Cisco Catalyst 2960 Software Configuration Manual page 499

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Configuring Optional Spanning-Tree Features

If link L1 fails as shown in

directly to link L1. However, because Switch B is directly connected to the root switch over L1, it detects

the failure, elects itself the root, and begins sending BPDUs to Switch C, identifying itself as the root.

When Switch C receives the inferior BPDUs from Switch B, Switch C assumes that an indirect failure

has occurred. At that point, BackboneFast 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.

reconfigures the topology to account for the failure of link L1.

If a new switch is introduced into a shared-medium topology as shown in

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.

BackboneFast Example Before Indirect Link Failure

Blocked port

18-7, Switch C cannot detect this failure because it is not connected

BackboneFast Example After Indirect Link Failure

Link failure

BackboneFast changes port

through listening and learning

states to forwarding state.

Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE

Understanding Optional Spanning-Tree Features

shows how BackboneFast

18-8, BackboneFast is

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