Multiple Spanning Tree Protocol: Difference between revisions

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{{commons category|MSTP}}

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 Protocol#Multiple Spanning Tree Instances .28MSTI.29(MSTI)|MSTI]] (Multiple Spanning Tree Instances) and in the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29(CST/CIST)|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 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]]'s Spanning Tree Protocol (STP) and the [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|Rapid Spanning Tree Protocol]] (RSTP). It has some similarities with [[Cisco Systems]]' Multiple Instances Spanning Tree Protocol (MISTP), buttherebut there are some differences.

If there is only one VLAN in the network, single (traditional) STP/RSTP will work appropriately but if the network contains more than one VLAN, the logical network configured by single STP/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 VLANs or groups of VLANs, here is where the necessity of hammering away a new extension of RSTP philosophy into multiple trees organized by VLANs' groups came up.

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[[File:MSTI.png|thumb|Different Spanning trees created by different MSTIs on the same physical layout.]]

As MSTP enables grouping and mapping VLANs into different spanning tree instances, there’s an urge of determining a group or set of VLANs, which are all using the same spanning tree, this is what we come to know as a MSTI. <br />

Each instance defines a single forwarding topology for an exclusive set of VLANs, by contrast, [[Spanning Tree Protocol|STP]] or [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] networks contains only a single spanning tree instance for the entire network, which contains all the VLANs. A region can include:<ref>{{cite book

|last = packard

|first = Hewlett

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=== MSTP Regions ===

[[File:MST Region.png|thumb|MSTIs in different regions.]]

A set of interconnected switches that must have configured the same VLANs and [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]], also the same following parameters:

* '''MST Configuration Name'''

* '''Revision Level'''

* '''Configuration Digest:''' Mapping of which VLAN are mapped to which MST instances.

An [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|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 [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]]. For achieving the task of assigning each bridge to a region, each switch/bridge must compare their '''MST Configuration Identifiers (Format Selector, Region Name, Revision Level and Configuration Digest)''', either of them represents VLAN to [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]] mapping for each bridge.

=== Common and Internal Spanning Tree (CST/CIST) ===

[[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 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.

=== MSTP Bridge Protocol Data Units (BPDU) ===

{{Main article|Bridge Protocol Data Unit}}

Its main function is enabling MSTP to select its root bridges for the proper [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] and each [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]]. MSTP includes all its spanning tree information in a single [[Bridge Protocol Data Unit|BPDU]] format. Not only does reduce the number of [[Bridge Protocol Data Unit|BPDUs]] required on a [[Local area network|LANs]] to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] (and in effect, classic [[Spanning Tree Protocol|STP]] too).

[[Bridge Protocol Data Unit|BPDUs']] general format comprises a common generic portion ''-octets 1 to 36-'' that are based on those defined in [[Institute of Electrical and Electronics Engineers|IEEE]] Standard [[IEEE 802.1D|802.1D]],2004,<ref>{{cite book|last = IEEE|first = Standard|title = IEEE Standard for Local and metropolitan area networks, Media Access Control (MAC) Bridges|publisher = IEEE Computer Society

|year = 2004|url = http://www.ccna-powertraining.de/wp-content/uploads/2014/10/802.1D-2004.pdf}}</ref> followed by components that are specific to [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] ''-octets 37 to 102.'' Components specific to each [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] are added to this [[Bridge Protocol Data Unit|BPDUs]] data block.

[[Bridge Protocol Data Unit|BPDUs']] general format comprises a common generic portion ''-octets 1 to 36-'' that are based on those defined in [[Institute of Electrical and Electronics Engineers|IEEE]] Standard [[IEEE 802.1D|802.1D]],2004,<ref>{{cite book|last = IEEE|first = Standard|title = IEEE Standard for Local and metropolitan area networks, Media Access Control (MAC) Bridges|publisher = IEEE Computer Society

[https://www.alliedtelesis.com/sites/default/files/stp_feature_config_guide.pdf BPDU table info] and [[Spanning Tree Protocol#Bridge Protocol Data Unit fields|STP BPDUs]] ''' show a deeper resume of the MSTP [[Bridge Protocol Data Unit|BPDU]] format''' and, besides, some additional information about how was this object structured in older or different versions of this protocol as [[Spanning Tree Protocol|STP]] and [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]], maintaining its compatibility.

|year = 2004|url = http://www.ccna-powertraining.de/wp-content/uploads/2014/10/802.1D-2004.pdf}}</ref> followed by components that are specific to [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] ''-octets 37 to 102.'' Components specific to each [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] are added to this [[Bridge Protocol Data Unit|BPDUs]] data block.

