 packets/100ms.
uplinkfast all-protocols field set to off.
uplinkfast enabled for bridge.
Console> (enable) show spantree uplinkfast
Station update rate set to 15 packets/100ms.
uplinkfast all-protocols field set to off.
VLAN          port list
-----------------------------------------------
1             1/1(fwd),1/2
100           1/2(fwd)
521           1/1(fwd),1/2
522           1/1(fwd),1/2
523           1/1(fwd),1/2
524           1/1(fwd),1/2
Console> (enable)
This example shows how to display the UplinkFast feature settings for all VLANs:
Console> show spantree uplinkfast
Station update rate set to 15 packets/100ms.
uplinkfast all-protocols field set to off.
VLAN port list 	
------------------------------------------------
1-20   1/1(fwd),1/2-1/5
21-50  1/9(fwd), 1/6-1/8, 1/10-1/12
51-100 2/1(fwd), 2/12
Console> 
Disabling UplinkFast

To disable UplinkFast and restore the default spanning tree bridge priority, port cost, and port-VLAN cost values to their default values, enter the clear spantree uplinkfast command.

Caution Use caution when entering the clear spantree uplinkfast command. This command restores the default port-VLAN costs on all ports. If you have configured per-VLAN load sharing on redundant trunk links, the load-sharing configuration could be affected by this command.

You can disable only spanning tree UplinkFast processing on the switch using the set spantree uplinkfast disable command. This command does not affect the bridge priority, port cost, and port-VLAN cost values on the switch.

Note When you disable spanning tree UplinkFast, it affects all VLANs on the switch. You cannot disable UplinkFast on a per-VLAN basis.

To disable UplinkFast on the switch, perform this task in privileged mode:

Task

Command

Step 1

(Optional) Disable UplinkFast processing on the switch and restore the default bridge priority, port cost, and port-VLAN cost values.

clear spantree uplinkfast

Step 2

(Optional) Disable UplinkFast processing on the switch without affecting the bridge priority, port cost, and port-VLAN cost values.

set spantree uplinkfast disable

Step 3

Verify that UplinkFast is enabled.

show spantree uplinkfast

This example shows how to disable UplinkFast on the switch and restore the default bridge priority, port cost, and port-VLAN cost values:
Console> (enable) clear spantree uplinkfast
This command will cause all portcosts, portvlancosts, and the 
bridge priority on all vlans to be set to default.
Do you want to continue (y/n) [n]? y
VLANs 1-1005 bridge priority set to 32768.
The port cost of all bridge ports set to default value.
The portvlancost of all bridge ports set to default value.
uplinkfast all-protocols field set to off.
uplinkfast disabled for bridge.
Console> (enable) show spantree uplinkfast
uplinkfast disabled for bridge.
Console> (enable)
Understanding How BackboneFast Works

The BackboneFast feature provides fast convergence in the network backbone after a spanning tree topology change occurs. A switch detects an indirect link failure (the failure of a link to which the switch is not directly connected) when the switch receives inferior BPDUs from its designated bridge on its root port or blocked ports. These inferior BPDUs indicate that the designated bridge has lost its connection to the root bridge. An inferior BPDU identifies a single switch as both the root bridge and the designated bridge. Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured maximum aging time (specified by the set spantree maxage command).

The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, which causes the maximum aging time on the root to expire, and the switch becomes the root switch according to normal spanning tree rules.

If the switch has alternate paths to the root bridge, it transmits Root Link Query PDUs out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves them out of the blocking state (if they were in blocking state), through the listening and learning states, and into the forwarding state.

Figure 9-3 shows an example BackboneFast network topology. Switch A, the root switch, connects directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that connects directly to Switch B over link L3 is in the blocking state.

Figure 9-3 BackboneFast Example Before Indirect Link Failure

If link L1 fails, Switch C detects this failure as an indirect failure, because it is not connected directly to link L1. Switch B no longer has a path to the root switch. BackboneFast allows the blocked port on Switch C to move immediately to the listening state without waiting for the maximum aging time for the port to expire. BackboneFast then transitions the port on Switch C to the forwarding state, providing a path from Switch B to Switch A. This switchover takes approximately 30 seconds. Figure 9-4 shows how BackboneFast reconfigures the topology to account for the failure of link L1.

