Cisco ACI Mis-Cabling Protocol (MCP) – Loop Detection Simplified

Cisco ACI uses Mis-Cabling Protocol (MCP) to detect and mitigate Layer 2 loops, replacing traditional STP participation. MCP sends special Layer 2 packets across access ports, VPCs, and virtual ports. If the fabric receives its own MCP packet, it identifies a loop and can either log the event or error-disable the port.

✅ Key Highlights:

• Global MCP policies are disabled by default; port-level policies are enabled.
• Global MCP Policy:
  This is the master switch that controls whether MCP is active across the entire fabric.
• Disabled by default: Even though individual ports may be configured to support MCP, no MCP packets are sent unless this global policy is explicitly enabled.
• Port-Level MCP Policy:
  These are the interface-specific settings that determine how each port behaves when MCP is active.
• Enabled by default: Ports are ready to participate in MCP loop detection, but they won’t actually send or process MCP packets unless the global policy is turned on.

• MCP works complementarily with STP on external switches.
• BPDU filtering or disabling loopguard on external switches helps prevent loop-related issues.
• Endpoint move loop detection is available but disabled by default.
• MCP supports native VLAN mode and per-VLAN mode (from APIC 2.0(2)) for granular loop detection.
• Faster detection introduced in APIC 3.2(1) with transmission intervals as low as 100 ms.
• Scalability limits: 256 VLANs per interface and 2000 logical ports per leaf switch. Per-VLAN MCP will only run on 256 VLANs per interface. If there are more than 256 VLANs, then the first numerical 256 VLANs are chosen.

🔐 MCP Modes:

• Non-Strict Mode: Allows traffic while monitoring for loops; default detection time is 7 seconds.
• Strict Mode (from APIC 5.2(4)):
• Performs early loop detection before allowing data traffic.
• Uses initial delay and grace period timers for STP convergence and aggressive MCP checks.
• Requires port flap to activate on already-up ports.

⚠️ Strict Mode Guidelines:

• Not supported on FEX or QinQ edge ports.
• Requires APIC 5.2(4) or later on all participating leaf switches.
• May impact vPC convergence time.
• Must be disabled before downgrading the fabric.
• Can cause both ports to error-disable if loops are detected simultaneously.

MCP Mode Comparison Table

Feature	Non-Strict Mode	Strict Mode
Traffic Acceptance	Accepts data and control traffic immediately	Initially blocks data traffic; only control packets allowed
Loop Detection Timing	MCP packets sent every 2 seconds; loop detection in ~7 seconds	Aggressive MCP packet transmission during grace period (default 3 sec)
Early Loop Detection	Not performed	Performed before allowing data traffic
Port Behavior on Loop Detection	Port is error-disabled	Port is error-disabled and shut down
Activation Requirement	Active immediately	Requires port flap to activate if port is already up
Timers Used	Global MCP instance policy	Initial delay timer + grace period timer
Default Initial Delay	Not applicable	0 seconds (can be set to 45–60 sec for STP convergence)
Default Grace Period	Not applicable	3 seconds
STP Compatibility	Works with STP	Accepts STP BPDUs even if VLAN is not enabled
Use Case	General loop detection	Early and aggressive loop prevention before traffic forwarding