Multiple Spanning Tree Protocol: Difference between revisions

Revision as of 14:21, 26 September 2017

Revision as of 20:41, 26 September 2017

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== 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 [[IEEE 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), but 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.

== Main Entities ==

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=== 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 MSTI. MSTP includes all its spanning tree information in a single [[Bridge Protocol Data Unit|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 [[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 BPDUs data block.

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== 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 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.

== Protocol Configuration ==

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==== Protocol Operation ====

To set up these trees, AMSTP relies in one basic tree which will be used to obtain instances (named Alternate Multiple Spanning Tree Instances – AMSTI), until one of them is built per switch for the network. The process applied to build up the main/basic tree is the same as in [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]]. In summary, firstly a bridge must be elected as the Root Bridge (this is done by the emission of [[Bridge Protocol Data Unit|BPDUs]] from each switch on the network periodically, every “Hello Time”, and selecting the lowest Bridge ID). Then, every switch will compute and calculate its cost to the Root Bridge and, afterwards, the root [[Port (computer networking)|Ports]] must be elected by selecting the one which receives the best [[Bridge Protocol Data Unit|BPDU]], this is, the one that announces minimum path cost to root bridge.

==== BPDUs ====

{{Main article|Bridge Protocol Data Unit}}

AMSTP [[Bridge Protocol Data Unit|BPDUs]] use the same local multicast protocol addresses than [[Spanning Tree Protocol|STP]] and have a structure that resembles MSTP [[Bridge Protocol Data Unit|BPDUs]] since both are comprised essentially of a basic [[Bridge Protocol Data Unit|BPDU]] and several AM-Records, allowing full-backwards compatibility with [[Spanning Tree Protocol#Rapid Spanning Tree Protocol|RSTP]] and [[Spanning Tree Protocol|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 [[Port (computer networking)|Ports]] of neighboring switches to negotiate the transitions of each tree instance with a proposal/agreement mechanism.

=== ABRIDGES ===