LAN Switching Configuration Guide, Cisco IOS XE Release 3S
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For conceptual information about Spanning Tree Protocol, see the “Using the Spanning Tree Protocol with the EtherSwitch Network
Module” section of the EtherSwitch Network feature module.
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Information About Spanning Tree Protocol
Using the Spanning Tree Protocol with the EtherSwitch Network Module
The EtherSwitch Network Module uses Spanning Tree Protocol (STP) (the IEEE 802.1D bridge protocol) on all VLANs. By default,
a single instance of STP runs on each configured VLAN (provided that you do not manually disable STP). You can enable and
disable STP on a per-VLAN basis.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a network. The spanning
tree algorithm calculates the best loop-free path throughout a switched Layer 2 network. Switches send and receive spanning
tree frames at regular intervals. The switches do not forward these frames but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network, end stations might
receive duplicate messages and switches might learn endstation MAC addresses on multiple Layer 2 interfaces. These conditions
result in an unstable network.
STP defines a tree with a root switch and a loop-free path from the root to all switches in the Layer 2 network. STP forces
redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails and a redundant path
exists, the spanning tree algorithm recalculates the spanning tree topology and activates the standby path.
When two ports on a switch are part of a loop, the spanning tree port priority and port path cost setting determine which
port is put in the forwarding state and which port is put in the blocking state. The spanning tree port priority value represents
the location of an interface in the network topology and how well located it is to pass traffic. The spanning tree port path
cost value represents media speed.
Spanning Tree Port States
Propagation delays occur when protocol information passes through a switched LAN. As a result, topology changes take place
at different times and at different places in a switched network. When a Layer 2 interface changes from nonparticipation in
the spanning tree topology to the forwarding state, it creates 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 are forwarded using the old topology.
Each Layer 2 interface on a switch using Spanning Tree Protocol (STP) exists in one of the following states:
Blocking—The Layer 2 interface does not participate in frame forwarding.
Disabled—The Layer 2 interface does not participate in spanning tree and is not forwarding frames.
Forwarding—The Layer 2 interface forwards frames.
Learning—The Layer 2 interface prepares to participate in frame forwarding.
Listening—First transitional state after the blocking state when spanning tree determines that the Layer 2 interface must
participate in frame forwarding.
A Layer 2 interface moves through the following states:
From blocking state to listening or disabled state.
From forwarding state to disabled state.
From initialization to blocking state.
From learning state to forwarding or disabled state.
From listening state to learning or disabled state.
The figure below illustrates how a port moves through these five states.
Boot-up Initialization
When you enable Spanning Tree Protocol (STP), every port in the switch, VLAN, or network goes through the blocking state
and transitory states of listening and learning at power up. If properly configured, each Layer 2 interface stabilizes to
the forwarding or blocking state.
When the spanning tree algorithm places a Layer 2 interface in the forwarding state, the following process occurs:
The Layer 2 interface is put into the listening state while it waits for protocol information to go to the blocking state.
The Layer 2 interface waits for the forward delay timer to expire, moves the Layer 2 interface to the learning state, and
resets the forward delay timer.
The Layer 2 interface continues to block frame forwarding in the learning state as it learns end station location information
for the forwarding database.
The Layer 2 interface waits for the forward delay timer to expire and then moves the Layer 2 interface to the forwarding
state, where both learning and frame forwarding are enabled.
Blocking State
A Layer 2 interface in the blocking state does not participate in frame forwarding, as shown in the figure below. After initialization,
a bridge protocol data unit (BPDU) is sent out to each Layer 2 interface in the switch. The switch initially assumes it is
the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root or
root bridge. If only one switch is in the network, no exchange occurs, the forward delay timer expires, and the ports move
to the listening state. A port enters the blocking state following switch initialization.
A Layer 2 interface in the blocking state performs as follows:
Discards frames received from the attached segment.
Discards frames switched from another interface for forwarding.
Does not incorporate end station location into its address database. (There is no learning on a blocking Layer 2 interface,
so there is no address database update.)
Does not transmit BPDUs received from the system module.
Receives BPDUs and directs them to the system module.
Receives and responds to network management messages.
Listening State
The listening state is the first transitional state a Layer 2 interface enters after the blocking state. The Layer 2 interface
enters this state when STP determines that the Layer 2 interface must participate in frame forwarding. The figure below shows
a Layer 2 interface in the listening state.
A Layer 2 interface in the listening state performs as follows:
Discards frames received from the attached segment.
Discards frames switched from another interface for forwarding.
