To detect and prevent Layer 2 loops in a Cisco ACI fabric, the engineer must configure the Mis-Cabling Protocol (MCP) and related policies. MCP is designed to detect loops in Layer 2 network segments connected to ACI access ports and operates in conjunction with Spanning Tree Protocol (STP) running on external Layer 2 networks. The steps to accomplish these goals include:

Enable MCP Globally: Ensure that MCP is enabled globally on all access ports, virtual ports, and virtual port channels (VPCs) unless they are disabled at the individual port level1.

Configure MCP Transmit Frequency: Set the transmit frequency to a value that allows for quick loop detection. Starting with the 3.2(1) release, the Cisco ACI fabric provides faster loop detection with transmit frequencies from 100 milliseconds to 300 seconds1.

Set Action on Loop Detection: Decide how the MCP policies will act upon loop detection. Options include generating a syslog message or disabling the port upon detection of a loop1.

Implement Error Disabled Recovery Policy: Configure an error disabled recovery policy to automatically re-enable ports that were disabled due to loop detection after a configurable interval1.

By following these steps, the engineer can ensure that the Cisco ACI fabric will detect and prevent Layer 2 loops, thereby maintaining a stable and efficient network environment.

References :=

Cisco APIC Online Help - Loop Detection1

ACI Layer 2 loop detection and Mitigation - Cisco Video Portal2