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20-9
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 20 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
If link L1 fails as shown in Figure 20-7, Switch C cannot detect this failure because it is not connected
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. Figure 20-7 shows
how BackboneFast reconfigures the topology to account for the failure of link L1.
Figure 20-7 BackboneFast Example After Indirect Link Failure
If a new switch is introduced into a shared-medium topology as shown in Figure 20-8, 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 20-8 Adding a Switch in a Shared-Medium Topology
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Link failure
44964
BackboneFast changes port
through listening and learning
states to forwarding state.
Switch A
(Root)
Switch C
Switch B
(Designated bridge)
Added switch
44965
Blocked port