Ethernet Connectivity Fault Management – CFM, is an end-to-end per-service-instance Ethernet operation, administration, and management (OAM) protocol. CFM provides an equivalent of the ping and traceroute commands at layer 2 for large Ethernet metropolitan-area networks (MANs) and WANs.
CFM has its own Ether type (0x8902) and destination MAC address (for multicast messages). CFM employs regular Ethernet frames that travel in-band with the customer traffic so devices that cannot interpret CFM Messages forward them as normal data frames.
CFM defines three protocols:
- Continuity Check Protocol: This is a neighbor discovery and health check protocol that discovers and maintains adjacencies at the VLAN or link level.
- Link trace Protocol: Like IP traceroute, this protocol maps the path taken to a destination MAC address through one or more bridged networks between the source and destination.
- Loopback Protocol: Like IP ping, this protocol works with the continuity check protocol during troubleshooting.
- Maintenance Domain (MD): A Maintenance Domain (MD) is an administrative area of a network that is owned by a Maintenance Entity (ME). MDs can be non-overlapping, but they can also overlap if they are completely nested. A unique maintenance level from 0 to 7 is assigned to each domain. The larger the domain, the higher the value.
- Maintenance Point (MP): A Maintenance Point (MP) is a demarcation point for the CFM frames. It is an interface that participates in an MD. There are two types of maintenance points:
- Maintenance Endpoint (MEP): MEPs points at the edge of the domain, define the boundary for the domain. MEPs initiate and terminate CFM messages through the relay function. MEPs drops all CFM frames of its level or lower that come from the wire side.
- Maintenance Intermediate Point (MIP): MEPs points internal to a domain, not at the boundary. MIPs receive CFM messages and respond to originating MEPs. A MIP never initiates messages and does not expect any CFM messages. MIPs drops all CFM frames at lowers.
- Maintenance Association (MA): A set of MEPs that have the same MA identifier and MD level within one service instance to verify the integrity of the service.
Ethernet CFM exchanges messages and performs operations on a per-domain basis. For instance, CFM running at Operator 1 level does not allow discovery of the network by Provider or Customer level.
CFM Message Types
- Loopback Message (LBM)/ Loopback Response (LBR): A MEP transmits the LBM frame. This frame can be addressed to another MEP. The MP for which the frame is addressed responds with an LBR message.
- Link Trace Message (LTM)/ Link Trace Response (LTR): A MEP transmits the LTM packet. This packet specifies the target MAC address of an MP. MPs on the path to the target address respond with an LTR.
- Continuity Check Message (CCM): The Continuity Check Message (CCM) represents the “Heart beat” message for CFM. The CCM provides a means to detect connectivity failures in an MA. CCMs are multicast messages, confined to a domain (MD). These messages are unidirectional and do not solicit a response. Each MEP transmits a periodic multicast CCM towards the other MEPs.
The following rules apply to CFM message processing:
- If the MD level of the CFM message is higher than the MD level of the MEP/MIP, the MEP/MIP transparently passes the CFM message.
- If the MD level of the CFM message is lower than the MD level of the MEP/MIP, the MEP/MIP discards the CFM message.
- If the MD level of the CFM message is equal to the MD level of the MEP/MIP, the MEP/MIP processes the CFM message. Depending on the type of CFM message, the MEP/MIP responds to, transports to or accepts the message.
A MEP/MIP with a particular MD level generates a CFM message only at that level. The customer is interested in only the health of the end-to-end connection. Therefore, the MA has a high MD level (for example, MD=7). The MEPs/MIPs in provider and operator networks transparently pass customers CFM messages. The Provider is interested in the health of the service it provides. Therefore, the MA has a higher MD level (for example, MD=5) than the operators MD level but lower than the customer MD level. So, the MEPs/MIPs in operator network transparently pass the provider’s CFM messages. The operators are interested only in the health of their own networks. Therefore, their MAs have lowest MD level (for example, MD=0). So, the CFM messages are confined to their own network.