Figure 9-4 BackboneFast Example After Indirect Link Failure

If a new switch is introduced into a shared-medium topology, BackboneFast is not activated. Figure 9-5 shows a shared-medium topology in which a new switch is added. The new switch begins sending inferior BPDUs, which indicate that it is the root switch. However, the other switches ignore these inferior BPDUs and the new switch learns that Switch B is the designated bridge to Switch A, the root switch.

Figure 9-5 Adding a Switch in a Shared-Medium Topology

Configuring BackboneFast

These sections describe how to configure the BackboneFast feature:

•Enabling BackboneFast

•Displaying BackboneFast Statistics

•Disabling BackboneFast

Enabling BackboneFast

Note You must enable BackboneFast on all switches in the network. BackboneFast is not supported on Token Ring VLANs. This feature is supported for use with third-party switches.

To enable BackboneFast on the switch, perform this task in privileged mode:

Task

Command

Step 1

Enable BackboneFast on the switch.

set spantree backbonefast enable

Step 2

Verify that BackboneFast is enabled.

show spantree backbonefast

This example shows how to enable BackboneFast on the switch and how to verify the configuration:
Console> (enable) set spantree backbonefast enable
Backbonefast enabled for all VLANs
Console> (enable) show spantree backbonefast
Backbonefast is enabled.
Console> (enable)
Displaying BackboneFast Statistics

To display BackboneFast statistics, perform this task in privileged mode:

Task

Command

Display BackboneFast statistics.

show spantree summary

This example shows how to display BackboneFast statistics:
Console> (enable) show spantree summary
Summary of connected spanning tree ports by vlan
Uplinkfast disabled for bridge.
Backbonefast enabled for bridge.
Vlan  Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
   1         0         0        0          1          1
      Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total        0         0        0          1          1
BackboneFast statistics
-----------------------
Number of inferior BPDUs received (all VLANs)  : 0
Number of RLQ req PDUs received (all VLANs)    : 0
Number of RLQ res PDUs received (all VLANs)    : 0
Number of RLQ req PDUs transmitted (all VLANs) : 0
Number of RLQ res PDUs transmitted (all VLANs) : 0
Console> (enable)
Disabling BackboneFast

To disable BackboneFast on the switch, perform this task in privileged mode:

Task

Command

Step 1

Disable BackboneFast on the switch.

set spantree backbonefast disable

Step 2

Verify that BackboneFast is disabled.

show spantree backbonefast

This example shows how to disable BackboneFast on the switch and how to verify the configuration:
Console> (enable) set spantree backbonefast disable
Backbonefast enabled for all VLANs
Console> (enable) show spantree backbonefast
Backbonefast is disable.
Console> (enable)
Understanding How Loop Guard Works

Unidirectional link failures may cause a root port or alternate port to become designated as root if BPDUs are absent. Some software failures may introduce temporary loops in the network. The loop guard feature checks if a root port or an alternate root port receives BPDUs. If the port is not receiving BPDUs, the loop guard feature puts the port into an inconsistent state until it starts receiving BPDUs again. Loop guard isolates the failure and lets spanning tree converge to a stable topology without the failed link or bridge.

You can enable loop guard on a per-port basis. When you enable loop guard, it is automatically applied to all of the active instances or VLANs to which that port belongs. When you disable loop guard, it is disabled for the specified ports. Disabling loop guard moves all loop-inconsistent ports to the listening state.

If you enable loop guard on a channel and the first link becomes unidirectional, loop guard blocks the entire channel until the affected port is removed from the channel. Figure 9-6 shows loop guard in a triangle switch configuration.

Figure 9-6 Triangle Switch Configuration with Loop Guard

Figure 9-6 illustrates the following configuration:

•Switches A and B are distribution switches.

•Switch C is an access switch.

•Loop guard is enabled on ports 3/1 and 3/2 on Switches A, B, and C.

Use loop guard only in topologies where there are blocked ports. Topologies that have no blocked ports, which are loop free, do not need to enable this feature. Enabling loop guard on a root switch has no effect but provides protection when a root switch becomes a nonroot switch.

