Cisco WS-C2950SX-48-SI Configuration Manual page 161

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
Figure 8-3
Switch A
(Root)
If link L1 fails, Switch C detects this failure as an indirect failure, since it is not connected directly to
link L1. Switch B no longer has a path to the root switch. 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 port on Switch C to the forwarding state, providing a
path from Switch B to Switch A. This switchover takes approximately 30 seconds.
how BackboneFast reconfigures the topology to account for the failure of link L1.
Figure 8-4
Switch A
(Root)
If a new switch is introduced into a shared-medium topology, BackboneFast is not activated.
shows a shared-medium topology in which a new switch is added. The new switch begins sending
inferior BPDUs, which indicate that it is the root switch. However, the other switches ignore these
inferior BPDUs and the new switch learns that Switch B is the designated bridge to Switch A, the root
switch.
78-15486-01
Example of BackboneFast before Indirect Link Failure
L1
L2
Blocked port
Switch C
Example of BackboneFast after Indirect Link Failure
L1
Link failure
L2
Switch C
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
Switch B
L3
Switch B
L3
BackboneFast transitions port
through listening and learning
states to forwarding state
Understanding How BackboneFast Works
Figure 8-4
Figure 8-5
—
Release 8.1
shows
8-5