762 Configuring Connectivity Fault Management802.3ah), where the faults are detected and notified on a single point-to-pointIEEE Std. 802.3 LAN, Dot1ag addresses fault diagnosis at the service layeracross networks comprising multiple LANs, including LANs other than 802.3media.How Does Dot1ag Work Across a Carrier Network?A typical metropolitan area network comprises operator, service provider, andcustomer networks. To suit this business model, CFM relies on a functionalmodel of hierarchical maintenance domains (MDs). These domains areassigned a unique MD level. There is a maximum of 8 levels, which can benested but cannot overlap. Each organization can have its own maintenancedomain. The MD level limits administrator access to the appropriate domain.Figure 26-1 depicts three domains: the customer subscribes to the services ofa provider, who, in turn, subscribes to the services of two operators. Thisscenario is a likely one, since no operator has complete coverage of a largeregion. A service instance would span the provider network covering one ormore operators. Every domain has its own network management system.Dot1ag defines OAM services that operate across these domains (the verticalarrow) and within them (the horizontal arrow)Figure 26-1. Organization of DomainsProviderCustomerOperator 1 Operator 3Operator 2CustomerSite 1 CustomerSite 2AccessLinks,802.3ah802.1ag
