API Guide

BPDU guard Blocks the L2 bridged ports and LAG ports connected to end hosts and servers from receiving any
BPDUs. When you enable BPDU guard and when the BPDU frames are being received on the interface,
the bridge or LAG is placed in the blocking state. In case of a LAG, ports are either STP blocked or
shutdown based on the error disable command action. The data traffic is dropped but the port continues
to forward BPDUs to the CPU that are later dropped. To prevent further reception of BPDUs, configure a
port to shut down using the error disable command. For more information on this command.
Root guard Preserves the root bridge position during network transitions. STP selects the root bridge with the lowest
priority value. During network transitions, another bridge with a lower priority may attempt to become the
root bridge and cause unpredictable network behavior. To avoid such an attempt and to preserve the
position of the root bridge, configure the spanning-tree guard root command. This configuration
places the port in an inconsistent state if the port receives superior BPDU. Root guard is enabled only on
designated ports. The root guard configuration applies to all VLANs configured on the port.
Loop guard Prevents L2 forwarding loops caused by a cable or interface hardware failure. When a hardware failure
occurs, a participating spanning-tree link becomes unidirectional and the port stops receiving BPDUs.
When the blocked port stops receiving BPDUs, it transitions to a Forwarding state causing spanning-tree
loops in the network. Enable loop guard using the spanning-tree guard loop command on an
interface so that it transitions to the Loop-Inconsistent state until it receives BPDUs. After BPDUs are
received, the port moves out of the Loop-Inconsistent or Blocking state and transitions to an appropriate
state determined by STP. Enabling loop guard on a per-port basis enables it on all VLANs configured on
the port.
NOTE:
1. Root guard and Loop guard are mutually exclusive.
2. Configuring one overwrites the other from the active configuration.
1. Enable spanning-tree BPDU filter in INTERFACE mode.
spanning-tree bpdufilter enable
2. Enable STP BPDU guard in INTERFACE mode.
spanning-tree bpduguard enable
BPDU guard violation causes the system to perform the following actions in the port channel:
● The interface and all member ports are disabled in the hardware.
● When the port is added to the port channel that is in the Error Disable state, the new member port is disabled in the
hardware.
● When the port is removed from the port channel that is in the Error Disable state, the system clears the Error_Disabled
state on the physical port and enables it in the hardware.
To clear the Error Disabled state:
● Use the shutdown command on the interface.
● Use the spanning-tree bpduguard disable command to disable the BPDU guard on the interface.
● Use the spanning-tree disable command to disable STP on the interface.
3. Set the guard types to avoid loops in INTERFACE mode.
spanning-tree guard {loop | root | none}
● loop — Set the guard type to loop.
● root — Set the guard type to root.
● none — Set the guard type to none.
Port enabled with loop guard conditions
● Loop guard is supported on any STP-enabled port or port-channel interface.
● You cannot enable root guard and loop guard at the same time on an STP port. The loop guard configuration overwrites
an existing root guard configuration and vice versa.
● Enabling BPDU guard and loop guard at the same time on a port results in a port that remains in blocking state and
prevents traffic from flowing through it. For example, when you configure both Portfast BPDU guard and loop guard:
○ If a BPDU is received from a remote device, BPDU guard places the port in the Err-Disabled Blocking state and no
traffic forwards on the port.
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