Connectivity Guide

CPU that are later dropped. To prevent further reception of BPDUs, configure a port to shut down using
the shutdown command. The port can only resume operation from the Shutdown state after manual
intervention.
Root guard Avoids bridging loops and 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 preserve the position of the root bridge, configure the spanning-tree guard root
command. Root guard is enabled on ports that are 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 a port stops receiving BPDUs. When
a blocked port stops receiving BPDUs, it transitions to a Forwarding state causing spanning tree loops
in the network. You can enable loop guard on a port that transitions to the Loop-Inconsistent state
until it receives BPDUs using the spanning-tree guard loop command. 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. If you disable loop guard on a port, it is moved to the Listening state.
If you enable BPDU Filter and BPDU Guard on the same port, the BPDU Filter configuration takes precedence. Root Guard and
Loop Guard are mutually exclusive. Configuring one overwrites the other from the active configuration.
● Enable spanning-tree BPDU filter in INTERFACE mode. Use the spanning-tree bpdufilter disable command to
disable the BPDU filter on the interface.
spanning-tree bpdufilter enable
● Enable spanning-tree BPDU guard in INTERFACE mode.
spanning-tree bpduguard enable
○ Use the shutdown command to shut down the port channel interface, all member ports that are disabled in the
hardware.
○ Use the spanning-tree bpduguard disable command to add a physical port to a port-channel already in the
Error Disable state, the new member port is also disabled in the hardware.
● Set the guard types to avoid loops in INTERFACE mode.
spanning-tree guard {loop | root | none}
○ loop — Set the guard type to loop.
○ none — Set the guard type to none.
○ root — Set the guard type to root.
BPDU filter
OS10(conf-if-eth1/1/4)# spanning-tree bpdufilter enable
OS10(conf-if-eth1/1/4)# do show spanning-tree interface ethernet 1/1/4
ethernet1/1/4 of vlan1 is designated Blocking
Edge port:no (default) port guard :none (default)
Link type is point-to-point (auto)
Boundary: NO bpdu filter : Enable bpdu guard : bpduguard shutdown-on-
violation :disable RootGuard: enable LoopGuard disable
Bpdus (MRecords) sent 134, received 138
Interface Designated
Name PortID Prio Cost Sts Cost Bridge ID PortID
--------------------------------------------------------------------------
ethernet1/1/4 128.272 128 500 BLK 500 32769 90b1.1cf4.a911 128.272
BPDU guard
OS10(config)# interface ethernet 1/1/4
OS10(conf-if-eth1/1/4)# spanning-tree bpduguard enable
OS10(conf-if-eth1/1/4)# exit
OS10(config)# interface ethernet 1/1/4
OS10(conf-if-eth1/1/4)# do show spanning-tree interface ethernet 1/1/4
ethernet1/1/4 of vlan1 is designated Blocking
Edge port:no (default) port guard :none (default)
344
Layer 2