Quick Definition
Protects against STP loops by detecting unidirectional link failures where BPDUs stop arriving.
Loop Guard prevents switching loops caused by unidirectional link failures. Normally, if a non-designated port stops receiving BPDUs, STP assumes the Root Bridge path failed and the port transitions to Forwarding — potentially creating a loop. With Loop Guard, a port that stops receiving BPDUs transitions to the loop-inconsistent state instead of Forwarding, preventing the loop. Loop Guard is typically enabled on Root Ports and non-designated ports.
interface GigabitEthernet0/1 spanning-tree guard loop spanning-tree loopguard default ! global
Loop Guard and Root Guard cannot be configured together on the same port — they are mutually exclusive. Loop Guard is for Root/Alternate ports (non-designated). Root Guard is for Designated ports.
A Layer 2 loop-prevention protocol that blocks redundant paths in a switched network.
The STP control frames that switches exchange to elect the Root Bridge and maintain the spanning tree.
Shuts down a PortFast-enabled port if it receives a BPDU, protecting the spanning tree.
Prevents a port from becoming the Root Port, protecting the Root Bridge position.
Loop Guard prevents switching loops caused by unidirectional link failures. Normally, if a non-designated port stops receiving BPDUs, STP assumes the Root Bridge path failed and the port transitions to Forwarding — potentially creating a loop. With Loop Guard, a port that stops receiving BPDUs transitions to the loop-inconsistent state instead of Forwarding, preventing the loop. Loop Guard is typically enabled on Root Ports and non-designated ports.
Loop Guard and Root Guard cannot be configured together on the same port — they are mutually exclusive. Loop Guard is for Root/Alternate ports (non-designated). Root Guard is for Designated ports.
Loop Guard falls under the Spanning Tree domain of the 200-301 exam. Understanding it in context with related terms like stp and bpdu is essential for answering scenario-based questions correctly.