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Spanning Tree Protocol

Understanding and Configuring the Cisco UplinkFast Feature

Document ID: 10575

Updated: Aug 18, 2008

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Introduction

UplinkFast is a Cisco specific feature that improves the convergence time of the Spanning-Tree Protocol (STP) in the event of the failure of an uplink. The UplinkFast feature is supported on Cisco Catalyst 4500/4000, 5500/5000, and 6500/6000 series switches running CatOS. This feature is also supported on Catalyst 4500/4000 and 6500/6000 switches that run Cisco IOS® System Software and 2900 XL/3500 XL, 2950, 3550, 3560 and 3750 series switches. The UplinkFast feature is designed to run in a switched environment when the switch has at least one alternate/backup root port (port in blocking state), that is why Cisco recommends that UplinkFast be enabled only for switches with blocked ports, typically at the access-layer. Do not use on switches without the implied topology knowledge of a alternative/backup root link typically to distribution and core switches in Cisco multilayer design.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information

This diagram illustrates a typical redundant network design. Users are connected to an access switch. The access switch is dually attached to two core, or distribution, switches. As the redundant uplink introduces a loop in the physical topology of the network, the Spanning-Tree Algorithm (STA) blocks it.

51a.gif

In the event of failure of the primary uplink to core switch D1, the STP recalculates and eventually unblocks the second uplink to switch D2, therefore it restores connectivity. With the default STP parameters, the recovery takes up to 30 seconds, and with aggressive timer tuning, this lapse of time can be reduced to 14 seconds. The UplinkFast feature is a Cisco proprietary technique that reduces the recovery time further down to the order of one second.

This document details how the standard STP performs when the primary uplink fails, how UplinkFast achieves faster reconvergence than the standard reconvergence procedure, and how to configure UplinkFast. This document does not cover the basic knowledge of STP operation. Refer to Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches in order to learn more about STP operation and configuration:

Uplink Failure Without Uplink Fast Enabled

In this section, refer to the previous diagram, which uses a minimal backbone. The behavior of the STP is inspected in the event of uplink failure. Each step is followed with a diagram.

D1 and D2 are core switches. D1 is configured as the root bridge of the network. A is an access switch with one of its uplinks in blocking mode

  1. Assume that the primary uplink from A to D1 fails.

    51b.gif

  2. Port P1 goes down immediately and switch A declares its uplink to D1 as down.

    Switch A considers its link to D2, which still receives BPDUs from the root, as an alternate root port. Bridge A can start to transition port P2 from the blocking state to the forwarding state. In order to achieve this, it has to go through the listening and learning stages. Each of these stages last forward_delay (15 seconds by default), and holds port P2 blocking for 30 seconds.

  3. Once port P2 reaches the forwarding state, the network connectivity is re-established for hosts attached to switch A.

    The network outage lasted 30 seconds.

    51d.gif

    The minimum value allowed for the forward_delay timer is seven seconds. Tuning the STP parameters can lead to a recovery time of 14 seconds. This is still a noticeable delay for a user, and this kind of tuning should be done with caution. This section of this document shows how UplinkFast dramatically reduces the downtime.

Uplink Fast Theory of Operation

The UplinkFast feature is based on the definition of an uplink group. On a given switch, the uplink group consists in the root port and all the ports that provide an alternate connection to the root bridge. If the root port fails, which means if the primary uplink fails, a port with next lowest cost from the uplink group is selected to immediately replace it.

This diagram helps to explain on what the UplinkFast feature is based:

51e.gif

In this diagram, root ports are represented with a blue R and designated ports are represented with a green d. The green arrows represent the BPDUs generated by the root bridge and retransmitted by the bridges on their designated ports. Without the entrance a formal demonstration, you can determine these about BPDUs and ports in a stable network:

  • When a port receives a BPDU, it has a path to the root bridge. This is because BPDUs are originated from the root bridge. In this diagram, check switch A: three of its ports are receiving BPDUs, and three of its ports lead to the root bridge. The port on A that sends BPDU is designated and does not lead to the root bridge.

  • On any given bridge, all ports that receive BPDUs are blocking, except the root port. A port that receives a BPDU leads to the root bridge. If you had a bridge with two ports leading to the root bridge, you have a bridging loop.

  • A self-looped port does not provide an alternate path to the root bridge. See switch B in the diagram. Switch B blocked port is self-looped, which means that it cannot receive its own BPDUs. In this case, the blocked port does not provide an alternate path to the root.

