Multiple Spanning Tree Protocol: Difference between revisions

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{{IPstack}}

{{commons category|Multiple Spanning Tree Protocol}}

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 Instances (MSTI)|Multiple Spanning Tree instance]] (MSTI) and in the [[#Common and Internal Spanning Tree (CST/CIST)|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.

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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 STP or RSTP mode.

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

{{Main articleFurther|BridgeSpanning Tree Protocol#Bridge protocol Datadata Unitunits}}

ItsThe main function of bridge protocol data units (BPDUs) is enabling 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 reduce the number of BPDUs required on a LANs 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,<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 CIST ''-octets 37 to 102.'' Components specific to each MSTI are added to this BPDUs data block.

BPDUs' general format comprises a common generic portion, ''-octets 1 to 36-'', that are based on those defined in IEEE Standard [[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 CIST, octets 37 to 102. Components specific to each MSTI are added to this BPDUs data block.<ref>[https://www.alliedtelesis.com/sites/default/files/stp_feature_config_guide.pdf BPDU table info]</ref>

[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 BPDU format''' and, besides, some additional information about how was this object structured in older or different versions of this protocol as STP and RSTP, maintaining its compatibility.

=== MSTP Configuration Identification ===

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==== BPDUs ====

{{Main article|Bridge Protocol Data Unit}}

AMSTP BPDUs use the same local multicast protocol addresses than STP and have a structure that resembles MSTP BPDUs since both are comprised essentially of a basic BPDU and several AM-Records, allowing full-backwards compatibility with RSTP and STP standard protocols. Each of the AM-Records contains the data used to negotiate a specific tree instance (AMSTI). Every ABridge, except for the elected root bridge, creates an AM-Record for its own spanning tree instances. They are used by connected ports of neighboring switches to negotiate the transitions of each tree instance with a proposal/agreement mechanism.

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==See also==

* [[Bridge Protocol Data Unit]]

* [[Distributed minimum spanning tree]]

* [[EtherChannel]]