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Spanning tree protocolWikipedia

Spanning tree protocol

The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning TreeProtocol, which is also referred to as STP, is defined in the IEEE Standard 802.1D. Spanning tree is based onan algorithm invented by Radia Perlman while working for Digital Equipment Corporation. STP is used inswitched networks to prevent loops, and has been standardized by IEEE 802.1D. As the name suggests it findsa spanning tree within the mesh network formed by the installer, whether purposefully created or not, and

disables the links not part of that tree. There are 2 different standards of STP; IEEE and DEC. IEEE is themost common, and is recommended to be the one used in a network. Problems can be caused in networkswhere both standards of STP are implemented. It is likely that both standards will elect their own rootbridge and cause loops in the topology. The way in which both standards handle Bridge Protocol Data Units(see below) also differs.

If more than one open path were to be active at once then there would be several problems. First, a

broadcast storm caused by broadcast packets looping between switches would reduce available CPUresources and bandwidth (many modern bridges can detect and limit the effects of this, although it usuallystill causes a breakdown in connectivity). Second, the traditional source-based location system (filteringdatabase) used by switches would fail to operate correctly. However, a good network design should includespare (redundant) links to provide an alternate path if one fails. Hence the need for spanning tree in aswitched network.

Protocol operation

Elect a root bridge. Find paths to root bridge. Determine least cost (measured as sum of all traversed port costs) path to root bridge. Disable all other root paths.

Electing a root bridge

Each switch has a MAC address and a configurable priority number; both of these numbers make up theBridge Identification or BID. The BID is used to elect a root bridge based upon the lowest priority number; ifthis is a tie then the numerically lowest MAC address wins. Because it is next to impossible that two MACaddresses will be the same (they are uniquely assigned), one switch should always be successfully elected asthe root bridge with the switch that has the next lowest priority/cost acting as a secondary (backup) rootbridge. Other switches in the network that use spanning tree will use the same algorithmic process tocalculate the shortest path to the root bridge and so produce a loop free tree topology where multiple pathsto the root bridge exist. The priority number is normally left at its default value but can be reconfigured toa lower number if the network administrator wishes a particular switch to be elected; otherwise the wholeprocess is fully automated.

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RSTP switch port roles:

Root - A forwarding port that has been elected for the spanning-tree topology Designated - A forwarding port for every LAN segment Alternate - An alternate path to the root bridge. This path is different than using the root port. Backup - A backup/redundant path to a segment where another switch port already connects. Disabled - Not strictly part of STP, a network administrator can manually disable a port

RSTP is a refinement of STP and therefore share most of its basic operation characteristics. However thereare some notable differences as summarized below

Detection of root switch failure is done in 3 hello times or 6 seconds if default hello time have notbeen changed

Ports may be configured as Edge ports if they are attached to a LAN which has no other bridgesattached. These edge ports transition directly to the forwarding state. RSTP still continues to monitorthe port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detectedge ports.

Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge. An RSTP bridgewill "propose" to its designated ports its spanning tree information. If another RSTP bridge receivesthis information, determines this is the superior root information, and sets all its other ports to

discarding the bridge may send an "agreement" to the first bridge confirming its superior spanningtree information. The first bridge, upon receiving this agreement, knows it can rapidly transition thatport to the forwarding state bypassing the traditional listening/learning state transition. Thisessentially creates a cascading effect away from the root bridge where each designated bridgeproposes to its neighbors to determine if it can make a rapid transition. This is one of the majorelements which allows RSTP to achieve faster convergence times than STP.

As port roles details above shows, RSTP maintains back up details regarding discarding ports. This

avoids timeouts if the current forwarding ports were to fail or BPDUs were not recieved on the rootport in a certain interval.

Multiple Spanning Tree Protocol (MSTP)

The Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into IEEE802.1Q -2003, defines an extension to the RSTP protocol to further develop the usefulness of virtual LANs(VLANS). This "Per-VLAN" Multiple Spanning Tree Protocol configures a separate Spanning Tree for each VLAN

group and blocks the links that are redundant within each Spanning Tree.

