Configuring Spanning Tree

STP is a Layer 2 link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path must exist at Layer 2 between two stations. STP operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched LAN of multiple segments.

The Catalyst 6000 and 6500 series switches use STP (IEEE 802.1D bridge protocol) on all Ethernet virtual LANS (VLANs). When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a network. In STP, an algorithm calculates the best loop-free path throughout a Catalyst-switched network. The switches send and receive spanning-tree packets at regular intervals. The switches do not forward the packets, but use the packets to identify a loop-free path. The default configuration has STP enabled for all VLANs.

Multiple active paths between stations cause loops in the network. If a loop exists in the network, you might receive duplicate messages. When loops occur, some switches see stations on both sides of the switch. This condition confuses the forwarding algorithm and allows duplicate frames to be forwarded.

To provide path redundancy, STP defines a tree that spans all switches in an extended network. STP forces certain redundant data paths into a standby (blocked) state. If one network segment in the STP becomes unreachable, or if STP costs change, the spanning-tree algorithm reconfigures the spanning-tree topology and reestablishes the link by activating the standby path.

Election of the Root Switch

All switches in an extended LAN participating in STP gather information on other switches in the network through an exchange of data messages called Bridge Protocol Data Units (BPDUs). This exchange of messages results in the following actions:

The election of a unique root switch for the stable spanning-tree network topology
The election of a designated switch for every switched LAN segment
The removal of loops in the switched network by placing redundant switch ports in a backup state

The STP root switch is the logical center of the spanning-tree topology in a switched network. All paths that are not needed to reach the root switch from anywhere in the switched network are placed in STP backup mode. Table 7-1 describes the root switch variables that affect the entire spanning-tree performance.

Table 7-1: STP Root Switch Parameters
Variable Description

Hello Time

Determines how often the switch broadcasts its hello message to other switches.

Maximum Age Timer

Measures the age of the received protocol information recorded for a port and ensures that this information is discarded when its age limit exceeds the value of the maximum age parameter recorded by the switch. The timeout value is the maximum age parameter of the switches.

Forward Delay Timer

Monitors the time spent by a port in the learning and listening states. The timeout value is the forward delay parameter of the switches.

**Table 7-1: STP Root Switch Parameters**
Variable	Description
Hello Time	Determines how often the switch broadcasts its hello message to other switches.
Maximum Age Timer	Measures the age of the received protocol information recorded for a port and ensures that this information is discarded when its age limit exceeds the value of the maximum age parameter recorded by the switch. The timeout value is the maximum age parameter of the switches.
Forward Delay Timer	Monitors the time spent by a port in the learning and listening states. The timeout value is the forward delay parameter of the switches.

BPDUs contain information about the transmitting switch and its ports, including switch and port Media Access Control (MAC) addresses, switch priority, port priority, and port cost. The STP uses this information to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment.

Figure 7-1 shows how BPDUs enable an STP topology.

Figure 7-1: BPDUs Enabling a Stable STP Topology

Bridge Protocol Data Units

The stable active topology of a switched network is determined by the following:

The unique switch identifier (MAC address) associated with each switch
The path cost to the root associated with each switch port
The port identifier (MAC address) associated with each switch port

Each configuration BPDU contains the following minimal information:

The unique identifier of the switch that the transmitting switch believes to be the root switch
The cost of the path to the root from the transmitting port
The identifier of the transmitting port

The switch broadcasts configuration BPDUs to communicate and compute the spanning-tree topology. A MAC frame conveying a BPDU sends the switch group address to the destination address field. All switches connected to the LAN on which the frame is transmitted receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU, and, if topology changes, initiate a BPDU transmission.

A BPDU exchange results in the following:

One switch is elected as the root switch.
The shortest distance to the root switch is calculated for each switch.
A designated switch is selected. This is the switch closest to the root switch through which frames will be forwarded to the root.
A port for each switch is selected. This is the port providing the best path from the switch to the root switch.
Ports included in the STP are selected.

