Specifications

Configuring STP and RSTP
March 14, 2011 Page 13 of 29
configuredwithvariationsoftheSpanningTreeportconfigurationcommands.Defaultsettings
arelistedinTable 4:
Usethecommandsinthefollowing sectionstoadjustthesedefaults.
Setting Bridge Priority Mode and Priority
Bridgeprioritymodeaffectstherangeofpriorityvaluesusedtodeterminewhichdeviceis
selectedastheSpanningTreeroot.Bydefault,switchingdevicesaresetto802.1tmodeas
describedin“Updated802.1t”onpage5.802.1tmodeusesbridgepriorityvaluesof0to61440,in
increments
of4096,with0indicatinghighpriorityand6 1440lowpriority.Legacy(802.1D)
priorityvaluesare0to65535.
Usethiscommandtosetthebridgeprioritymode:
set spantree bridgepriortymode 802.1t | 802.1d
Inadditiontosettingprioritymode,youcangloballyconfigurethepriorityofanindividual
bridge.Whentwobridgestieforpositionastherootbridge,thissettingaffectsthelikelihoodthat
abridgewillbeselected.Thelowerthebridge’spriority,themorelikelythebridgewillbeselected
as
therootbridge.
Usethiscommandtosetthebridgepriority:
set spantree priority priority [sid]
Validpriority valuesare:
–For802.1tprioritymode:0
–61440(inincrementsof4096),with0indicatinghighpriority
and61440lowpriority.Valueswillautomaticallyberoundedupordown,dependingon
the802.1tvaluetowhichtheenteredvalueisclosest.
–For802.1Dprioritymode:0
–65535(inincrementsof1),with0indicatinghighpriorityand
65535lowpriority.
Validsidvaluesare0
–4094.Ifnotspecified,SID0willbeassumed.
Table 4 Spanning Tree Port Default Settings
Setting Default Value
Bridge priority mode 802.1t
Bridge priority 32768
Port priority 128
Port cost 0 (automatically calculated based on port speed)
Hello time (bridge and ports) 2 seconds
Bridge forward delay 15 seconds
Bridge maximum aging time 20 seconds
Note: Poorly chosen adjustments to these parameters can have a negative impact on network
performance. Please refer to the IEEE 802.1D specification for guidance.