Technical Documentation
Ethernet OAM Connectivity Fault Management
The most complete connectivity fault management (CFM) is defined in IEEE 802.1ag. This topic emphasizes the use of CFM in a Metro Ethernet environment.
The major features of CFM are:
- Fault monitoring using the continuity check protocol. This is a neighbor discovery and health check protocol which discovers and maintains adjacencies at the VLAN or link level.
- Path discovery and fault verification using the linktrace protocol. Similar to IP traceroute, this protocol maps the path taken to a destination MAC address through one or more bridged networks between the source and destination.
- Fault isolation using the loopback protocol. Similar to IP ping, this protocol works with the continuity check protocol during troubleshooting.
CFM partitions the service network into various administrative domains. For example, operators, providers, and customers may be part of different administrative domains. Each administrative domain is mapped into one maintenance domain providing enough information to perform its own management, thus avoiding security breaches and making end-to-end monitoring possible. Each maintenance domain is associated with a maintenance domain level from 0 through 7. Level allocation is based on the network hierarchy, where outermost domains are assigned a higher level than the innermost domains. Customer end points have to highest maintenance domain level. In a CFM maintenance domain, each service instance is called a maintenance association. A maintenance association can be thought as a full mesh of maintenance endpoints (MEPs) having similar characteristics. MEPs are active CFM entities generating and responding to CFM protocol messages. There is also a maintenance intermediate point (MIP), which is a CFM entity similar to the MEP, but more passive (MIPs only respond to CFM messages).
MEPs can be up MEPs or down MEPs. A link can connect a MEP at level 5 to a MEP at level 7. The interface at level 5 is an up MEP (because the other end of the link is at MEP level 7) and the interface at level 7 is a down MEP (because the other end of the link is at MEP level 5).
In a Metro Ethernet network, CFM is commonly used at two levels:
- By the service provider to check the connectivity among its provider edge (PE) routers
- By the customer to check the connectivity among its customer
edge (CE) routers
Note: The configured customer CFM level must be greater than service provider CFM level.
In many Metro Ethernet networks, CFM is used to monitor connectivity over a VPLS and bridge network.