Creating a Stable STP Topology

If all switches are enabled with default settings, the switch with the lowest MAC address in the network becomes the root switch. In Figure 7-2, Switch A is the root switch because it has the lowest MAC address. However, due to traffic patterns, number of forwarding ports, or line types, Switch A might not be the ideal root switch. By increasing the priority (lowering the numerical priority number) of the ideal switch so that it becomes the root switch, you force STP to recalculate a new, stable topology.

Figure 7-2: Configuring a Stable STP Topology

When the stable STP topology is based on default parameters, the path between source and destination stations in a switched network might not be the most ideal. For instance, connecting higher-speed links to a port that has a higher number than the current root port can cause a root-port change. The goal is to make the fastest link the root port.

For example, assume that port 2 on Switch B is a fiber-optic link, and that port 1 on Switch B (an unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the high-speed fiber-optic link. By changing the Port Priority parameter for port 2 to a higher priority (lower numerical value) than port 1, port 2 becomes the root port. The same change can occur by changing the Port Cost parameter for port 2 to a lower value than that of port 1.

STP Port States

Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When a switch port transitions directly from nonparticipation in the stable topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for frames that have been forwarded using the old topology.

Each port on a switch using STP exists in one of the following five states:

Blocking
Listening
Learning
Forwarding
Disabled

A port moves through these five states as follows:

From initialization to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
From learning to forwarding or to disabled
From forwarding to disabled

Figure 7-3 illustrates how a port moves through the five states.

Figure 7-3: STP Port States

You can modify each port state by using management software. When you enable STP, every switch in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, the ports then stabilize to the forwarding or blocking state.

When the spanning-tree algorithm places a port in the forwarding state, the following occurs:

The port is put into the listening state while it waits for protocol information that suggests it should go to the blocking state.
The port waits for the expiration of a protocol timer that moves the port to the learning state.
In the learning state, the port continues to block frame forwarding as it learns station location information for the forwarding database.
The expiration of a protocol timer moves the port to the forwarding state, where both learning and forwarding are enabled.

Blocking State

A port in the blocking state does not participate in frame forwarding, as shown in Figure 7-4. After initialization, a BPDU is sent to each port in the switch. A switch initially assumes it is the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is really the root. If only one switch resides in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A switch always enters the blocking state following switch initialization.

Figure 7-4: Port 2 in Blocking State

A port in the blocking state performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)
Receives BPDUs and directs them to the system module.
Does not transmit BPDUs received from the system module.
Receives and responds to network management messages.

Listening State

The listening state is the first transitional state a port enters after the blocking state (see Figure 7-5). The port enters this state when STP determines that the port should participate in frame forwarding. Learning is disabled in the listening state.

Figure 7-5: Port 2 in Listening State

A port in the listening state performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)
Receives BPDUs and directs them to the system module.
Processes BPDUs received from the system module.
Receives and responds to network management messages.

Learning State

A port in the learning state prepares to participate in frame forwarding. The port enters the learning state from the listening state.

A port in the learning state performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Incorporates station location into its address database.
Receives BPDUs and directs them to the system module.
Receives, processes, and transmits BPDUs received from the system module.
Receives and responds to network management messages.

Forwarding State

A port in the forwarding state forwards frames (see Figure 7-6). The port enters the forwarding state from the learning state.

Figure 7-6: Port 2 in Forwarding State

A port in the forwarding state performs as follows:

Forwards frames received from the attached segment.
Forwards frames switched from another port for forwarding.
Incorporates station location information into its address database.
Receives BPDUs and directs them to the system module.
Processes BPDUs received from the system module.
Receives and responds to network management messages

Caution
Use the immediate-forwarding (portfast) mode only on ports connected to individual workstations to allow these ports to come up and go directly to the forwarding state, instead of going through the entire spanning-tree initialization process. To prevent illegal topologies, enable STP on ports connected to switches or other devices that forward messages.

