Cisco 7604 Configuration Manual page 321
Ios software configuration guide
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Chapter 21
Configuring Optional STP Features
Understanding How BackboneFast Works
If the network device has alternate paths to the root bridge, it uses these alternate paths to transmit a new
kind of Protocol Data Unit (PDU) called the Root Link Query PDU. The network device sends the Root
Link Query PDU out all alternate paths to the root bridge. If the network device determines that it still
has an alternate path to the root, it causes the maximum aging time to expire on the ports on which it
received the inferior BPDU. If all the alternate paths to the root bridge indicate that the network device
has lost connectivity to the root bridge, the network device 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 network device 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 21-3
shows an example topology with no link failures. Switch A, the root bridge, connects
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 LAN interface on Switch C
that connects directly to Switch B is in the blocking state.
Figure 21-3 BackboneFast Example Before Indirect Link Failure
Switch A
Switch B
(Root)
L1
L2
L3
Blocked port
Switch C
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.
Figure 21-4
shows how BackboneFast reconfigures the topology to
account for the failure of link L1.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
21-5
OL-4266-08
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