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Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter 9 Configuring STP
Understanding Advanced STP Features
switchover takes approximately 30 seconds, twice the Forward Delay time if the default Forward Delay
time of 15 seconds is set. Figure 9-11 shows how BackboneFast reconfigures the topology to account for
the failure of link L1.
Figure 9-11 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure 9-12, 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 learns that Switch B is the designated
bridge to Switch A, the root switch.
Figure 9-12 Adding a Switch in a Shared-Medium Topology
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Link failure
44964
BackboneFast transitions port
through listening and learning
states to forwarding state.
Switch A
(Root)
Switch C
Switch B
(Designated bridge)
Added switch
44965
Blocked port
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