computer science

Virtual Link Trunking

Virtual Link Trunking is a computer 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 Virtual Link Trunking rather than just read about it. In short: Virtual Link Trunking (VLT) is a name that has been used for at least two proprietary network protocols. A link aggregation protocol developed by Force10 and an early VLAN tagging capability from 3Com.

Key takeaways

Virtual Link Trunking 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 Virtual Link Trunking to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Virtual Link Trunking from memory before moving on to harder problems.

Reference excerpt

Virtual Link Trunking (VLT) is a name that has been used for at least two proprietary network protocols. A link aggregation protocol developed by Force10 and an early VLAN tagging capability from 3Com.

Force10 Virtual Link Trunking or VLT is a proprietary aggregation protocol developed by Force10 (now Dell Networking) and available in their datacenter-class or enterprise-class network switches. VLT is implemented in the latest firmware releases of legacy (FTOS) OS9 for their high-end switches like the S-, Z- and E-series 10/25,40 and 100 Gbit/s datacenter switches. VLT is also implemented on the current OS10 Smart Fabric OS. Although VLT is a proprietary protocol from Dell Networking (formerly Force10) other vendors offer similar features to allow users to set up an aggregated link towards two (logical) different switches, where a standard aggregated link can only terminate on a single logical switch (thus either a single physical switch or on different members in a stacked switch setup) like Cisco vpc or MLAG. The latest Dell supported OS for their ONIE based PowerSwitch devices, SONiC, also offers a similar protocol as MLAG VLT is a layer-2 link aggregation protocol between end-devices (servers) connected to (different) access-switches, offering these servers a redundant, load-balancing connection to the core-network in a loop-free environment, eliminating the requirement for the use of Spanning Tree Protocol. Where existing link aggregation protocols like (static) LAG (IEEE 802.3ad) or LACP (IEEE 802.1ax) require the different (physical) links to be connected to the same (logical) switch (such as stacked switches), the VLT, for example, allows link connectivity between a server and the network via two different switches. Instead of using VLT between end-devices like servers it can also be used for uplinks between (access/distribution) switches and the core switches. A major complication of existing link aggregation or bonding technologies is that all members interfaces of such a team/group need to terminate on one single logical switch. Beside increasing bandwidth another reason for link aggregation is redundancy. To make it possible to connect a LAG to different physical switches is to combine more than one physical switch into one logical switch using switch stacking techniques where the different physical boxes are seen as one logical switch for management and (spanning-tree) topology. The switches running in a stacked configuration always have to run the same firmware, which means that in case of a firmware upgrade the network manager has to implement the new firmware on all stack-members at the same time, resulting in an outage of the entire stack. The alternative is to have different logical switches, but then one of the used links will have to be blocked to ensure a loop-free topology (which can partially be overcome by using Multiple Spanning Tree or Cisco's proprietary per VLAN spanning tree. Spanning Tree Protocol is relatively slow in convergence, which can result in periods that traffic can't be forwarded over the network - and thus leads to outages of the applications. VLT should address all these issues, making it possible to create a loop free redundant network topology without using the Spanning Tree Protocol. An example configuration how it works can be found on the Blog Geslinux written by Grzegorz Witkowski

3Com Prior to supporting standardized IEEE 802.1Q tagging, 3Com used proprietary Virtual LAN Trunking (VLT). 3Com VLT supported VLAN IDs 1–16 with 15 being reserved for Autoselect VLAN Mode (where a VLAN server decides port membership) and 16 reserved for Spanning Tree Protocol.

See also IEEE 802.1aq (Shortest Path Bridging) Multi-chassis link aggregation group TRILL (Transparent Interconnection of Lots of Links) Virtual PortChannel - the Cisco (Nexus) and Dell Networking (DNOS6.x) proprietary implementations of MC-LAG

References

Worked examples

Example 1 — a first encounter with Virtual Link Trunking

Start with the simplest possible case. Write down what Virtual Link Trunking 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 Virtual Link Trunking 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 Virtual Link Trunking 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 Virtual Link Trunking

In research: Virtual Link Trunking 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 Virtual Link Trunking 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: Virtual Link Trunking is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ethernet standards, Fault-tolerant computer systems, Link protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Virtual Link Trunking 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 Virtual Link Trunking in 20 minutes

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

Frequently asked questions

What is Virtual Link Trunking in simple terms?

Why does Virtual Link Trunking 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 Virtual Link Trunking?

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 Virtual Link Trunking.