Disabled State

A port in the disabled state does not participate in frame forwarding or STP (see Figure 7-7).

Figure 7-7: Port 2 in Disabled State

A disabled port performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Does not incorporate station location into its address database. (There is no learning, so there is no address database update.)
Receives BPDUs, but does not direct them to the system module.
Does not receive BPDUs for transmission from the system module.
Receives and responds to network management messages.

Default Spanning-Tree Configuration

Table 7-2 shows the default STP configuration.

Feature	Default Value
Enable state	STP enabled for all VLANs.
Port priority	128
Port cost	62
Bridge Priority	32,768

Table 7-2: STP Default Configuration

Feature Default Value

Enable state

STP enabled for all VLANs.

Port priority

128

Port cost

62

Bridge Priority

32,768

Configuring Spanning-Tree

These sections describe how to configure STP:

Enabling STP

Note STP is enabled by default on all VLANs and on all newly created VLANs.

To enable STP for all VLANs, perform this task in privileged mode:

Task Command

Step 1 Enable spanning tree.

set spantree enable {vlan | all}

Step 2 Verify that spanning tree is enabled.

show spantree [vlan]

Task	Command
Step 1 Enable spanning tree.	set spantree enable {vlan \| all}
Step 2 Verify that spanning tree is enabled.	show spantree [vlan]

This example shows how to enable spanning tree and verify that it is enabled on a selected VLAN:

Console> (enable) set spantree enable all
Spanning tree is enabled.
Console> (enable) show spantree 100
VLAN 100
Spanning tree enabled
Spanning tree type          ieee
Designated Root             00-10-0d-40-34-63
Designated Root Priority    32768
Designated Root Cost        19
Designated Root Port        1/2
Root Max Age   20 sec    Hello Time 2  sec   Forward Delay 15 sec
Bridge ID MAC ADDR          00-10-0d-aa-cc-63
Bridge ID Priority          32768
Bridge Max Age 20 sec    Hello Time 2  sec   Forward Delay 15 sec
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      100   forwarding        19        32   disabled              
Console> (enable)

Configuring the Port Priority

You can change the port priority of switch ports. The port with the lowest priority value forwards frames for all VLANs. The possible port-priority range is 0 through 63. The default is 32. If all ports have the same priority value, the port with the lowest port number forwards frames.

To change the port priority for a port, perform this task in privileged mode:

Task Command

Step 1 Change the port priority for a switch port.

set spantree portpri mod_num/port_num priority [vlans]

Step 2 Verify the port priority setting.

show spantree [mod_num/port_num]

Task	Command
Step 1 Change the port priority for a switch port.	set spantree portpri mod_num/port_num priority [vlans]
Step 2 Verify the port priority setting.	show spantree [mod_num/port_num]

This example shows how to change the port priority for a port and verify the configuration:

Console> (enable) set spantree portpri 1/2 20
Bridge port 1/2 port priority set to 20.
Console> (enable) show spantree 1/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      1     blocking          19        20   disabled              
 1/2      100   forwarding        19        20   disabled              
 1/2      521   blocking          19        20   disabled              
 1/2      522   blocking          19        20   disabled              
 1/2      523   blocking          19        20   disabled              
 1/2      524   blocking          19        20   disabled              
 1/2      1003  not-connected     19        20   disabled              
 1/2      1005  not-connected     19         4   disabled              
Console> (enable)

Configuring the Port VLAN Priority

You can set the port priority for a port on a per-VLAN basis. The port with the lowest priority value for a specific VLAN forwards frames for that VLAN. The possible port-VLAN priority range is 0 through 63. The default is 32. If all ports have the same priority value for a particular VLAN, the port with the lowest port number forwards frames for that VLAN.

