In IEEE 802.1ag, how are loops detected?

In IEEE 802.1ag Connectivity Fault Management (CFM), a loop is detected when a Maintenance End Point (MEP) receives a CFM frame that it originally transmitted. The most common frame for this is the Continuity Check Message (CCM). Each MEP in a Maintenance Association periodically multicasts CCMs. If a loop exists within the service path, a MEP's own CCM will be forwarded back to itself. Upon receiving its own CCM, the MEP raises a loop fault alarm, indicating an unintended closed path in the network. This mechanism is specific to the configured Maintenance Domain and provides a deterministic method for fault detection.

A. Spanning-tree BPDUs are used by the IEEE 802.1D Spanning Tree Protocol for loop prevention, which is a separate and distinct function from 802.1ag fault detection.
C. While 802.1D (STP) is for loop prevention, IEEE 802.1ag is explicitly designed for fault management, which includes the detection of loops as a specific fault condition.
D. The Linktrace Message (LTM) in CFM traces a path by collecting responses from intermediate points, but the primary loop detection mechanism is a MEP receiving its own frames.

1. Nokia 7750 SR OS OAM and Diagnostics Guide, Release 21.10.R1.

Section: Ethernet Connectivity Fault Management (ETH-CFM) > ETH-CFM Functions > Continuity Check

Reference: "When a MEP receives a CCM frame that it transmitted, it will raise a loop indication alarm. The loop indication is a primary alarm that is enabled by default." (This directly supports answer B).

2. IEEE Std 802.1ag-2007, "IEEE Standard for Local and metropolitan area networks-- Virtual Bridged Local Area Networks-- Amendment 5: Connectivity Fault Management."

Section: 21.5, Continuity Check Protocol

Reference: Clause 21.5.3 describes the CCM reception state machine. It specifies that if a MEP receives a CCM with its own MAC address as the source (ccmSourceMacAddress), it indicates a loop. The standard states: "If a CCM is received from the local MEP's MAC Address, there is a loop in the network..."

3. Stallings, W. (2016). Foundations of Modern Networking: SDN, NFV, and Cloud Computing. Pearson Education.

Chapter: 11, Carrier Ethernet

Section: 11.4, Ethernet OAM

Reference: The text explains that CFM's Continuity Check function involves MEPs transmitting multicast messages. If a MEP receives its own message, it is an indication of a loop in the network. This confirms that receiving its own frame is the detection method.