Specifications

Understanding How Spanning Tree Operates
March 14, 2011 Page 7 of 29
Rootportsanddesignatedportsareleftintheforwardingstate.Redundantportsareplacedinthe
blockingstatetoensurethetopologyremainsloop‐free.Table 2liststheseandadditionalport
stateswhichcontroltheforwardingandlearningprocesseswithinatopology.
Typically,switchportsareeitherinblockingor
forwardingstate.Asstatedpreviously,a
forwardingportisaportthathasthelowestpathcosttotherootbridge.Aportwillneverbe
placedinforwardingstateunlesstherearenoredundantlinksandSpanningTreedeterminesthat
itisthebestpathtotherootbridge.
Ifthenetworktopologychanges(forexample,duetoafailed
linkortheadditionofanewswitchingdevicetothe network),theportsonaswitchwillbein
listeningandlearningstates.Blockingportsareusedtopreventnetworkloops.Onceaswitch
determinesthebestpathto
therootbridge,allotherportswillbeinblockingstate.Blockedports
donotforwardframes,buttheystillreceiveBPDUs.Ifaswitchdeterminesthatablockedport
shouldnowbethedesignatedport,itwillgointolisteningstate.ItwillcheckalltheBPDUsto
make
surealoopwillnotbecreatedoncetheportgoestoforwardingstate.
MSTP Operation
MSTPmakesitpossibleforVLANswitchingdevicestousemultipleSpanningTrees,allowing 
trafficbelongingtodifferentVLANstoflowoverpotentiallydifferentpathswithintheLAN.It
buildsupontheadvancementsofRSTPwithitsdecreasedtimefornetworkre‐spans.MSTP’s
principleobjectiveistoincreasebandwidthutilization
byallowing:
•FramesassignedtodifferentVLANstofollowdifferentdataroutes
•PortstoblockforsomeSpanningTreesandforwardforothers
•EveryISLinthetopologytobeforwardingforatleastoneSpanningTree
Table 1 Spanning Tree Port Roles
Port Role Description
Root The one port that is used to connect to the root bridge. It is elected based on its least
“path-cost” to the root bridge and is forwarding traffic.
Alternate Any redundant upstream port that provides an alternate path to the root bridge (other
than the root port).
Designated Any downstream port that provides a path back to the root bridge for a downstream
bridge. This port is forwarding traffic.
Backup A port that acts as a redundant designated port on a shared LAN.
Table 2 Spanning Tree Port States
Port State Behavior
Blocking Actively preventing traffic from using this path. Still receiving BPDUs, so continuing to
monitor for management and STA information.
Listening Continuing to block traffic while waiting for protocol information to determine whether
to go back to the blocking state, or continue to the learning state. Listens to BPDUs
to ensure no loops occur on the network.
Learning Learning station location information but continuing to block traffic.
Forwarding Forwarding traffic and continuing to learn station location information.
Disabled Disabled administratively or by failure.