Cisco Catalyst 2960 Series, Configuration Manual

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allows the blocked interface on Switch C to move immediately to the listening state without waiting for the
maximum aging time for the interface to expire. BackboneFast then transitions the Layer 2 interface on
Switch C to the forwarding state, providing a path from Switch B to Switch A. The root-switch election takes
approximately 30 seconds, twice the Forward Delay time if the default Forward Delay time of 15 seconds is
set. BackboneFast reconfigures the topology to account for the failure of link L1.
Figure 26: BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology, BackboneFast is not activated because the
inferior BPDUs did not come from the recognized designated switch (Switch B). The new switch begins
sending inferior BPDUs that indicate it is the root switch. However, the other switches ignore these inferior
BPDUs, and the new switch learns that Switch B is the designated switch to Switch A, the root switch.
Figure 27: Adding a Switch in a Shared-Medium Topology
Related Topics
Specifying the MST Region Configuration and Enabling MSTP , on page 261
MSTP Configuration Guidelines, on page 243
Multiple Spanning-Tree Regions, on page 245
Enabling BackboneFast , on page 309
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
296
Information About Optional Spanning-Tree Features