Cisco 7609 Configuration Manual page 239

Chapter 16 Configuring Optional STP Features
Figure 16-3 BackboneFast Example Before Indirect Link Failure
Switch A
(Root)
If link L1 fails, 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 bridge over L1, it detects the failure and
elects itself the root and begins sending BPDUs to Switch C indicating itself as the root. When Switch C
receives the inferior BPDUs from Switch B, Switch C infers that an indirect failure has occurred. At that
point, BackboneFast allows the blocked port on Switch C to move immediately to the listening state
without waiting for the maximum aging time for the port to expire. BackboneFast then transitions the
Layer 2 LAN 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.
account for the failure of link L1.
Figure 16-4 BackboneFast Example After Indirect Link Failure
Switch A
(Root)
If a new network device is introduced into a shared-medium topology as shown in
BackboneFast is not activated because the inferior BPDUs did not come from the recognized designated
bridge (Switch B). The new network device begins sending inferior BPDUs that indicate that it is the
root bridge. However, the other network devices ignore these inferior BPDUs and the new network
device learns that Switch B is the designated bridge to Switch A, the root bridge.
78-14064-04
L1
L2
Blocked port
Switch C
Figure 16-4
L1
Link failure
L2
BackboneFast transitions port
through listening and learning
states to forwarding state
Switch C
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
Switch B
L3
shows how BackboneFast reconfigures the topology to
Switch B
L3
Understanding How BackboneFast Works
Figure 16-5,
16-5