science

IEEE 802.1ah

IEEE 802.1ah is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand IEEE 802.1ah rather than just read about it. In short: IEEE 802.1ah is an amendment to the IEEE 802.1Q networking standard which adds support for Provider Backbone Bridges. It includes an architecture and a set of protocols for routing over a provider's network, allowing interconnection of multiple provider bridge networks without losing each customer's individually defined VLANs.

Key takeaways

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

Reference excerpt

IEEE 802.1ah is an amendment to the IEEE 802.1Q networking standard which adds support for Provider Backbone Bridges. It includes an architecture and a set of protocols for routing over a provider's network, allowing interconnection of multiple provider bridge networks without losing each customer's individually defined VLANs. It was initially created by Nortel before being submitted to the IEEE 802.1 committee for standardization. The final version was approved by the IEEE in June 2008 and has been integrated into IEEE 802.1Q-2011.

History The now-ubiquitous Ethernet was initially defined as a local area network (LAN) technology to interconnect the computers within a small organization in which these host computers were very close in proximity to each other. Over the years, Ethernet has become such a popular technology that it became the default Data Link Layer (OSI Layer 2) mechanism for data transport. This created a need for extending the Ethernet from a customer LAN bridging domain to service provider MAN, also known as the Provider bridging domain. For this, a 4 byte S-Tag or Service Tag, a type of Virtual LAN tag, was added to the header of the Ethernet frame in IEEE 802.1ad standard. In the service provider domain, switching was based on S-Tag and destination MAC address, and C-tag was used to create virtual LAN within the customer domain. This technology is also known as QinQ or Q-tunneling. QinQ does not offer true separation of customer and provider domains but is merely a way to overcome the limitations on the VLAN identifier space. It can also help in separation of the customer and provider control domains when used with other features like control protocol tunneling or Per-VLAN Spanning Tree etc. There is still the problem of having too little control on the MAC addresses, since QinQ forwarding is still based on the customer destination addresses. Thus, better mechanisms are needed.

Description The idea of PBB is to offer complete separation of customer and provider domains. For this purpose, a new Ethernet header has been defined. This header may take multiple different forms, but the main components of the header are:

Backbone Destination Address (B-DA): 48 bits Backbone destination MAC address. Backbone Source Address (B-SA): 48 bits Backbone source MAC address. Backbone EtherType: 16 bits Backbone EtherType. 0x88A8 indicates Service VLAN tag identifier (S-Tag). Backbone Flags: 4 bits Backbone flag bits. They are (in order, from most significant to least significant): Priority Code Point (PCP): 3 bits Priority Code Point. Drop Eligible Indicator (DEI): 1 bit Drop Eligible Indicator. Backbone VLAN ID (B-VID): 12 bits Backbone VLAN identifier. MIM EtherType: 16 bits MIM EtherType. 0x887E indicates Provider Backbone Bridges. MIM Flags: 8 bits Flag bits. They are (in order, from most significant to least significant): Priority Code Point (PCP): 3 bits Priority Code Point. Drop Eligible Indicator (DEI): 1 bit Drop Eligible Indicator. No Customer Access (NCA): 1 bit E.g. OAM frames. Reserved1 (Res1): 1 bit Reserved. Reserved2 (Res2): 2 bits Reserved. MIM Service Identifier (I-SID): 24 bits Service Instance VLAN ID. Customer Destination Address (C-DA): 48 bits Customer destination MAC address. Customer Source Address (C-SA): 48 bits Customer source MAC address.

The bridges in the PBB domain switch based on the B-VID and B-DA values, which contain 60 bits total. Bridges learn based on the B-SA and ingress port value and hence is completely unaware of the customer MAC addresses. I-SID allows distinguishing the services within a PBB domain. PBB is the foundation for the IEEE 802.1Qay PBB-TE standard, which was standardized in 2009. PBB is sometimes referred to as MAC-in-MAC.

See also Carrier Ethernet IEEE 802.1 IEEE 802.1aq, Shortest Path Bridging Metro Ethernet

References

External links http://www.ieee802.org/1/pages/802.1ah.html The right transport foundation for carrier Ethernet services

Worked examples

Example 1 — a first encounter with IEEE 802.1ah

Start with the simplest possible case. Write down what IEEE 802.1ah claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In 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 802.1ah 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 802.1ah 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 802.1ah

In research: IEEE 802.1ah appears in 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 802.1ah 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 802.1ah is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ethernet standards, IEEE 802, Telecommunication protocols, so understanding it makes those chapters shorter.
In everyday life: Look for IEEE 802.1ah 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.

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How to study IEEE 802.1ah in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what IEEE 802.1ah 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 802.1ah out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is IEEE 802.1ah in simple terms?

Why does IEEE 802.1ah matter?

Because it connects several 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 802.1ah?

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 802.1ah.