On a given bridge, the root port and all blocked ports that are not self-looped form the uplink group. This section describes step-by-step how UplinkFast achieves fast convergence with the use of an alternate port from this uplink group.

Note: UplinkFast only works when the switch has blocked ports. The feature is typically designed for an access switch that has redundant blocked uplinks. When you enable UplinkFast, it is enabled for the entire switch and cannot be enabled for individual VLANs.

Uplink Failure With Uplink Fast Enabled

This section details the steps for UplinkFast recovery. Use the network diagram that was introduced at the beginning of the document.

Immediate Switch Over to the Alternate Uplink

51f.gif

Complete these steps for an immediate switch over to the alternate uplink:

  1. The uplink group of A consists of P1 and its non-self-looped blocked port, P2.

  2. When the link between D1 and A fails, A detects a link down on port P1.

    It knows immediately that its unique path to the root bridge is lost, and other paths are through the uplink group, for example, port P2 , which is blocked.

    51g.gif

  3. A places port P2 in forwarding mode immediately, thus it violates the standard STP procedures.

    There is no loop in the network, as the only path to the root bridge is currently down. Therefore, recovery is almost immediate.

    51h.gif

CAM Table Update

Once UplinkFast has achieved a fast-switchover between two uplinks, the Content-Addressable Memory (CAM) table in the different switches of the network can be momentarily invalid and slow down the actual convergence time.

In order to illustrate this, two hosts are added, named S and C, to this example:

51j.gif

The CAM tables of the different switches are represented in the diagram. You can see that, in order to reach C, packets originated from S have to go through D2, D1, and then A.

As shown in this diagram, the backup link is brought up:

51k.gif

The backup link is brought up so quickly, however, that the CAM tables are no longer accurate. If S sends a packet to C, it is forwarded to D1, where it is dropped. Communication between S and C is interrupted as long as the CAM table is incorrect. Even with the topology change mechanism, it can take up to 15 seconds before the problem is solved.

In order to solve this problem, switch A begins to flood dummy packets with the different MAC addresses that it has in its CAM table as a source. In this case, a packet with C as a source address is generated by A. Its destination is a Cisco proprietary multicast MAC address that ensures that the packet is flooded on the whole network and updates the necessary CAM tables on the other switches.

51m.gif

The rate at which the dummy multicasts are sent can be configured.

New Uplink Added

In the event of failure of the primary uplink, a replacement is immediately selected within the uplink group. What happens when a new port comes up, and this port, in accordance with STP rules, should rightfully become the new primary uplink (root port)? An example of this is when the original root port P1 on switch A goes down, port P2 takes over, but then port P1 on switch A comes back up. Port P1 has the right to regain the root port function. Should UplinkFast immediately allow port P1 to take over and put P2 back in blocking mode?

51n.gif

No. An immediate switchover to port P1, which immediately blocks port P2 and put port P1 in forwarding mode, is not wanted, for these reasons:

  • Stability—if the primary uplink is flapping, it is better to not introduce instability in the network by re-enabling it immediately. You can afford to keep the existing uplink temporarily.

  • The only thing UplinkFast can do is to move port P1 in forwarding mode as soon as it is up. The problem is that the remote port on D1 also goes up and obeys the usual STP rules.

    51o.gif

Immediately blocking port P2 and moving port P1 to forwarding does not help in this case. Port P3 does not forward before it goes through the listening and learning stages, which take 15 seconds each by default.

The best solution is to keep the current uplink active and hold port P1 blocked until port P3 begins forwarding. The switchover between port P1 and port P2 is then delayed by 2*forward_delay + 5 seconds (which is 35 seconds by default). The five seconds leave time for other protocols to negotiate, for example, DTP of EtherChannel.

Uplink Failure Repeated After Primary Uplink is Brought Back Up

When the primary uplink comes back up, it is first kept blocked for about 35 seconds by uplinkfast, before it is immediately switched to a forwarding state, as was explained previously. This port is not able to do another uplinkfast transition for roughly the same period of time. The idea is to protect against a flapping uplink that keeps triggering UplinkFast too often, and can cause too many dummy multicasts to be flooded through the network

Changes Implied by Uplink Fast

In order to be effective, the feature needs to have blocked ports that provides redundant connectivity to the root. As soon as Uplink Fast is configured on a switch, switch automatically adjusts some STP parameters in order to help achieve this:

  • The bridge priority of the switch is increased to a significantly higher value than the default. This ensures that the switch is not likely to be elected root bridge, which does not have any root ports (all ports are designated).

