Cisco WS-C3750-48PS-S Software Configuration Manual page 413

Chapter 19 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. This switchover 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.
Figure 19-7 BackboneFast Example After Indirect Link Failure
Switch A
(Root)
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.
Figure 19-8 Adding a Switch in a Shared-Medium Topology
Switch C
78-16180-02
Figure 19-7, Switch C cannot detect this failure because it is not connected
L1
Link failure
L2
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch C
Switch A
(Root)
Blocked port
Understanding Optional Spanning-Tree Features
Figure 19-7
Switch B
L3
Switch B
(Designated bridge)
Added switch
Catalyst 3750 Switch Software Configuration Guide
shows how BackboneFast
Figure 19-8, BackboneFast is
19-9