Loop guard prevents a switch port from transitioning to the forwarding state when it stops receiving bridge protocol data units (BPDUs)? this prevents Layer 2 switching loops from occurring. A port that is configured with loop guard that stops receiving BPDUs will be put into the loopinconsistent state, as shown in the following output:
%SPANTREE-4-LOOPGUARDBLOCK: No BPDUs were received on port 0/1 in vlan 4. Moved to loop inconsistent state
After the port starts receiving BPDUs again, loop guard enables the port to transition through the normal Spanning Tree Protocol (STP) states. Loop guard is only used on interfaces that STP considers to be pointtopoint links.
Root guard, not loop guard, prevents a switch port from becoming the root port, thereby influencing where the root bridge is located on the network. When a port receives a superior BPDU, it will normally attempt to become a root port. However, a root guard port that receives a superior BPDU will be put into the rootinconsistent state, as shown in the following output:
%SPANTREE-2-ROOTGUARDBLOCK: Port 0/1 tried to become non-designated in VLAN 4. Moved to root-inconsistent state

When the port stops receiving superior BPDUs, the port will be enabled, as shown in the following output:
[#codeE]%SPANTREE-2-ROOTGUARDUNBLOCK: Port 0/1 restored in VLAN 4

BPDU guard disables a switch port that receives BPDUs. Access ports should never receive BPDUs? to prevent access ports from receiving BPDUs, you can enable BPDU guard on the access ports, thereby defining the edge of the STP domain. When a port that is configured with BPDU guard receives a BPDU, BPDU guard immediately puts the port into the errdisable state and shuts down the port, as shown in the following output:

%SPANTREE-2-RX_PORTFAST: Received BPDU on PortFast enabled port. Disabling FastEthernet0/1.
%PM-4-ERR_DISABLE: bpduguard error detected on Fa0/1, putting Fa0/1 in err-disable state
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down


The port must be manually re-enabled, or it can be recovered automatically through the err0disable timeout function.
Although loop guard can be enabled on trunk ports, it does not prevent a switch port from becoming a trunk port. Loop guard should not be enabled on access ports. By contrast, BPDU guard and root guard can be enabled on access ports.