  • All the ports of the switch have their cost increased by 3000. This ensures that switch ports are not likely be elected designated ports.

warning Warning: Be careful before you configure Uplink Fast feature because the automatic changes of STP parameters can change the current STP topology.

Uplink Fast Feature Limitations and Interfacing with Other Features

Sometimes a Switching hardware or software feature causes the UplinkFast feature not to function properly. These are some examples of these limitations.

  • Uplink fast does not do the fast transition during a High Availability supervisor switchover on 6500/6000 switches that run CatOS. When the root port is lost on failed-resetting supervisor, the situation after a switchover is similar to when the switch boots up the first time because you do not sync the root port information between Supervisors. High Availability (HA) maintains only spanning tree port state, not the root port information, so when the HA switchover occurs, the new sup has no idea that it has lost a port on one of the uplink ports of the failed supervisor. A common workaround is the use of a port channel (EtherChannel). Root port status is maintained when a Port Channel is built across both supervisors, 1/1-2/1 or 1/2-2/2, for example, or root port is on the port of any Line Card. As no spanning tree topology change occurs when failing-resetting the active supervisor, no UplinkFast transition is necessary.

  • Uplink fast does not do the fast transition during an RPR or RPR+ switchover on a 6500/6000 switch that runs Cisco IOS System Software. There is no workaround because Layer 2 port must go through spanning tree convergence states of listening, learning, and forwarding.

  • Uplink fast implementation on gigastack of 2900/3500XL/2950/3550/3560/3750 is called Cross Stack Uplink Fast Feature (CSUF), general UplinkFast feature on gigastack setup is not supported. CSUF does not implement generation of dummy multicast packets after UplinkFast transition for the update of CAM tables.

  • Do not change spanning tree priority on the switch when UplinkFast is enabled because, it depends on the platform, and it can cause UplinkFast feature to be disabled, or it can cause a loop as the UplinkFast feature automatically changes the priority to a higher value in order to prevent the switch from becoming Root Bridge.

Uplink Fast Configuration

This section gives a step-by-step example of UplinkFast configuration and operation. Use this network diagram:

51p_new.gif

Switches A, D1, and D2 are all Catalyst switches that support the UplinkFast feature. Focus on switch A, while you perform these steps:

Note: Here, the configuration is tested with switch A that runs CatOS and Cisco IOS software.

Viewing the STP Parameter Default

These are the default parameters that are set for the STP on our access switch A:

Note: Port that connects to switch D2 is currently blocking, the current cost value for the ports depends on the bandwidth, for example, 100 for an Ethernet port, 19 for a Fast Ethernet port, 4 for a Gigabit Ethernet port, and the priority of the bridge is the default 32768.

CatOS

A>(enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type          ieee

Designated Root             00-40-0b-cd-b4-09
Designated Root Priority    8192
Designated Root Cost        100
Designated Root Port        2/1
Root Max Age   20 sec    Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-90-d9-5a-a8-00
Bridge ID Priority          32768
Bridge Max Age 20 sec    Hello Time 2  sec   Forward Delay 15 sec

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
 1/1                     1    not-connected    19       32 disabled   0         
 1/2                     1    not-connected    19       32 disabled   0         
 2/1                     1    forwarding      100       32 disabled   0

!--- Port connecting to D1
         
 2/2                     1    blocking        100       32 disabled   0

!--- Port connecting to D2

 2/3                     1    not-connected   100       32 disabled   0         
 2/4                     1    not-connected   100       32 disabled   0         
 2/5                     1    not-connected   100       32 disabled   0         
<snip>

Cisco IOS

A#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        19
             Port        130 (FastEthernet3/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Altn BLK 19        128.129  P2p

!--- Port connecting to D2
 
Fa3/2            Root FWD 19        128.130  P2p

!--- Port connecting to D1

Configure Uplink Fast and Check the Changes In the STP Parameters

CatOS

You enable UplinkFast on switch A with the set spantree uplinkfast enable command. These parameters are set:

A>(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 15 packets/100ms.
uplinkfast all-protocols field set to off.
uplinkfast enabled for bridge.

