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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