Configuring Connectivity Fault Management 861Higher levels have a broader, but less detailed, view of the network. As aresult, a provider could include multiple operators, provided that the domainsnever intersect. The operator transparently passes frames from the customerand provider, and the customer does not see the operator frames. Multiplelevels within a domain (say, operator) are supported for flexibility.What Entities Make Up a Maintenance Domain?Dot1ag defines three primary entities that make up the maintenance domain:Maintenance End Points (MEPs), Maintenance Intermediate Points (MIPs),and Maintenance Associations (MAs).MEPs, and MIPsMEPs and MIPs are software or sometimes hardware per-service entitieswhere CFM functionalities are present.• MEPs define the boundary of a maintenance domain. They initiate andrespond to CFM messages. MEPs prevent the leaking of CFM messagesbetween domains (for example, among operators or between operators andcustomers). Each MEP has a configurable unique identifier (MEPID) in amaintenance domain.MEPs periodically issue Continuity Check Messages (CCM) to discovereach other and issue SNMP traps to report connectivity losses ormalformed or incorrect CCMs.A MEP can be defined as “down MEP” or an “up MEP”. A down MEPsreside in a bridge that transmits CFM PDUs towards, and receives themfrom, the direction of the LAN. An up MEP resides in a bridge thattransmits CFM PDUs towards, and receives them from, the direction ofthe Bridge Relay Entity.• MIPs are entities within a domain that enable the outer domain to achieveend-to-end connectivity checks. MIPs passively receive CFM messages andrespond back to the originating MEP.NOTE: An entity at the boundary of maintenance domain that offersconnectivity and other services to systems outside the domain is referred toas a Domain Service Access Point (DoSAP). A MEP is a type of DoSAPwhose services relate to connectivity fault management.