Does not incorporate end station location into its address database. (There is no learning on a blocking Layer 2 interface,
so there is no address database update.)
Receives and directs BPDUs to the system module.
Receives, processes, and transmits BPDUs received from the system module.
Receives and responds to network management messages.
Learning State
The learning state prepares a Layer 2 interface to participate in frame forwarding. The Layer 2 interface enters the learning
state from the listening state. The figure below shows a Layer 2 interface in the learning state.
A Layer 2 interface in the learning state performs as follows:
Discards frames received from the attached segment.
Discards frames switched from another interface for forwarding.
Incorporates end 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 Layer 2 interface in the forwarding state forwards frames, as shown in the figure below. The Layer 2 interface enters the
forwarding state from the learning state.
A Layer 2 interface in the forwarding state performs as follows:
Forwards frames received from the attached segment.
Forwards frames switched from another Layer 2 interface for forwarding.
Incorporates end 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.
Disabled State
A Layer 2 interface in the disabled state does not participate in frame forwarding or spanning tree, as shown in the figure
below. A Layer 2 interface in the disabled state is virtually nonoperational.
A Layer 2 interface in the disabled state performs as follows:
Discards frames received from the attached segment.
Discards frames switched from another Layer 2 interface for forwarding.
Does not incorporate end station location into its address database. (There is no learning on a blocking Layer 2 interface,
so there is no address database update.)
Does not receive BPDUs for transmission from the system module.
Default Spanning Tree Configuration
The table below shows the default Spanning Tree Protocol (STP) configuration values.
Table 1. SPT Default Configuration Values
Feature
Default Value
Bridge priority
32768
Enable state
Spanning tree enabled for all VLANs
Forward delay time
15 seconds
Hello time
2 seconds
Maximum aging time
20 seconds
Spanning tree port cost (configurable on a per-interface basis; used on interfaces configured as Layer 2 access ports)
Fast Ethernet: 19
Ethernet: 100
Gigabit Ethernet: 19 when operated in 100 Mb mode, and 4 when operated in 1000 Mb mode
Spanning tree port priority (configurable on a per-interface basis; used on interfaces configured as Layer 2 access ports)
128
Spanning tree VLAN port cost (configurable on a per-VLAN basis; used on interfaces configured as Layer 2 trunk ports)
Fast Ethernet: 10
Ethernet: 10
Spanning tree VLAN port priority (configurable on a per-VLAN basis; used on interfaces configured as Layer 2 trunk ports)
128
Bridge Protocol Data
Units
The stable active
spanning tree topology of a switched network is determined by the following:
Port identifier
(port priority and MAC address) associated with each Layer 2 interface.
Spanning tree
path cost to the root bridge.
Unique bridge ID
(bridge priority and MAC address) associated with each VLAN on each switch.
The bridge protocol
data units (BPDUs) are transmitted in one direction from the root switch and
each switch sends configuration BPDUs to communicate and compute the spanning
tree topology. Each configuration BPDU contains the following minimal
information:
Bridge ID of the
transmitting bridge
Message age
Port identifier
of the transmitting port
Spanning tree
path cost to the root
Unique bridge
ID of the switch that the transmitting switch believes to be the root switch
Values for the
hello, forward delay, and max-age protocol timers
When a switch
transmits a BPDU frame, all switches connected to the LAN on which the frame is
transmitted receive the BPDU. When a switch receives a BPDU, it does not
forward the frame but uses the information in the frame to calculate a BPDU,
and, if the topology changes, begin a BPDU transmission.
A BPDU exchange
results in the following:
A designated
bridge for each LAN segment is selected. This is the switch closest to the root
bridge through which frames are forwarded to the root.
A root port is
selected. This is the port providing the best path from the bridge to the root
bridge.
One switch is
elected as the root switch.
Ports included
in the spanning tree are selected.
The shortest
distance to the root switch is calculated for each switch based on the path
cost.
For each VLAN, the
switch with the highest bridge priority (the lowest numerical priority value)
is elected as the root switch. If all switches are configured with the default
priority (32768), the switch with the lowest MAC address in the VLAN becomes
the root switch.
The spanning tree
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 spanning tree blocking mode.
BPDUs contain
information about the transmitting bridge and its ports, including bridge and
MAC addresses, bridge priority, port priority, and path cost. Spanning tree
uses this information to elect the root bridge and root port for the switched
network, as well as the root port and designated port for each switched
segment.