Use the show spantree command and you can see the main changes:

  • the priority of the bridge has increased to 49152

  • the cost of the ports has increased by 3000

A>(enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type          ieee

Designated Root             00-40-0b-cd-b4-09
Designated Root Priority    8192
Designated Root Cost        3100
Designated Root Port        2/1
Root Max Age   20 sec    Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-90-d9-5a-a8-00
Bridge ID Priority          49152
Bridge Max Age 20 sec    Hello Time 2  sec   Forward Delay 15 sec

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
 1/1                     1    not-connected  3019       32 disabled   0         
 1/2                     1    not-connected  3019       32 disabled   0         
 2/1                     1    forwarding     3100       32 disabled   0         
 2/2                     1    blocking       3100       32 disabled   0         
 <snip>

Cisco IOS

You can use the command spanning-tree uplinkfast in order to enable UplinkFast on switch A. These parameters are set:

A(config)#spanning-tree uplinkfast

Use the show spanning-tree command and you can see the main changes:

  • the priority of the bridge has increased to 49152

  • the cost of the ports has increased by 3000

A(config)#do show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        3019
             Port        130 (FastEthernet3/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49152
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
  Uplinkfast enabled

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Altn BLK 3019      128.129  P2p 
Fa3/2            Root FWD 3019      128.130  P2p

Increase the Logging Level on Switch A In Order to See the STP Debugging Information

CatOS

Use the set logging level command and increase the logging level for the STP, so that you can have detailed information displayed on the screen during the test:

A>(enable) set logging level spantree 7
System logging facility  for this session set to severity 7(debugging)
A>(enable)

Cisco IOS

Use the logging console debugging command and set the console logging of messages at the debugging level, which is the least severe level and which displays all logging messages.

A(config)#logging console debugging

Unplug the Primary Uplink Between A and D1

CatOS

At this stage, unplug the cable between A and D1. In the same second, you can see the port connect to D1 that goes down and the port connect to D2 that is transferred immediately into forwarding mode:

2000 Nov 21 01:34:55 %SPANTREE-5-UFAST_PORTFWD: Port 2/2 in vlan 1 moved to
forwarding(UplinkFast)
2000 Nov 21 01:34:55 %SPANTREE-6-PORTFWD: Port 2/2 state in vlan 1 changed to forwarding
2000 Nov 21 01:34:55 %SPANTREE-7-PORTDEL_SUCCESS:2/1 deleted from vlan 1 (LinkUpdPrcs)

Use the show spantree command in order to check that you have immediately updated the STP:

A>(enable) show spantree
<snip>
Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
 1/1                     1    not-connected  3019       32 disabled   0         
 1/2                     1    not-connected  3019       32 disabled   0         
 2/1                     1    not-connected  3100       32 disabled   0         
 2/2                     1    forwarding     3100       32 disabled   0         
<snip>

Cisco IOS

A#
00:32:45: %SPANTREE_FAST-SP-7-PORT_FWD_UPLINK: VLAN0001 FastEthernet3/1 moved to Forwarding (UplinkFast).
A#

Use the show spanning-tree command in order to check updated STP information:

A#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        3038
             Port        129 (FastEthernet3/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49152
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15 
  Uplinkfast enabled

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Root FWD 3019      128.129  P2p 

Plug Back the Primary Uplink

At this point, the primary uplink is manually plugged in and put back up. You can see that the UplinkFast feature forces the port into a blocking mode, whereas usual STP rules have put it in listening mode. At the same time, port that connects to D2, which should go immediately into blocking mode according to the standard STP, is kept in forwarding mode. UplinkFast forces the current uplink to stay up until the new one is fully operational:

CatOS

A>(enable) 2000 Nov 21 01:35:38 %SPANTREE-6-PORTBLK: Port 2/1
state in vlan 1 changed to blocking
2000 Nov 21 01:35:39 %SPANTREE-5-PORTLISTEN: Port 2/1 state in vlan 1 changed to listening
2000 Nov 21 01:35:41 %SPANTREE-6-PORTBLK: Port 2/1 state in vlan 1 changed to
blocking

A>(enable) show spantree
<snip>
Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
<snip>
 2/1                     1    blocking       3100       32 disabled   0         
 2/2                     1    forwarding     3100       32 disabled   0         
<snip>
A>(enable)

35 seconds after the port that connects to D1 is brought up, UplinkFast switches the uplinks, blocks port to D2 and moves port to D1 directly into forwarding mode:

