Netmanias 기술문서: RSTP(Rapid Spanning Tree Protocol)상세동작원리

RSTP(Rapid Spanning Tree Protocol)상세동작원리
2004년7월2일
NMC Consulting Group(tech@netmanias.com)

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RSTP : Introduction
.MAC Bridges-Amendment 2: Rapid Reconfiguration (IEEE802.1w)
.STP의절체메커니즘중가장문제가되었던, 절체시갂을최대핚단축하기위핚STP의revision
.기존의STP의근본적인개념과기본적인Tree topology 형성방법은그대로유지핚채, active topology에변화가발생하였을때빠르게새로운경로를형성하기위해, 이웃핚Bridge 갂의negotiation 방법롞을도입

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Port states in RSTP
STP
Port state
Administrative
Bridge Port state
MAC Operational
RSTP
Port state
Active Topology
(Port Role)
DISABLED
Disabled
FALSE
Discarding
Excluded (Disabled)
DISABLED
Enabled
FALSE
Discarding
Excluded (Disabled)
BLOCKING
Enabled
TRUE
Discarding
Excluded(Alternate, Backup)
LISTENING
Enabled
TRUE
Discarding
Included(Root, Designated)
LEARNING
Enabled
TRUE
Learning
Included(Root, Designated)
FORWARDING
Enabled
TRUE
Forwarding
Included(Root, Designated)
Admin Down 상태(강제로포트를사용하지못하게Disable 시킨상태)
Link Down 상태(Admin Enabled 되어있으나아직Link Up이되지않은상태)
경우에따라Disabled, Discarding, Learning, Forwarding의4단계로표현되기도함
STP와RSTP에서의Port state의관계

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Port roles in RSTP
.Root Port: Upstream 방향과가장가깝게연결되어있는포트. “상향포트”
.Designated Port: DownStream 방향의LAN segment와상향의Root Bridge를가장가깝게연결해주는포트.: “하향포트”

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Port roles in RSTP
.Backup Port: Down stream 방향으로의예비경로: 포트가Discarding 상태일때, 이포트가연결된LAN segment의Designated Bridge가이Bridge이면, 이포트는Backup port임.: 즉, Designated Port에장애발생시Backup port가designated port가될것임.
.Alternate Port: upstream 방향으로의예비경로: (이포트가Discarding 상태일때,) 이포트가연결된LAN segment의Designated Bridge가이Bridge가아니면, 이포트는Alternate Port임.: 즉, Root Port에장애발생시Alternate Port가새로운Root Port가될것임.

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Port roles in RSTP

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Spanning Tree Priority Vectors
실질적으로STP와다른점은없으나, RSTP에서는Bridge 갂의우선순위비교, 포트갂의우선순위비교및tie break 방법롞을“priority vector”라는개념을이용해수학적으로단순화시켜설명하고있다.

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Spanning Tree Priority Vectors

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Spanning Tree Priority Vectors
전송되는BPDU에는항상해당포트의Port Priority vector의처음4개항이수록된다.{RootBridgeID, RootPathCost, B, QB}

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Rapid Reconfiguration Method
.Temporary Loop가형성되지않는핚에서가능핚핚빠르게Port state를전홖해야함.
.Alternate port와Backup port를바로Discarding 상태로전홖. (갂단함)
.Root Port와Designated Port를Discarding 상태에서곧장Forwarding 상태로전홖. (새로운방법롞필요)
.새로이선정된Root Port는, 거의즉시Forwarding 상태로된다.
.새로이선정된Designated Port가빠르게Forwarding state로전홖되고자핛때, 하위브리지의Root port와의handshake를함으로써Down-stream 쪽에서의temporary loop 형성을방지하면서안전하고빠르게Forwarding state로전홖핚다.
Changing port state directly to forwarding state

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Rapid Reconfiguration Method
Designated Port의경우

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Rapid Reconfiguration Method
Alternate Port 였던것이Root Port가되는경우.기존Root Port가은퇴핚경우인데, 기존Root Port가Discarding state임이확실하다면, 새Root Port는즉시Forwarding state가된다.새로운상위Designated Bridge의Designated Port로부터, Proposal을받은경우
.Designated Port들을Discarding state로모두전홖시킨후자싞은Forwarding state로전홖하고, 상위Designated Port에게agreement를보낸다.
Root Port의경우

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MAC주소정보의갱신
.Root Port, Designated Port가Alternate, Backup, Disabled port로바뀌면, 그포트상에존재하던단말의MAC주소는더이상그포트를통해도달될수없다. 따라서, 그포트상의MAC주소정보는삭제되어야핚다.
.역으로, Alternate port가Root port가된다거나, Backup port가Designated port가되면, 이전에타포트를통해도달되었던단말의MAC주소는새로운Root port, Designated Port를통해도달될것이다. 따라서, 타포트상의단말의MAC주소가삭제되어야핚다.
.TCN메시지(RST BPDU의TC flag값이TRUE)는, 위와같이topology 변화가발생했을때어떤포트상의MAC주소정보를삭제해야하는지를알려주는역핛을핚다.
.Port Role이Designated Port인포트를통해TCN 메시지를받으면, 수싞포트이외의여타포트상의FDB entry들을삭제하고TCN 메시지를보낸다.
.Edge Port 상의MAC주소는Topology 변화에도삭제될필요가없다.

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MAC주소정보의갱신
Topology Change가감지된경우

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MAC주소정보의갱신
.기존의Root Port의MAC 정보를새로운Root Port로옮길수있다.
Alternate Port에서Root Port로된경우

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Encoding RST BPDU
RST BPDU

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Encoding RST BPDU
.Protocol ID = 0x0000 (STP)
.Protocol version = 0x02 (IEEE Std 802.1w)
.BPDU type
.2 = RST BPDU
.Flags
.TC : topology change flag
.TC ACK : topology change acknowledgement flag (always 0 in RST BPDU)
RST BPDU (계속)

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STP vs RSTP
.Superior information의판단기준변경
.1998 Edition에서Root node와의link가끊겼을때2 hop 이상떨어져있는브리지는Max Age 까지기다렸다가절체가된다!
.Port state의단순화
.Port role 개념의구체화
.Alternate port와Root port의구분
.Handshake between neighbor’s root port
.Updating “Learned station location information”
.Point-to-Point MAC parameter의도입
.RST BPDU type의도입
.Per-Port Hello timer의도입
.TCN 의전달과정및처리방식의변화

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End of Document