computer science

Multiple Spanning Tree Protocol

Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol rather than just read about it. In short: The Multiple Spanning Tree Protocol (MSTP) and algorithm, provides both simple and full connectivity assigned to any given virtual LAN (VLAN) throughout a bridged local area network. MSTP uses bridge protocol data unit (BPDUs) to exchange information between spanning-tree compatible devices, to prevent loops in each Multiple Spanning Tree instance (MSTI) and in the Common and Internal Spanning Tree (CIST), by select…

Key takeaways

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

Reference excerpt

The Multiple Spanning Tree Protocol (MSTP) and algorithm, provides both simple and full connectivity assigned to any given virtual LAN (VLAN) throughout a bridged local area network. MSTP uses bridge protocol data unit (BPDUs) to exchange information between spanning-tree compatible devices, to prevent loops in each Multiple Spanning Tree instance (MSTI) and in the Common and Internal Spanning Tree (CIST), by selecting active and blocked paths. This is done as well as in Spanning Tree Protocol (STP) without the need of manually enabling backup links and getting rid of switching loop danger. MSTP allows frames assigned to different VLANs to follow separate paths, each based on an independent MSTI, within MST regions composed of local area networks (LANs) and MST bridges. These regions and the other bridges and LANs are connected into a single common spanning tree (CST).

History and motivation It was originally defined in IEEE 802.1s as an amendment to 802.1Q, 1998 edition, and later merged into IEEE 802.1Q-2005 Standard, and clearly defines an extension or an evolution of Radia Perlman's Spanning Tree Protocol (STP) and the Rapid Spanning Tree Protocol (RSTP). It has some similarities with Cisco Systems' Multiple Instances Spanning Tree Protocol (MISTP), but there are some differences. The original STP and RSTP work on the physical link level, preventing bridge loops when redundant paths are present. However, when a LAN is virtualized using VLAN trunking, each physical link represents multiple logical connections. Blocking a physical link blocks all its logical links and forces all traffic through the remaining physical links within the spanning tree. Redundant links cannot be utilized at all. Moreover, without careful network design, seemingly redundant links on the physical level may be used to connect different VLANs and blocking any of them may disconnect one or more VLANs, causing bad paths. Instead, MSTP provides a potentially better utilization of alternate paths by enabling the use of alternate spanning trees for different VLANs or groups of VLANs.

Main entities

Multiple Spanning Tree instances (MSTI)

As MSTP enables grouping and mapping VLANs into different spanning tree instances, there is an urge to determine a group or set of VLANs that are all using the same spanning tree; this is an MSTI. Each instance defines a single forwarding topology for an exclusive set of VLANs; by contrast, STP or RSTP networks contain only a single spanning tree instance for the entire network, which contains all the VLANs. A region can include:

Internal spanning-tree instance (IST): Default spanning tree instance in any MST region. All VLANs in this IST instance form a single spanning tree topology, allowing only one forwarding path between any two nodes. It also provides the root switch for any VLAN-configured switches that are not specifically assigned to an MSTI. Multiple Spanning Tree instance (MSTI): Unlike IST, this kind of instance comprises all static VLANs specifically assigned to it and must include at least one VLAN. While each MSTI can have multiple VLANs, each VLAN can be associated with only one MSTI.

MSTP regions

A set of interconnected switches that must have been configured with the same VLANs and MSTIs, also have the same following parameters:

MST Configuration Name Revision Level Configuration Digest: Mapping of which VLAN are mapped to which MST instances. An MSTI is unable to span across MST regions because of its inherent locality to a single MST region. This is done by an identifying number for each MSTI. For achieving the task of assigning each bridge to a region, each switch/bridge must compare its MST Configuration Identifiers (Format Selector, Region Name, Revision Level and Configuration Digest), either of which represents VLAN to MSTIs mapping for each bridge.

Common and Internal Spanning Tree (CST/CIST)

We can differentiate two kinds of conformated Spanning Trees into the different networks created by MSTP, these are:

Common Spanning Tree (CST): Administers the connectivity among MST regions, STP LANs and RSTP LANs in a bridged network. Common Internal Spanning Tree (CIST): Identifies regions in a network and administers the CIST root bridge for the network, for each region, and for each spanning tree instance in each region. It's also the default spanning tree instance of MSTP so that any VLAN that isn't a member of a particular MSTI will be a member of the CIST. Furthermore, works as well as the spanning tree that runs between regions and between MST regions and Single Spanning Tree (SST) entities. The role of the Common Spanning Tree (CST) in a network, and the Common and Internal Spanning Tree (CIST) configured on each device, is to prevent loops within a wider network that may span more than one MSTP Region and parts of the network running in legacy STP or RSTP mode.

MSTP BPDUs

The main function of bridge protocol data units (BPDUs) is to enable MSTP to select its root bridges for the proper CIST and each MSTI. MSTP includes all its spanning tree information in a single BPDU format. Not only does it reduce the number of BPDUs required on a LAN to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP (and, in effect, classic STP too). BPDUs' general format comprises a common generic portion, octets 1 to 36, that are based on those defined in IEEE Standard 802.1D, 2004, followed by components that are specific to CIST, octets 37 to 102. Components specific to each MSTI are added to this BPDUs data block.

MSTP configuration identification In case there is an allocation of VIDs (VLAN IDs) into an MST Region which differs within the different bridges that compose it, frames for some VIDs might be duplicated or even not delivered to some LANs at all. To avoid this, MST Bridges check that they are allocating VIDs to the same spanning trees as their neighboring MST Bridges in the same Region by transmitting and receiving MST Configuration Identifiers along with the spanning tree information. These MST Configuration Identifiers, while compact, are designed so that two matching identifiers have a very high probability of denoting the same configuration even in the absence of any supporting management practice for identifier allocation. Either one of this objects contains the following:

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Illustrations

Worked examples

Example 1 — a first encounter with Multiple Spanning Tree Protocol

Start with the simplest possible case. Write down what Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol

In research: Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol 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: Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol in 20 minutes

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

Frequently asked questions

What is Multiple Spanning Tree Protocol in simple terms?

Why does Multiple Spanning Tree Protocol 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 Multiple Spanning Tree Protocol?

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 Multiple Spanning Tree Protocol.