Multiple Spanning Tree Protocol: Difference between revisions

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The '''''Multiple Spanning Tree Protocol''''' (MSTP) and [[algorithm]], provides both, simple and full, connectivity assigned to any given [[Virtual LAN|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 Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] (Multiple Spanning Tree Instances) and in the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] (Common and Internal Spanning Tree), 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 [[Bridging (networking)|bridge]] [[Switching loop|loops]] danger.

Moreover, MSTP allows frames/packets assigned to different [[Virtual LAN|VLANs]] to follow separate paths, each based on an independent [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]], within MST Regions composed of [[Local area network|LANs]] and or MST Bridges. These Regions and the other Bridges and [[Local area network|LANs]] are connected into a single Common Spanning Tree (CST).

== History ==

It was originally defined in [[Institute of Electrical and Electronics Engineers|IEEE]] 802.1s as an amendment to [[IEEE 802.1Q|802.1Q]], 1998 edition and later merged into [[Institute of Electrical and Electronics Engineers|IEEE]] [[IEEE 802.1Q|802.1Q-2005 Standard]], clearly defines an extension or an evolution of [[Radia Perlman|Radia Perlman’s]] [[Spanning Tree Protocol]] and the [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|Rapid Spanning Tree Protocol]]. It’s mandatory to highlight that it has some similarities with [[Cisco Systems|Cisco Systems']] Multiple Instances Spanning Tree Protocol (MISTP) but, in fact, as always occurs with providers implementations there are a couple of differences.

If there is only one [[Virtual LAN|VLAN]] in the network, single (traditional) [[Spanning Tree Protocol|STP]]/[[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] will work appropriately but, as it’s easily supposed, if the network contains more than one [[Virtual LAN|VLAN]], the logical network configured by single [[Spanning Tree Protocol|STP]]/[[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] wouldn’t work as efficiently as it is supposed to, even letting some errors ([[Switching loop|loops]], bad paths…) to appear. Instead, it is possible to make better use of the alternate paths available by using an alternate [[spanning tree]] for different [[Virtual LAN|VLANs]] or groups of [[Virtual LAN|VLANs]], here is where the necessity of hammering away a new extension of [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] philosophy into multiple trees organized by [[Virtual LAN|VLANs']] groups came up.

== Main Entities ==

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[[File:CIST.png|thumb|CIST operates links between regions and to SST devices.]]

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, [[Spanning Tree Protocol|STP]] [[Local area network|LANs]] and [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] [[Local area network|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 [[Virtual LAN|VLAN]] which isn’t a member of a particular [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|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 [[Multiple Spanning Tree Protocol#MSTP Regions|MSTP Region]] and parts of the network running in legacy [[Spanning Tree Protocol|STP]] or [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] mode.

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=== MSTP Configuration Identification ===

In case there is an allocation of [[IEEE 802.1Q#Double tagging|VIDs (VLAN IDs)]] into a MST Region which differs within the different bridges that compound it, '''frames for some [[IEEE 802.1Q#Double tagging|VIDs]] might be duplicated or even not delivered to some [[Local area network|LANs]] at all'''. To avoid this, MST Bridges check that they are allocating [[IEEE 802.1Q#Double tagging|VIDs]] to the same [[spanning tree]]s 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:

* '''Configuration Identifier Format Selector:''' Indicates the use which is going to be given to the following components.

* '''Configuration Name'''<ref>{{cite book

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MSTP configures for every [[Virtual LAN|VLAN]] a single spanning tree active topology in a manner that there’s at least one data route between any two end stations, eliminating data loops. It specifies various “objects” allowing out the algorithm to operate in a proper way. The different bridges in the various [[Virtual LAN|VLANs]] start advertising their own configuration to other bridges using the MST Configuration Identifier in order to allocate frames with given VIDs (VLAN ID) to any of the different [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]]. A priority vector is utilized to construct the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]], it connects all the bridges and [[Local area network|LANs]] in a Bridged [[Local area network|LAN]] and ensures that paths within each region are always preferred to paths outside the Region. Besides, there is a [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] priority vector, this one compromises the necessary information to build up a deterministic and independently manageable active topology for any given [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] within each region.

Additionally, comparisons and calculations done by each bridge select a [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] priority vector for each [[Port (computer networking)|Port]] (based on priority vectors, MST Configuration Identifiers and on an incremental Path Cost associated to each receiving port). This leads to one bridge been selected as the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root of the Bridged [[Local area network|LAN]]; then, a minimum cost path to the root is shifted out for each Bridge and [[Local area network|LANs]] (thus preventing loops and ensuring full connectivity between [[Virtual LAN|VLANs]]). Subsequently, in each region, the bridge whose minimum cost path to the root doesn’t pass through another bridge with the same MST Conf.ID will be identified as its Region’s [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Regional Root. Conversely, each Bridge whose minimum cost path to the Root is through a Bridge using the same MST Configuration Identifier is identified as being in the same MST Region as that Bridge.