MAC Address Allocation
MAC addresses are
allocated sequentially, with the first MAC address in the range assigned to
VLAN 1, the second MAC address in the range assigned to VLAN 2, and so forth.
For example, if the MAC address range is 00-e0-1e-9b-2e-00 to
00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge
ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so
forth.
BackboneFast
BackboneFast is
started when a root port or blocked port on a switch receives inferior bridge
protocol data units (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 means that a link to which the switch is
not directly connected is failed. That is, the designated bridge has lost its
connection to the root switch. Under Spanning Tree Protocol (STP) rules, the
switch ignores inferior BPDUs for the configured maximum aging time specified
by the
spanning-tree
max-age command.
The switch
determines if it has an alternate path to the root switch. 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 switch. If the inferior BPDU arrives on the
root port, all blocked ports become alternate paths to the root switch. If the
inferior BPDU arrives on the root port and there are no blocked ports, the
switch assumes that it lost connectivity to the root switch, causes the maximum
aging time on the root to expire, and becomes the root switch according to
normal STP rules.
Note
Self-looped
ports are not considered as alternate paths to the root switch.
If the switch
possesses alternate paths to the root switch, it uses these alternate paths to
transmit the protocol data unit (PDU) that is called the root link query PDU.
The switch sends the root link query PDU on all alternate paths to the root
switch. If the switch determines that it has an alternate path to the root, it
causes the maximum aging time on ports on which it received the inferior BPDU
to expire. If all the alternate paths to the root switch indicate that the
switch has lost connectivity to the root switch, the switch causes the maximum
aging time on the ports on which it received an inferior BPDU to expire. If one
or more alternate paths connect to the root switch, 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.
The figure below
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
interface on Switch C that connects directly to Switch B is in the blocking
state.
Figure 1. BackboneFast Example Before
Indirect Link Failure
If link L1 fails,
Switch C cannot detect this failure because it is not connected directly to
link L1. However, Switch B is directly connected to the root switch over L1 and
it detects the failure, elects itself as the root switch, and begins sending
BPDUs to Switch C. When Switch C receives the inferior BPDUs from Switch B,
Switch C assumes that an indirect failure has occurred. At that point,
BackboneFast allows the blocked port on Switch C to move to the listening state
without waiting for the maximum aging time for the port to expire. BackboneFast
then changes the interface on Switch C to the forwarding state, providing a
path from Switch B to Switch A. This switchover takes 30 seconds, twice the
forward delay time, if the default forward delay time of 15 seconds is set. The
figure below shows how BackboneFast reconfigures the topology to account for
the failure of link L1.
Figure 2. BackboneFast Example After
Indirect Link Failure
If a new switch
is introduced into a shared-medium topology as shown in the figure below,
BackboneFast is not activated because inferior BPDUs did not come from the
designated bridge (Switch B). 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 3. Adding a Switch in a
Shared-Medium Topology
STP Timers
The table below describes the Spanning Tree Protocol (STP) timers that affect the entire spanning tree performance.
Table 2. STP Timers
Timer
Purpose
Forward delay timer
Determines how long listening state and learning state last before the port begins forwarding.
Hello timer
Determines how often the switch broadcasts hello messages to other switches.
Maximum age timer
Determines how long a switch can store the protocol information received on a port.
Spanning Tree Port Priority
Spanning tree considers port priority when selecting an interface to put into the forwarding state if there is a loop. You
can assign higher priority values to interfaces that you want spanning tree to select first, and lower priority values to
interfaces that you want spanning tree to select last. If all interfaces possess the same priority value, spanning tree puts
the interface with the lowest interface number in the forwarding state and blocks other interfaces. The spanning tree port
priority range is from 0 to 255, configurable in increments of 4. The default value is 128.
Cisco software uses the port priority value when an interface is configured as an access port and uses VLAN port priority
values when an interface is configured as a trunk port.
Spanning Tree Port Cost
The spanning tree port path cost default value is derived from the media speed of an interface. if there is a loop, spanning
tree considers port cost value when moving an interface to the forwarding state. You can assign lower port cost values to
interfaces that you want spanning tree to select first and higher port cost values to interfaces that you want spanning tree
to select last. If all interfaces have the same port cost value, spanning tree puts the interface with the lowest interface
number to the forwarding state and blocks other interfaces.
The port cost range is from 0 to 65535. The default value is media-specific.
Spanning tree uses the port cost value when an interface is configured as an access port and uses VLAN port cost value when
an interface is configured as a trunk port.
Spanning tree port cost value calculations are based on the bandwidth of the port. There are two classes of port cost values.
