762 Configuring Connectivity Fault Management
How Does Dot1ag Work Across a Carrier Network?
A typical metropolitan area network comprises operator, service provider, and
customer networks. To suit this business model, CFM relies on a functional
model of hierarchical maintenance domains (MDs). These domains are
assigned a unique MD level. There is a maximum of 8 levels, which can be
nested but cannot overlap. Each organization can have its own maintenance
domain. The MD level limits administrator access to the appropriate domain.
Figure 26-1 depicts three domains: the customer subscribes to the services of
a provider, who, in turn, subscribes to the services of two operators. This
scenario is a likely one, since no operator has complete coverage of a large
region. A service instance would span the provider network covering one or
more operators. Every domain has its own network management system.
Dot1ag defines OAM services that operate across these domains (the vertical
arrow) and within them (the horizontal arrow)
Figure 26-1. Organization of Domains
Entities at different levels have different responsibilities. For example, the
lower level (operator) overlooks a subset of the network in detail and provides
information about its status to its higher levels such as the provider level).
Higher levels have a broader, but less detailed, view of the network. As a
result, a provider could include multiple operators, provided that the domains
Provider
Customer
Operator 1 Operator 3
Operator 2
Customer
Site 1
Customer
Site 2
Access
Links,
802.3ah
802.1ag
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