IEEE 802.1ag

Key takeaways

IEEE 802.1ag belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect IEEE 802.1ag to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of IEEE 802.1ag from memory before moving on to harder problems.

Reference excerpt

IEEE 802.1ag is an amendment to the IEEE 802.1Q networking standard which introduces Connectivity Fault Management (CFM). This defines protocols and practices for the operations, administration, and maintenance (OAM) of paths through 802.1 bridges and local area networks (LANs). The final version was approved by the IEEE in 2007. IEEE 802.1ag is a subset of 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 (MD) 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 (e.g., 7) Provider Domain: In between (e.g., 3) Operator Domain: Smallest (e.g., 1)

Maintenance Association (MA) Defined as a "set of MEPs, all of which are configured with the same MAID (Maintenance Association Identifier) 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 (MEP) 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 from the wire side. Maintenance domain Intermediate Point (MIP) 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 (Connectivity Fault Management) protocols comprise three protocols that work together to help administrators debug Ethernet networks. They are:

Continuity Check Protocol (CCP) "Heartbeating" messages for CFM. The Continuity Check Message (CCM) provides a means to detect connectivity failures in an MA. CCMs are multicast messages. CCMs are confined to a domain (MD). 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 (LT) Link Trace messages otherwise known as Mac Trace Route are Multicast frames that a MEP transmits to track the path (hop-by-hop) to a destination MEP which is similar in concept to User Datagram Protocol (UDP) Trace Route. Each receiving MEP sends a Trace Route Reply directly to the Originating MEP, and regenerates the Trace Route Message. Loop-back (LB) 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 (ICMP) Echo (Ping) 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.

Y.1731

ITU-T Y.1731 additionally supports the following:

Ethernet alarm indication signal (ETH-AIS) Ethernet remote defect indication (ETH-RDI) Ethernet locked signal (ETH-LCK) Ethernet test signal (ETH-Test) Ethernet automatic protection switching (ETH-APS) Ethernet maintenance communication channel (ETH-MCC) Ethernet experimental OAM (ETH-EXP) Ethernet vendor-specific OAM (ETH-VSP) Ethernet client signal fail (ETH-CSF) Ethernet bandwidth notification (ETH-BN) Ethernet expected defect function (ETH-ED) Frame loss measurement (ETH-LM) Frame delay measurement (ETH-DM) Delay measurement message (DMM) Loss measurement message (LMM)

References

External links 802 Committee website on 802.1ag IEEE Std 802.1ag-2007 Virtual Bridged Local Area Networks, Amendment 5: Connectivity Fault Management (through IEEE Get Program) ITU-T Recommendation Y.1731 OAM functions and mechanisms for Ethernet based networks

Worked examples

Example 1 — a first encounter with IEEE 802.1ag

Start with the simplest possible case. Write down what IEEE 802.1ag claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to IEEE 802.1ag before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about IEEE 802.1ag ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of IEEE 802.1ag

In research: IEEE 802.1ag appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses IEEE 802.1ag in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: IEEE 802.1ag is common in secondary-school and first-year university syllabi. It links to neighbouring topics IEEE 802, Networking standards, so understanding it makes those chapters shorter.
In everyday life: Look for IEEE 802.1ag outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study IEEE 802.1ag in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what IEEE 802.1ag means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain IEEE 802.1ag out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is IEEE 802.1ag in simple terms?

IEEE 802.1ag is an amendment to the IEEE 802.1Q networking standard which introduces Connectivity Fault Management (CFM). This defines protocols and practices for the operations, administration, and maintenance (OAM) of paths through 802.1 bridges and local area networks (LANs).

Why does IEEE 802.1ag matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study IEEE 802.1ag?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on IEEE 802.1ag.