MCP (Mis-Cabling Protocol)

How Loops Can Form in the ACI Fabric

Incorrect cabling or misconfigurations can cause loops in the Cisco ACI fabric.
A loop means there is more than one path for traffic to circulate endlessly, which can overload the network.

LLDP (Link Layer Discovery Protocol)
- When a port becomes active, the switches exchange LLDP information.
- If a leaf switch sees LLDP from another leaf (or a spine sees LLDP from another spine), it detects a wiring mismatch.
- In that case, the port is disabled (out-of-service) and a fault is raised (wiring-mismatch).
  
  Important: MCP (Mis-Cabling Protocol) is not needed for these kinds of leaf-to-leaf or spine-to-spine errors.
STP (Spanning Tree Protocol)
- ACI itself does not run STP inside the fabric.
- However, ACI leaf ports can connect to external Layer 2 networks that do run STP.
- STP in the external network will detect and block loops there.
  
  Important: MCP is not needed for loops that STP can handle.

A protocol designed to detect other types of loops that LLDP or STP cannot detect.
Sends a special Layer 2 frame (with a multicast MAC) that downstream devices flood.
Can be sent on a per-VLAN basis.
If a leaf port in the fabric receives its own MCP frame back:
- It checks if the MD5 digest is the same, and
- If the send time matches (within 2 seconds).
- If either condition is met, the port goes into error-disable (mcp-loop-err-disable).

MCP Packet is a lightweight Layer 2 frame with low overhead.
MCP Design:
- Automatically disables any ports that form a loop within the fabric.
- Prevents user traffic from using these looped ports.
- Generates syslog events and faults so you can troubleshoot problems.

In the APIC GUI:
- Fabric → Access Policies → Policies → Global → MCP instance Policy (default) → Policy
Set:
- Admin State = Enabled
- Controls = Enable MCP PDU per VLAN
- Loop Protection Action = Port Disable
  ( If you choose “disabled,” you will only see logs, and ports will not be disabled.)

Note : MCP must be globally enabled for any interface-specific MCP policy to actually apply.

Create an MCP Interface Policy
- Fabric → Access Policies → Policies → Interface → MCP Interface
- Right-click → Create Mis-cabling Protocol Interface Policy
- Give it a name and enable Admin State.
Assign MCP Policy

Leaf Access Port
- Fabric → Access Policies → Interfaces → Leaf Interfaces → Policy Groups → Leaf Access Port
- Under Policy → MCP, select either the default global policy or the new MCP Interface Policy.
Port Channel (PC)
- Fabric → Access Policies → Interfaces → Leaf Interfaces → Policy Groups → PC Interface
- In Advanced Policies → MCP Policy, select the global or custom MCP policy.
Virtual Port Channel (vPC)
- Fabric → Access Policies → Interfaces → Leaf Interfaces → Policy Groups → vPC Interface
- In Advanced Policies → MCP Policy, pick the global or custom MCP policy.

Check MCP interface status
leaf# moquery -c mcpIf -f 'mcp.If.id=="eth1/1"'
Show global MCP info
leaf# show mcp internal info global
Show MCP info for a specific interface
leaf# show mcp internal info interface ethernet 1/1
leaf# show mcp internal info interface port-channel X
Verify if a loop was detected

leaf# show mcp internal event-history interface port-channelX
- If loop detected, you may see:
MCP loop detected at: Wed Sep 21 18:00:09 2021

leaf# show interface ethernet 1/1
- If loop detected, you may see:
  Ethernet1/1 is down (mcp-loop-err-disable)
Fault Codes
- F1607 and F0532 may appear if MCP detects a loop.

Summary

LLDP handles leaf-to-leaf or spine-to-spine cabling errors.
STP handles loops in the external Layer 2 network.
MCP handles all other loops inside or just outside the fabric that LLDP/STP don’t catch.
MCP sends special frames, and if those frames return to the sender, it flags a loop and can disable the port.
Always enable MCP globally, and then enable it on interfaces or port groups to fully protect against loops.