Cisco WS-C6506 Software Manual page 225

Chapter 9 Configuring Spanning-Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of
PDU called the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines
that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it
received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch
has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on
which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root
bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves
them out of the blocking state (if they were in the blocking state) through the listening and learning states
and into the forwarding state.
Figure 9-3
to Switch B over link L1 and to Switch C over link L2. The port on Switch C that connects directly to
Switch B is in the blocking state.
Figure 9-3
Switch A
(Root)
If link L1 fails, Switch C detects this failure as an indirect failure, because 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 9-4
Switch A
(Root)
OL-8978-04
shows an example topology with no link failures. Switch A, the root switch, connects directly
BackboneFast Example Before Indirect Link Failure
L1
L2
Blocked port
Switch C
BackboneFast Example After Indirect Link Failure
L1
Link failure
L2
BackboneFast transitions port
through listening and learning
states to forwarding state
Switch C
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Switch B
L3
Switch B
L3
Understanding How BackboneFast Works
Figure 9-4 shows
9-5