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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter14 Configuring STP Features
Overview of BackboneFast
Figure14-5 BackboneFast after Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure14-6, BackboneFast is
not activated, because the inferior BPDUs did not come from the recognized designated bridge
(Switch B). The new switch begins sending inferior BPDUs that say it is the root switch. However, the
other switches ignore these inferior BPDUs, and the new switch lear ns that Switch B is the designated
bridge to SwitchA, the root switch.
Figure14-6 Adding a Switch in a Shared-Medium Topology
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Link failure
11244
BackboneFast transitions port
through listening and learning
states to forwarding state
Switch A
(Root)
Switch C Switch B
(Designated Bridge)
Added switch
11245
Blocked port