If there is only one Virtual LAN (VLAN) in the network, single (traditional) STP works appropriately. If thenetwork contains more than one VLAN, the logical network configured by single STP would work, but it ispossible to make better use of the redundant links available by using an alternate spanning tree for different(groups of) VLANs.

MSTP allows formation of MST regions which can run multiple MST instances (MSTI). Multiple regions andother STP bridges are interconnected using one single common spanning tree (CST).

MSTP was inspired by Cisco Systems' Multiple Instances Spanning Tree Protocol, and is an evolution of theSpanning Tree Protocol and the Rapid Spanning Tree Protocol. It was introduced in IEEE 802.1s asamendment to 802.1Q, 1998 edition. Standard IEEE 802.1Q-2003 now includes MSTP.

Unlike some proprietary per-vlan spanning tree implementations, MSTP includes all of its spanning treeinformation in a single BPDU format. Not only does this reduce the number of BPDUs required on a LAN tocommunicate spanning tree information for each VLAN but also ensures backward compatibility with RSTP(and in effect classical STP too). MSTP does this by encoding additional region information after the standardRSTP BPDU as well as a number of MSTI messages (from 0 to 64 instances, although in practice many bridgessupport less). Each of these MSTI configuration messages conveys the spanning tree information for eachinstance. Each instance can be assigned a number of configured VLANs and frames (packets) assigned to




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these VLANs operate in this spanning tree instance whenever they are inside the MST region. In order toavoid conveying their entire VLAN to spanning tree mapping in each BPDU, bridges encode an MD5 digest oftheir VLAN to instance table in the MSTP BPDU. This digest is then used by other MSTP bridges, along withother administratively configured values, to determine if the neighboring bridge is in the same MST region asitself.

MSTP is fully compatible with RSTP bridges, in that an MSTP BPDU can be interpreted by an RSTP bridge as

an RSTP BPDU. This not only allows compatibility with RSTP bridges without configuration changes but alsocauses any RSTP bridges outside of an MSTP region to see the region as a single RSTP bridge, regardless ofthe number of MSTP bridges inside the region itself. In order to further facilitate this view of an MST regionas a single RSTP bridge, the MSTP protocol utilizes a variable known as remaining hops as a time to livecounter instead of the message age timer used by RSTP. The message age time is only incremented oncewhen spanning tree information enters an MST region and therefore RSTP bridges will see a region as only 1"hop" in the spanning tree. Ports at the edge of an MST region connected to either a RSTP or STP bridge or anendpoint are known as boundary ports. Like RSTP these ports can be configured as edge ports to facilitate

rapid changes to the forwarding state when connected to endpoints.

Trivia

Radia Perlman, the inventor of the algorithm summarized it in the form of a poem, titled "Algorhyme":(This poem was modified from the original entitled "Trees", by Joyce Kilmer).

I think that I shall never seeA graph more lovely than a tree.A tree whose crucial propertyIs loop-free connectivity.A tree which must be sure to span.So packets can reach every LAN.

First the Root must be selectedBy ID it is elected.Least cost paths from Root are tracedIn the tree these paths are placed.

A mesh is made by folks like meThen bridges find a spanning tree.

References

Perlman, Radia (2000). Interconnections, Second Edition. USA: Addison-Wesley. ISBN 0-201-63448-1.

See also

minimum spanning tree distributed minimum spanning tree

IEEE 802.1D VLAN

External links

Radia Perlman at Sun Labs ANSI/IEEE 802.1D-2004 standard Cisco's version of 'Understanding STP' RFCs

RFC 2674-1999, proposed standard, Definitions of Managed Objects for Bridges with Traffic




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Classes, Multicast Filtering and Virtual LAN Extensions RFC 1525-1993, - SBRIDGEMIB, proposed standard, Definitions of Managed Objects for Source

Routing Bridges RFC 1493-1993 - BRIDGEMIB, draft standard, Definitions of Managed Objects for Bridges

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