User manual

 16‐PortGigabitEthernetPoE+Web‐ManagedSwitchwith2SFPPorts
51
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STPcommunicatesbetweenswitchesonthenetworkusingBridgeProtocolDataUnits(BPDUs).EachBPDUcontainsthe
followinginformation:
 Theuniqueidentifieroftheswitchthatthetransmittingswitchcurrentlybelievesistherootswitch
 Thepathcosttotherootfromthetransmittingport
 Theportidentifierofthetransmittingport
TheswitchsendsBPDUstocommunicateandconstructthespanning‐treetopology.AllswitchesconnectedtotheLAN
onwhichthepacketistransmittedwillreceivetheBPDU.BPDUsarenotdirectlyforwardedbytheswitch,butthe
receivingswitchusestheinformationintheframetocalculateaBPDU,and,ifthetopologychanges,initiatesaBPDU
transmission.
ThecommunicationbetweenswitchesviaBPDUsresultsinthefollowing:
 Oneswitchiselectedastherootswitch
 Theshortestdistancetotherootswitchiscalculatedforeachswitch
 Adesignatedswitchisselected.Thisistheswitchclosesttotherootswitchthroughwhichpacketswillbe
forwardedtotheroot.
 Aportforeachswitchisselected.Thisistheportprovidingthebestpathfromtheswitchtotherootswitch.
 PortsincludedintheSTPareselected.
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CreatingaStableSTPTopology
IfallswitcheshaveSTPenabledwithdefaultsettings,theswitchwiththelowestMACaddressinthenetworkwill
becometherootswitch.Byincreasingthepriority(loweringtheprioritynumber)ofthebestswitch,STPcanbeforcedto
selectthebestswitchastherootswitch.WhenSTPisenabledusingthedefaultparameters,thepathbetweensource
anddestinationstationsinaswitchednetworkmightnotbeideal.Forinstance,connectinghigher‐speedlinkstoaport
thathasahighernumberthanthecurrentrootportcancausearoot‐portchange.
STPPortStates
BPDUstakesometimetopassthroughanetwork.Thispropagationdelaycanresultintopologychangeswhereaport
thattransitioneddirectlyfromaBlockingstatetoaForwardingstatecouldcreatetemporarydataloops.Portsmustwait
fornewnetworktopologyinformationtopropagatethroughoutthenetworkbeforestartingtoforwardpackets.They
mustalsowaitforthepacketlifetimetoexpireforBPDUpacketsthatwereforwardedbasedontheoldtopology.The
forwarddelaytimerisusedtoallowthenetworktopologytostabilizeafteratopologychange.Inaddition,STPspecifiesa
seriesofstatesaportmusttransitionthroughtofurtherensurethatastablenetworktopologyiscreatedaftera
topologychange.
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