MisCabling Protocol (MCP) detects loops from external sources (i.e., misbehaving servers, external
networking equipment running STP, etc.) and will err-disable the interface on which ACI receives its
own packet. Enabling this feature is a best practice, and it should be enabled globally and on all
interfaces, regardless of the end device. For MCP to be enabled, you need to have it enabled globally
and on a per-interface basis. While MCP is enabled on all interfaces by default, it is not turned “on”
until you also enable it globally. The global configuration knob for MCP can be enabled by configuring
the global settings here: Fabric > Access Policies > Global Policies > MCP Instance Policy default.
https://www.cisco.com/c/dam/en/us/solutions/collateral/data-center-virtualization/applicationcentric-infrastructure/aci-guide-using-mcp-mis-cabling-protocol.pdf