In summary, MSTP encodes some additional information in its [[Bridge Protocol Data Unit|BPDU]] regarding region information and configuration, each of these messages conveys the spanning tree information for each instance. Each instance can be assigned several configured [[Virtual LAN|VLANs]], frames (packets) assigned to these [[Virtual LAN|VLANs]] operate in this spanning tree instance whenever they are inside the MST region. To avoid conveying their entire [[Virtual LAN|VLAN]] to spanning tree mapping in each [[Bridge Protocol Data Unit|BPDU]], bridges encode an MD5 digest of their [[Virtual LAN|VLAN]] to instance table in the MSTP [[Bridge Protocol Data Unit|BPDU]]. This digest is then used by other MSTP bridges, along with other administratively configured values, to determine if the neighboring bridge is in the same MST region as itself.

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* '''Root:''' Provides the minimum cost path from the Bridge to the [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] Regional Root.

* '''Designated:''' Provides the least cost path from the attached [[Local area network|LANs]] through the Bridge to the Regional Root.

* '''Master:''' Provides connectivity from the Region to a [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root that lies outside the Region. The Bridge [[Port (computer networking)|Port]] that is the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root [[Port (computer networking)|Port]] for the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Regional Root is the Master [[Port (computer networking)|Port]] for all [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]].

* '''Alternate or Backup:''' Provides connectivity if other Bridges, Bridges [[Port (computer networking)|Ports]] or [[Local area network|LANs]] fail or are erased.

== RSTP compatibility ==

MSTP is designed to be [[Spanning Tree Protocol|STP]] and [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] compatible and interoperable without additional operational management practice, this is due to a set of measurements based on [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] (Clause 17 of [[Institute of Electrical and Electronics Engineers|IEEE]] Std [[IEEE 802.1D|802.1D]], 2004 Edition) intending to provide the capability for frames assigned to different [[Virtual LAN|VLANs]], to be transmitted along different paths within MST Regions.<br />

Both protocols have in common various issues such as: the selection of the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root Bridge (it uses the same fundamental algorithm, 17.3.1 of [[Institute of Electrical and Electronics Engineers|IEEE]] Std [[IEEE 802.1D|802.1D]], 2004 Edition, but with extended priority vector components within MST Regions), the selection of the [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] Root Bridge and computation of [[Port (computer networking)|Port]] roles for each [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]], the [[Port (computer networking)|Port]] roles used by the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] are the same as those of [[Spanning Tree Protocol|STP]] and [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] (with the exception of the Master Port), and the state variables associated with each port. <br />

Into the bargain, they also share some problems as, for instance: MSTP can’t protect against temporary loops caused by the inter-connection of two [[Local area network|LANs]] segments by devices other than the Bridges that operate invisibly with respect to support of the Bridges’ [[MAC address|MAC]] Internal Sublayer Service.

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Be sure of having configured [[Virtual LAN|VLANs]] and having associated them with switch ports, afterwards determine: [[Multiple Spanning Tree Protocol#MSTP Regions|MSTP Regions]], revision level and instances; which [[Virtual LAN|VLANs]] and switch [[Port (computer networking)|Ports]] will belong to which [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]] and, finally, which devices do you want to be root bridges for each [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]].

=== Configuration guidelines for MSTP ===

[[File:MSTP config.png|thumb|Simple network topology for MSTP trials.]]

# Switches must have the same MST configuration identification elements (region name, revision level and [[Virtual LAN|VLAN]] to [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] mapping) to be in the same MST region. When configuring multiple MST regions for MSTP, [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]] are locally significant within an MST region. [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]] will not span from one region to another region.

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Bridge address 000e.8316.f500 priority 32768 (32768 sysid 0)

Root address 0013.c412.0f00 priority 0 (0 sysid 0)

port Fa0/13 path cost 0

Regional Root address 0013.c412.0f00 priority 0 (0 sysid 0)

internal cost 200000 rem hops 19

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=== ABRIDGES ===

This protocol, developed in <ref>{{cite book

|last = Ibáñez, García, Azcorra, Soto

|first = Guillermo, Alberto, Arturo, Ignacio

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== External links ==

{{commons category|MSTP}}

* [http://standards.ieee.org/about/get/802/802.1.html IEEE "Home Page" for 802.1] (Related Standards of the 802.1 family)

* [http://blog.ine.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/ MSTP Tutorial] (Brief Tutorial for the comprehension of SMTP)

* [http://www.postel.org/pipermail/rbridge/ RBridge]

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** [https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html] (Cisco Implementation and brief tutorial about SMTP)

** [http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html Cisco home page for the Spanning-Tree protocol family] (discusses CST, MISTP, PVST, PVST+, RSTP, STP)

** [http://www.cisco.com/image/gif/paws/10556/spanning_tree1.swf Educational explanation of STP] www.cisco.com

* {{cite web

| last = Perlman