Connectivity Guide

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, congure the spanning-tree guard root command. Root guard is enabled on
ports that are designated ports. The root guard conguration applies to all VLANs congured 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. 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 congured on the port. If you disable loop guard on a port, it moves to the Listening state.
If you enable BPDU lter and BPDU guard on the same port, the BPDU lter conguration takes precedence. Root guard and Loop guard
are mutually exclusive. Conguring one overwrites the other from the active conguration.
1 Enable spanning-tree BPDU lter in INTERFACE mode.
spanning-tree bpdufilter enable
• To shut down the port channel interface, all member ports are disabled in the hardware.
• 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.
• To remove a physical port from a port-channel in Error Disable state, the Error Disabled state clears on this physical port. The
physical port is enabled in the hardware.
To clear the Error Disabled state:
• Use the shutdown command on the interface.
• Use the spanning-tree bpdufilter disable command to disable the BPDU guard on the interface.
• Use the spanning-tree disable command to disable STP on the interface.
2 Enable STP BPDU guard in INTERFACE mode.
spanning-tree bpduguard enable
• To shut down the port channel interface, all member ports are disabled in the hardware.
• 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.
• To remove a physical port from a port-channel in Error Disable state, the Error Disabled state clears on this physical port. The
physical port is enabled 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.
• none — Set the guard type to none.
• root — Set the guard type to root.
BPDU lter
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
Layer 2
225