Follow these guidelines when using loop guard:

•You cannot enable loop guard on PortFast-enabled or dynamic VLAN ports.

•You cannot enable PortFast on loop guard-enabled ports.

•You cannot enable loop guard if root guard is enabled.

Loop guard interacts with other features as follows:

•Loop guard does not affect the functionality of UplinkFast or BackboneFast.

•Do not enable loop guard on ports that are connected to a shared link.

Note We recommend that you enable loop guard on root ports and alternate root ports on access switches.

•Root guard forces a port to be always designated as the root port. Loop guard is effective only if the port is a root port or an alternate port. You cannot enable loop guard and root guard on a port at the same time.

•PortFast transitions a port into a forwarding state immediately when a link is established. Because a PortFast-enabled port will not be a root port or alternate port, loop guard and PortFast cannot be configured on the same port. Assigning dynamic VLAN membership for the port requires that the port is PortFast enabled. You cannot configure a loop guard-enabled port with dynamic VLAN membership.

•If your network has a type-inconsistent port or a PVID-inconsistent port, all BPDUs are dropped until the misconfiguration is corrected. The port transitions out of the inconsistent state after the message age expires. Loop guard ignores the message age expiration on type-inconsistent ports and PVID-inconsistent ports. If the port is already blocked by loop guard, misconfigured BPDUs received on the port make loop guard recover, but the port is moved into the type-inconsistent state or PVID-inconsistent state.

•In high-availability switch configurations, if a port is put into the blocked state by loop guard, it remains blocked even after switchover to the redundant supervisor engine. The newly activated supervisor engine recovers the port only after receiving a BPDU on that port.

•Loop guard uses the ports known to spanning tree. Loop guard can take advantage of logical ports provided by the Port Aggregation Protocol (PAgP). However, to form a channel, all the physical ports grouped in the channel must have compatible configurations. PAgP enforces uniform configurations of root guard or loop guard on all the physical ports to form a channel.

These caveats apply to loop guard:

–Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that link becomes unidirectional, loop guard blocks the channel, even if other links in the channel are functioning properly.

–If a set of ports that are already blocked by loop guard are grouped together to form a channel, spanning tree loses all the state information for those ports and the new channel port may obtain the forwarding state with a designated role.

–If a channel is blocked by loop guard and the channel breaks, spanning tree loses all the state information. The individual physical ports may obtain the forwarding state with the designated role, even if one or more of the links that formed the channel are unidirectional.

Note You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure. A loop may occur until UDLD detects the failure, but loop guard will not be able to detect it.

•Loop guard has no effect on a disabled spanning tree instance or a VLAN.

Configuring Loop Guard

These sections describe how to configure BackboneFast:

•Enabling Loop Guard

•Disabling Loop Guard

Enabling Loop Guard

Use the set spantree guard command to enable or disable the spanning tree loop guard feature on a per-port basis.

To enable loop guard on the switch, perform this task in privileged mode:

Task

Command

Step 1

Enable loop guard on a port.

set spantree guard {root | loop | none} mod/port

Step 2

Verify that loop guard is enabled.

show spantree guard {mod/port | vlan} mistp-instance instance

This example shows how to enable loop guard:
Console> (enable) set spantree guard loop 5/1
Rootguard is enabled on port 5/1, enabling loopguard will disable rootguard on
 this port.
Do you want to continue (y/n) [n]? y
Loopguard on port 5/1 is enabled.
Console> (enable)
Disabling Loop Guard

To disable loop guard on the switch, perform this task in privileged mode:

Task

Command

Step 1

Disable loop guard on a port.

set spantree guard {root | loop | none} mod/port

Step 2

Verify that loop guard is disabled.

show spantree guard {mod/port | vlan} mistp-instance instance

This example shows how to disable loop guard:
Console> (enable) set spantree guard none 5/1
Rootguard is disabled on port 5/1, disabling loopguard will disable rootguard on
 this port.
Do you want to continue (y/n) [n]? y
Loopguard on port 5/1 is disabled.
Console> (enable)