Short (16-bit) values are specified by the IEEE 802.1D specification and the range is from 1 to 65535. Long (32-bit) values
are specified by the IEEE 802.1t specification and the range is from 1 to 200,000,000.
Assigning Short Port Cost Values
You can manually assign port cost values in the range of 1 to 65535. Default port cost values are listed in Table 2.
Table 3. Default Port Cost Values
Port Speed
Default Port Cost Value
10 Mbps
100
100 Mbps
19
Assigning Long Port Cost Values
You can manually assign port cost values in the range of 1 to 200,000,000. Default port cost values are listed in Table 3.
Table 4. Default Port Cost Values
Port Speed
Recommended Value
Recommended Range
10 Mbps
2,000,000
200,000 to 20,000,000
100 Mbps
200,000
20,000 to 2,000,000
Spanning Tree Root Bridge
The EtherSwitch HWIC maintains a separate instance of spanning tree for each active VLAN configured on the device. A bridge
ID, consisting of the bridge priority and the bridge MAC address, is associated with each instance. For each VLAN, the device
with the lowest bridge ID will become the root bridge for that VLAN.
To configure a VLAN instance to become the root bridge, the bridge priority can be modified from the default value (32768)
to a lower value so that the bridge becomes the root bridge for the specified VLAN. Use the
spanning-tree vlan root command to alter the bridge priority.
The device checks the bridge priority of current root bridges for each VLAN. The bridge priority for specified VLANs is set
to 8192, if this value is caused the device to become the root for specified VLANs.
If any root device for specified VLANs has a bridge priority lower than 8192, the device sets the bridge priority for specified
VLANs to 1 less than the lowest bridge priority.
For example, if all devices in a network have the bridge priority for VLAN 100 set to the default value of 32768, entering
the
spanning-treevlan100rootprimary command on a device sets the bridge priority for VLAN 100 to 8192, causing the device to become the root bridge for VLAN
100.
Note
The root device for each instance of spanning tree must be a backbone or distribution device. Do not configure an access
device as the spanning tree primary root.
Use the
diameter keyword to specify the Layer 2 network diameter. That is, the maximum number of bridge hops between any two end stations
in the Layer 2 network. When you specify the network diameter, the device automatically picks an optimal hello time, a forward
delay time, and a maximum age time for a network of that diameter, which reduces the spanning tree convergence time. You can
use the
hello keyword to override the automatically calculated hello time.
Note
We recommend that you do not configure the hello time, forward delay time, and maximum age time manually after you configure
the device as the root bridge.
How to Configure Spanning Tree Protocol
Enabling Spanning Tree Protocol
You can enable spanning tree protocol on a per-VLAN basis. The device maintains a separate instance of spanning tree for
each VLAN except for which you disable spanning tree.
SUMMARY STEPS
enable
configureterminal
spanning-tree vlan
vlan-id
end
showspanning-treevlanvlan-id
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Device# configure terminal
Enters global configuration mode.
Step 3
spanning-tree vlan
vlan-id
Example:
Device(config)# spanning-tree vlan 200
Enables spanning tree on a per-VLAN basis.
Step 4
end
Example:
Device(config)# end
Exits global configuration mode and enters privileged EXEC mode.
Step 5
showspanning-treevlanvlan-id
Example:
Device# show spanning-tree vlan 200
Verifies spanning tree configuration.
Configuring the Bridge Priority of a VLAN
SUMMARY STEPS
enable
configureterminal
spanning-treevlanvlan-idprioritybridge-priority
showspanning-treevlanbridge
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Device# configure terminal
Enters global configuration mode.
Step 3
spanning-treevlanvlan-idprioritybridge-priority
Example:
Device(config)# spanning-tree vlan 200 priority 2
Configures the bridge priority of a VLAN. The bridge priority value ranges from 0 to 65535.
Caution
Use the
spanning-treevlanvlan-idrootprimary command and the
spanning-treevlanvlan-idrootsecondary command to modify the bridge priority.
Use this command with the
vlan keyword to display the spanning tree information about a specified VLAN.
Example:
Device# show spanning-tree vlan 200
VLAN200 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0050.3e8d.6401
Configured hello time 2, max age 20, forward delay 15
Current root has priority 16384, address 0060.704c.7000
Root port is 264 (FastEthernet5/8), cost of root path is 38
Topology change flag not set, detected flag not set
Number of topology changes 0 last change occurred 01:53:48 ago
Times: hold 1, topology change 24, notification 2
hello 2, max age 14, forward delay 10
Timers: hello 0, topology change 0, notification 0
Example:
Port 264 (FastEthernet5/8) of VLAN200 is forwarding
Port path cost 19, Port priority 128, Port Identifier 129.9.
