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CCNP Enterprise Core (350-401) Cisco Datalink Layer

Spanning Tree Protocol – Loop Guard

Loop Guard is a technology that assists with physical layer technologies that can end up in a scenario of one way traffic only (only send or only receive). In such a scenario happening the switch may detect an interface in an ‘up’ state since it can receive traffic from its peer, but may not be able to transmit traffic due to the other direction not functioning correctly. This can result in spanning tree scenarios where BPDUs can be received but not sent.

Loop Guard prevents alternative or root ports from becoming designated ports due to a loss of BPDUs on a root port.

Loop Guard will place a root interface in an error disabled state whilst BPDUs are not being received on that interface. When BPDUs begin to transmit through again on that interface, the port will recover automatically and transfer through each of the Spanning Tree Protocol processes.

Loopguard can be enabled globally with the command spanning-tree loopguard default or enabled on an interface basis with the command spanning-tree guard loop

Ports blocked by Loop Guard can be viewed with the command show spanning-tree inconsistent-ports or displayed as type *LOOP_Inc when running the command show spanning-tree

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