To change the port-VLAN priority for a port, perform this task in privileged mode:

Task Command

Step 1 Change the port-VLAN priority for a VLAN on a switch port.

set spantree portvlanpri mod_num/port_num priority [vlans]

Step 2 Verify the port-VLAN priority setting.

show spantree [mod_num/port_num]

Task	Command
Step 1 Change the port-VLAN priority for a VLAN on a switch port.	set spantree portvlanpri mod_num/port_num priority [vlans]
Step 2 Verify the port-VLAN priority setting.	show spantree [mod_num/port_num]

This example shows how to change the port-VLAN priority on a port and verify the configuration:

Console> (enable) set spantree portvlanpri 1/2 1 100
Port 1/2 vlans 1-99,101-1004 using portpri 32.
Port 1/2 vlans 100 using portpri 1.
Port 1/2 vlans 1005 using portpri 4.
Console> (enable) show spantree 1/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      1     blocking          19        32   disabled              


 
   1/2      100   forwarding        19         1   disabled              
 1/2      521   blocking          19        32   disabled              
 1/2      522   blocking          19        32   disabled              
 1/2      523   blocking          19        32   disabled              
 1/2      524   blocking          19        32   disabled              
 1/2      1003  not-connected     19        32   disabled              
 1/2      1005  not-connected     19         4   disabled              
Console> (enable)

Configuring Port Cost

You can change the port cost of switch ports. Ports with lower port costs are more likely to be chosen to forward frames. Assign lower numbers to ports attached to faster media (such as full duplex) and higher numbers to ports attached to slower media.The possible range is 1 to 65535. The default differs for different media. Path cost is typically 1000 ÷ LAN speed in megabits per second.

To change the port cost for a port, perform this task in privileged mode:

Task Command

Step 1 Change the port cost for a switch port.

set spantree portcost mod_num/port_num cost

Step 2 Verify the port cost setting.

show spantree [mod_num/port_num]

Task	Command
Step 1 Change the port cost for a switch port.	set spantree portcost mod_num/port_num cost
Step 2 Verify the port cost setting.	show spantree [mod_num/port_num]

This example shows how to change the port-VLAN priority on a port and verify the configuration:

Console> (enable) set spantree portcost 1/2 10
Spantree port 1/2 path cost set to 10.
Console> (enable) show spantree 1/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      1     forwarding        10        20   disabled              
 1/2      100   forwarding        10        20   disabled              
 1/2      521   forwarding        10        20   disabled              
 1/2      522   forwarding        10        20   disabled              
 1/2      523   forwarding        10        20   disabled              
 1/2      524   forwarding        10        20   disabled              
 1/2      1003  not-connected     10        20   disabled              
 1/2      1005  not-connected     10         4   disabled              
Console> (enable)

Configuring Port-VLAN Cost

You can change the port cost for a port on a per-VLAN basis. Ports with lower port-VLAN costs are more likely to be chosen to forward frames. You should assign lower numbers to ports attached to faster media (such as full duplex) and higher numbers to ports attached to slower media. The possible range is 1 to 65535. The default differs for different media.

To change the port-VLAN cost for a port, perform this task in privileged mode:

Task Command

Step 1 Change the port-VLAN cost for a VLAN on a switch port.

set spantree portvlancost mod_num/port_num cost cost [vlans]

Step 2 Verify the port-VLAN cost setting.

show spantree [mod_num/port_num]

Task	Command
Step 1 Change the port-VLAN cost for a VLAN on a switch port.	set spantree portvlancost mod_num/port_num cost cost [vlans]
Step 2 Verify the port-VLAN cost setting.	show spantree [mod_num/port_num]

This example shows how to change the port-VLAN priority on a port and verify the configuration:

Console> (enable) set spantree portvlancost 1/2 cost 10 100
Port 1/2 VLANs 1-99,101-1005 have path cost 19.
Port 1/2 VLANs 100 have path cost 10.
Console> (enable) show spantree 1/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      1     blocking          19        20   disabled              


 
   1/2      100   forwarding        10        20   disabled              
 1/2      521   blocking          19        20   disabled              
 1/2      522   blocking          19        20   disabled              
 1/2      523   blocking          19        20   disabled              
 1/2      524   blocking          19        20   disabled              
 1/2      1003  not-connected     19        20   disabled              
 1/2      1005  not-connected     19         4   disabled              
Console> (enable)

Configuring Spanning-Tree PortFast

With spanning-tree PortFast, a switch port connected to a single workstation or server can connect faster by causing spanning tree to enter the forwarding state immediately, bypassing the listening and learning states.