[https://www.alliedtelesis.com/sites/default/files/stp_feature_config_guide.pdf BPDU table info] and [[Spanning Tree Protocol#Bridge Protocol Data Unit fields|STP BPDUs]] ''' show a deeper resume of the MSTP [[Bridge Protocol Data Unit|BPDU]] format''' and, besides, some additional information about how was this object structured in older or different versions of this protocol as [[Spanning Tree Protocol|STP]] and [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]], maintaining its compatibility.

=== 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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|url = https://tools.ietf.org/html/rfc2104

}}</ref> A 16B signature [[Hash-based message authentication code|HMAC]]-[[MD5|MD5 Algorithms]] created from the MST Configuration Table.<br />

This object is specific and unique of MSTP, neither [[Spanning Tree Protocol|STP]] or [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] use it.

== Protocol Operation ==

MSTP configures for every 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 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 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 VLANs, frames (packets) assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. To avoid conveying their entire VLAN to spanning tree mapping in each [[Bridge Protocol Data Unit|BPDU]], bridges encode an MD5 digest of their 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.

== Port Roles ==

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=== Common Internal Spanning Tree Ports ===

* '''Root:''' Provides the minimum cost path from the Bridge to the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root through the Regional Root.

* '''Designated:''' Provides the least cost path from the attached [[Local area network|LAN]] through the Bridge to the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] Root.

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

=== Multiple Spanning Tree Instance Ports ===

* '''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]]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]]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]]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 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.

For all the above, it can be concluded that MSTP is fully compatible with [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] bridges, an MSTP [[Bridge Protocol Data Unit|BPDU]] can be interpreted by an [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] bridge as an [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] [[Bridge Protocol Data Unit|BPDU]]. This not only allows compatibility with [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] bridges without configuration changes, but also causes any [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] bridges outside of an [[Multiple Spanning Tree Protocol#MSTP Regions|MSTP Region]] to see the region as a single [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] bridge, regardless of the number of MSTP bridges inside the region itself.

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# Common and Internal Spanning Tree (CIST) is the default spanning tree instance for MSTP. This means that all VLANs that are not explicitly configured into another [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] are members of the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]].

# The software supports a single instance of the MSTP Algorithm consisting of the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] and up to 15 [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTIs]].

A VLAN can only be mapped to one [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]] or to the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]]. One VLAN mapped to multiple spanning trees is not allowed. All the VLANs are mapped to the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]] by default. Once a VLAN is mapped to a specified [[Multiple Spanning Tree Protocol#Multiple Spanning Tree Instances .28MSTI.29|MSTI]], it is removed from the [[Multiple Spanning Tree Protocol#Common and Internal Spanning Tree .28CST.2FCIST.29|CIST]].To avoid unnecessary [[Spanning Tree Protocol|STP]] processing, a [[Port (computer networking)|Port]] that is attached to a [[Local area network|LAN]] with no other bridges/switches attached, can be configured as an edge port.

An example of how to configure a simple, three switch SMTP topology wherein a layer-two access switch carries four VLANs and has two uplinks to two distribution switches, can be found here: [http://packetlife.net/blog/2010/apr/26/multiple-spanning-tree-mst/ MSTP Configuration Guide]<br />

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==== ARP and ABridge Resolution ====

As any layer-two based protocol, ABridges uses [[Address Resolution Protocol|ARP]] broadcasts to obtain the [[link layer]] address associated to an IP address at the same [[Local area network|LAN]] or VLAN. That is the main cause why avoiding overflooding is a matter of paramount priority; to limit this broadcast traffic, is recommended the use of distributed load [[Address Resolution Protocol|ARP]] servers, although its use is not compulsory.

==See also==