IEEE 802.1ag is a standard defined by IEEE. It defines protocols and practices for OAM for paths through 802.1 bridges and local area networks. It is an amendment to IEEE 802.1Q-2005 and was approved in 2007. IEEE 802.1ag is copied from the earlier ITU-T Recommendation Y.1731, which additionally addresses performance monitoring. The standard:
Defines maintenance domains, their constituent maintenance points, and the managed objects required to create and administer them
Defines the relationship between maintenance domains and the services offered by VLAN-aware bridges and provider bridges
Describes the protocols and procedures used by maintenance points to maintain and diagnose connectivity faults within a maintenance domain;
Provides means for future expansion of the capabilities of maintenance points and their protocols
Definitions
The document defines various terms: ;Maintenance Domain : Maintenance Domains are management space on a network, typically owned and operated by a single entity. MDs are configured with Names and Levels, where the eight levels range from 0 to 7. A hierarchical relationship exists between domains based on levels. The larger the domain, the higher the level value. Recommended values of levels are as follows:
Customer Domain: Largest
Provider Domain: In between
Operator Domain: Smallest
;Maintenance Association : Defined as a "set of MEPs, all of which are configured with the same MAID and MD Level, each of which is configured with a MEPID unique within that MAID and MD Level, and all of which are configured with the complete list of MEPIDs." ;Maintenance association End Point : Points at the edge of the domain, define the boundary for the domain. A MEP sends and receives CFM frames through the relay function, drops all CFM frames of its level or lower that come fromthe wire side. ;Maintenance domain Intermediate Point : Points internal to a domain, not at the boundary. CFM frames received from MEPs and other MIPs are cataloged and forwarded, all CFM frames at a lower level are stopped and dropped. MIPs are passive points, respond only when triggered by CFM trace route and loop-back messages.
CFM Protocols
IEEE 802.1ag Ethernet CFM protocols comprise three protocols that work together to help administrators debug Ethernet networks. They are: ;Continuity Check Protocol : "Heartbeating" messages for CFM. The Continuity Check Message provides a means to detect connectivity failures in an MA. CCMs are multicast messages. CCMs are confined to a domain. These messages are unidirectional and do not solicit a response. Each MEP transmits a periodic multicast Continuity Check Message inward towards the other MEPs. ;Link Trace : Link Trace messages otherwise known as Mac Trace Route are Multicast frames that a MEP transmits to track the path to a destination MEP which is similar in concept to User Datagram Protocol Trace Route. Each receiving MEP sends a Trace Route Reply directly to the Originating MEP, and regenerates the Trace Route Message. ;Loop-back : Loop-back messages otherwise known as MAC ping are Unicast frames that a MEP transmits, they are similar in concept to an Internet Control Message Protocol Echo messages, sending Loopback to successive MIPs can determine the location of a fault. Sending a high volume of Loopback Messages can test bandwidth, reliability, or jitter of a service, which is similar to flood ping. A MEP can send a Loopback to any MEP or MIP in the service. Unlike CCMs, Loop back messages are administratively initiated and stopped.