Designated root has priority 16384, address 0060.704c.7000
Designated bridge has priority 32768, address 00e0.4fac.b000
Designated port id is 128.2, designated path cost 19
Timers: message age 3, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 3, received 3417
Use this command with the
interface keyword to display spanning tree information about a specified interface.
Example:
Device# show spanning-tree interface fastethernet 5/8
Port 264 (FastEthernet5/8) of VLAN200 is forwarding
Port path cost 19, Port priority 100, Port Identifier 129.8.
Designated root has priority 32768, address 0010.0d40.34c7
Designated bridge has priority 32768, address 0010.0d40.34c7
Designated port id is 128.1, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 0, received 13513
Use this command with the
bridge,
brief, and
vlan keywords to display the bridge priority information.
Example:
Device# show spanning-tree bridge brief vlan 200
Hello Max Fwd
Vlan Bridge ID Time Age Delay Protocol
---------------- -------------------- ---- ---- ----- --------
VLAN200 33792 0050.3e8d.64c8 2 20 15 ieee
Configuration Examples for Spanning Tree Protocol
Example: Enabling Spanning Tree Protocol
The following example shows how to enable spanning tree protocol on VLAN 20:
Device# configure terminal
Device(config)# spanning-tree vlan 20
Device(config)# end
Device#
Note
Because spanning tree is enabled by default, the
showrunning command will not display the command you entered to enable spanning tree protocol.
The following example shows how to disable spanning tree protocol on VLAN 20:
Device# configure terminal
Device(config)# no spanning-tree vlan 20
Device(config)# end
Device#
Example: Configuring the Bridge Priority of a VLAN
The following example shows how to configure the bridge priority of VLAN 20 to 33792:
The following example shows how to configure VLAN port priority on an interface:
Device# configure terminal
Device(config)# interface fastethernet 0/3/2
Device(config-if)# spanning-tree vlan 20 port priority 64
Device(config-if)# end
The following example shows how to verify the configuration of VLAN 20 on an interface when it is configured as a trunk port:
Device#show spanning-tree vlan 20
VLAN20 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 00ff.ff90.3f54
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32768, address 00ff.ff10.37b7
Root port is 33 (FastEthernet0/3/2), cost of root path is 19
Topology change flag not set, detected flag not set
Number of topology flags 0 last change occurred 00:05:50 ago
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 0
Port 33 (FastEthernet0/3/2) of VLAN20 is forwarding
Port path cost 18, Port priority 64, Port Identifier 64.33
Designated root has priority 32768, address 00ff.ff10.37b7
Designated bridge has priority 32768, address 00ff.ff10.37b7
Designated port id is 128.13, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 1, received 175
Example: Configuring Spanning Tree Port Cost
The following example shows how to change the spanning tree port cost of a Fast Ethernet interface:
Device# configure terminal
Device(config)# interface fastethernet0/3/2
Device(config-if)# spanning-tree cost 18
Device(config-if)# end
Device#
Device# show run interface fastethernet0/3/2
Building configuration...
Current configuration: 140 bytes
!
interface FastEthernet0/3/2
switchport access vlan 20
no ip address
spanning-tree vlan 20 port-priority 64
spanning-tree cost 18
end
The following example shows how to verify the configuration of a Fast Ethernet interface when it is configured as an access
port:
Device# show spanning-tree interface fastethernet0/3/2
Port 33 (FastEthernet0/3/2) of VLAN20 is forwarding
Port path cost 18, Port priority 64, Port Identifier 64.33
Designated root has priority 32768, address 00ff.ff10.37b7
Designated bridge has priority 32768, address 00ff.ff10.37b7
Designated port id is 128.13, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
BPDU: sent 1, received 175
Example: Configuring Spanning Tree Root Bridge
The following example shows how to configure the spanning tree root bridge for VLAN 10, with a network diameter of 4:
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The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
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Table 5. Feature Information for Spanning Tree Protocol
Feature Name
Releases
Feature Information
Spanning Tree Protocol
12.1(1)E
Spanning Tree Protocol (STP) is a Layer 2 link management protocol that provides path redundancy while preventing undesirable
loops in the network.
The following commands were introduced or modified:
spanning-tree vlan,
spanning-tree port-priority, and
spanning-tree cost.
Feedback