Note Use spanning-tree PortFast only when connecting a single end station to a switch port. Otherwise, you might create a network loop.

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_num/port_num {enable | disable}

Step 2 Verify the PortFast setting.

show spantree [mod_num/port_num]

Task	Command
Step 1 Enable PortFast on a switch port connected to a single workstation or server.	set spantree portfast mod_num/port_num {enable \| disable}
Step 2 Verify the PortFast setting.	show spantree [mod_num/port_num]

This example shows how to enable PortFast on a port and verify the configuration:

Console> (enable) set spantree portfast 1/2 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 1/2 fast start enabled.
Console> (enable) show spantree 1/2
Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method
--------- ----  -------------  -----  --------  ----------  ------------
 1/2      1     blocking          19        20   enabled              
 1/2      100   forwarding        10        20   enabled              
 1/2      521   blocking          19        20   enabled              
 1/2      522   blocking          19        20   enabled              
 1/2      523   blocking          19        20   enabled              
 1/2      524   blocking          19        20   enabled              
 1/2      1003  not-connected     19        20   enabled              
 1/2      1005  not-connected     19         4   enabled              
Console> (enable)

Configuring a Primary Root Switch

To configure a switch as the primary root switch, perform this task in privileged mode:

Task Command

Configure a switch as the primary root switch.

set spantree root vlans [dia network_diameter] [hello hello_time]

Task	Command
Configure a switch as the primary root switch.	set spantree root vlans [dia network_diameter] [hello hello_time]

This example shows how to specify the primary root switch for VLANs 1-10:

Console> (enable) set spantree root 1-10 dia 4
VLANs 1-10 bridge priority set to 8192
VLANs 1-10 bridge max aging time set to 14 seconds.
VLANs 1-10 bridge hello time set to 2 seconds.
VLANs 1-10 bridge forward delay set to 9 seconds.
Switch is now the root switch for active VLANs 1-6.
Console> (enable)

Note Run the set spantree root command on backbone switches or distribution switches only, not on access switches.

The set spantree root command reduces the bridge priority (the value associated with the switch) from the default (32,768) to a significantly lower value, which allows the switch to become the root switch.

When you specify a switch as the primary root, the default bridge priority is modified so that it becomes the root for the specified VLANs. Set the bridge priority to 8192. If this setting does not result in the switch becoming a root, modify the bridge priority to be 100 less than the bridge priority of the current root switch. Since different VLANs could potentially have different root switches, the bridge VLAN-priority chosen makes this switch the root for all the VLANs specified. If reducing the bridge priority as low as 1 still does not make the switch the root switch, the switch displays a message.

Configuring a Secondary Root Switch

To configure a switch as the secondary root switch, perform this task in privileged mode:

Task Command

Configure a switch as the secondary root switch.

set spantree root [secondary] vlans [dia network_diameter] [hello hello_time]

Task	Command
Configure a switch as the secondary root switch.	set spantree root [secondary] vlans [dia network_diameter] [hello hello_time]

This example shows how to specify the secondary root switch for VLANs 22 and 24:

Console> (enable) set spantree root secondary 22,24 dia 5 hello 1
VLANs 22,24 bridge priority set to 16384.
VLANs 22,24 bridge max aging time set to 10 seconds.
VLANs 22,24 bridge hello time set to 1 second.
VLANs 22,24 bridge forward delay set to 7 seconds.
Console> (enable)

The set spantree root secondary command reduces the bridge priority to 16,384, making it the probable candidate to become the root switch if the primary root switch fails. You can run this command on more than one switch to create multiple backup switches in case the primary root switch fails.