2000 Nov 21 01:36:15 %SPANTREE-6-PORTBLK: Port 2/2
state in vlan 1 changed to blocking
2000 Nov 21 01:36:15 %SPANTREE-5-UFAST_PORTFWD: Port 2/1 in vlan 1 moved to
forwarding(UplinkFast)
2000 Nov 21 01:36:15 %SPANTREE-6-PORTFWD: Port 2/1 state in vlan 1 changed to forwarding

A>(enable) show spantree
<snip>
Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
<snip>    
 2/1                     1    forwarding     3100       32 disabled   0         
 2/2                     1    blocking       3100       32 disabled   0         
<snip>

Cisco IOS

A#show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        3038
             Port        129 (FastEthernet3/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49152
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
  Uplinkfast enabled

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Root FWD 3019      128.129  P2p
Fa3/2            Altn BLK 3019      128.130  P2p

A#
01:04:46: %SPANTREE_FAST-SP-7-PORT_FWD_UPLINK: VLAN0001 FastEthernet3/2 moved to
 Forwarding (UplinkFast).

A#show spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        3019
             Port        130 (FastEthernet3/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49152
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
  Uplinkfast enabled

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Altn BLK 3019      128.129  P2p
Fa3/2            Root FWD 3019      128.130  P2p

Disable and Clear the Uplink Fast Feature From the Switch

CatOS

Use the set spantree uplinkfast disable command in order to disable UplinkFast. Only the feature is disabled when this command is issued. All the tuning that is done on the port cost and switch priority remains unchanged:

A>(enable) set spantree uplinkfast disable
uplinkfast disabled for bridge.
Use clear spantree uplinkfast to return stp parameters to default.
A>(enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type          ieee

Designated Root             00-40-0b-cd-b4-09
Designated Root Priority    8192
Designated Root Cost        3100
Designated Root Port        2/1
Root Max Age   20 sec    Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-90-d9-5a-a8-00
Bridge ID Priority          49152
Bridge Max Age 20 sec    Hello Time 2  sec   Forward Delay 15 sec

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
 1/1                     1    not-connected  3019       32 disabled   0         
 1/2                     1    not-connected  3019       32 disabled   0         
 2/1                     1    forwarding     3100       32 disabled   0         
 2/2                     1    blocking       3100       32 disabled   0
 <snip>

Use the clear spantree uplinkfast command. This command not only disables the feature, but also resets the parameters:

A>(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.
A>(enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type          ieee

Designated Root             00-40-0b-cd-b4-09
Designated Root Priority    8192
Designated Root Cost        100
Designated Root Port        2/1
Root Max Age   20 sec    Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-90-d9-5a-a8-00
Bridge ID Priority          32768
Bridge Max Age 20 sec    Hello Time 2  sec   Forward Delay 15 sec

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
 1/1                     1    not-connected    19       32 disabled   0         
 1/2                     1    not-connected    19       32 disabled   0         
 2/1                     1    forwarding      100       32 disabled   0         
 2/2                     1    blocking        100       32 disabled   0
 <snip>

Cisco IOS

Use the no spanning-tree uplinkfast command in order to disable UplinkFast. In Cisco IOS switches, unlike CatOS switches, all the tuning that is done on the port cost and switch priority revert to the old values automatically at this point:

A(config)#no spanning-tree uplinkfast
A(config)#do show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    8193
             Address     0016.4748.dc80
             Cost        19
             Port        130 (FastEthernet3/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     0009.b6df.c401
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15 

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa3/1            Altn BLK 19        128.129  P2p 
Fa3/2            Root FWD 19        128.130  P2p

Conclusion

The UplinkFast feature dramatically decreases the convergence time of the STP in the event of the failure of an uplink on an access switch. UplinkFast interacts with other switches that have a strict standard STP. UplinkFast is only effective when the configured switch has some non-self-looped blocked ports. In order to increase the chances to have blocked ports, the port cost and bridge priority of the switch are modified. This tuning is consistent for an access switch, but is not useful on a core switch.

UplinkFast only reacts to direct link failure. A port on the access switch must physically go down in order to trigger the feature. Another Cisco proprietary feature, Backbone Fast, can help to improve convergence time of a bridged network in case of indirect link failure.

Command Reference

Related Information

Updated: Aug 18, 2008
Document ID: 10575