Configuring Additional STP Parameters

To configure additional STP parameters, perform one of these tasks in privileged mode:

Task Command

Set the bridge forward delay for a VLAN.

set spantree fwddelay delay [vlan]

Set the bridge hello time for a VLAN.

set spantree hello interval

Set the bridge maximum aging time for a VLAN.

set spantree maxage agingtime [vlan]

Set the bridge priority for a VLAN.

set spantree priority bridge_priority [vlan]

Task	Command
Set the bridge forward delay for a VLAN.	set spantree fwddelay delay [vlan]
Set the bridge hello time for a VLAN.	set spantree hello interval
Set the bridge maximum aging time for a VLAN.	set spantree maxage agingtime [vlan]
Set the bridge priority for a VLAN.	set spantree priority bridge_priority [vlan]

Disabling STP

To disable STP across all VLANs, enter this command in privileged mode:

Task Command

Disable STP.

set spantree disable [vlan]

Task	Command
Disable STP.	set spantree disable [vlan]

Using Spanning-Tree Uplink Fast Switchover

This section describes the UplinkFast feature (also known as the spanning-tree Uplink Fast Switchover feature):

Understanding How UplinkFast Works

UplinkFast provides fast convergence after a spanning-tree topology change and achieves load balancing between redundant links using uplink groups. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. 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.

Note UplinkFast is most useful in wiring-closet switches. This feature may not be useful for other types of applications.

Figure 7-8 shows an example topology with no link failures. 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 directly to Switch B is in the blocking state.

Figure 7-8: UplinkFast 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 without going through the listening and learning states (see Figure 7-9). This switchover takes approximately one to five seconds.

Figure 7-9: UplinkFast After Direct Link Failure

Configuring UplinkFast

Note When you enable the set spantree uplinkfast command, it affects all VLANs on the switch. You cannot configure UplinkFast on an individual VLAN.

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

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

The set spantree uplinkfast enable command increases the path cost of all ports on the switch, making it unlikely that the switch becomes the root switch. The station_update_rate value represents the number of multicast packets transmitted per 100 milliseconds (the default is 15 packets per millisecond).

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 15 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)

Using Spanning-Tree Backbone Fast Convergence

This section describes the BackboneFast feature (also known as the spanning-tree Backbone Fast Convergence feature):

Understanding How BackboneFast Works

BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. An inferior BPDU identifies one switch as both the root bridge and the designated bridge. When a switch receives an inferior BPDU, it indicates that a link to which the switch is not directly connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root bridge). Under normal spanning-tree rules, the switch ignores inferior BPDUs for the configured maximum aging time, as specified by the agingtime variable of 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. (Self-looped ports are not considered 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, causes the maximum aging time on the root to expire, and becomes the root switch according to normal spanning-tree rules.

If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of PDU called the Root Link Query PDU. The switch sends the Root Link Query PDU 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 the blocking state), through the listening and learning states, and into the forwarding state.

Figure 7-10 shows an example topology with no link failures. 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 is in the blocking state.

Figure 7-10: BackboneFast Before Indirect Link Failure

If link L1 fails, Switch C detects this failure as an indirect failure, since 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 7-11 shows how BackboneFast reconfigures the topology to account for the failure of link L1.

Figure 7-11: BackboneFast After Indirect Link Failure

If a new switch is introduced into a shared-medium topology, BackboneFast is not activated. Figure 7-12 shows a shared-medium topology in which a new switch is added. The new switch begins sending inferior BPDUs that say 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 7-12: Adding a Switch in a Shared-Medium Topology

Configuring BackboneFast

These sections describe how to configure Backbone Fast Convergence:

Enabling BackboneFast

Note For BackboneFast to work, 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.

To enable BackboneFast, 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

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

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, 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

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)

Table of Contents

Configuring Spanning Tree