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13-10
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 13 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Understanding BackboneFast
BackboneFast detects indirect failures in the core of the backbone. BackboneFast is a complementary
technology to the UplinkFast feature, which responds to failures on links directly connected to access
switches. BackboneFast optimizes the maximum-age timer, which determines the amount of time the
switch stores protocol information received on an interface. When a switch receives an inferior BPDU
from the designated port of another switch, the BPDU is a signal that the other switch might have lost
its path to the root, and BackboneFast tries to find an alternate path to the root.
BackboneFast, which is enabled by using the spanning-tree backbonefast global configuration
command, starts when a root port or blocked port on a switch receives inferior BPDUs from its
designated bridge. An inferior BPDU identifies one switch as both the root bridge and the designated
bridge. When a switch receives an inferior BPDU, it means that a link to which the switch is not directly
connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root
switch). Under spanning-tree rules, the switch ignores inferior BPDUs for the configured maximum
aging time specified by the spanning-tree max-age global configuration command. The BackboneFast
feature is supported only when the switch is running PVST.
The switch tries to determine if it has an alternate path to the root switch. If the inferior BPDU arrives
on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root
switch. (Self-looped ports are not considered alternate paths to the root switch.) If the inferior BPDU
arrives on the root port, all blocked ports become alternate paths to the root switch. If the inferior BPDU
arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity
to the root switch, causes the maximum aging time on the root port to expire, and becomes the root
switch according to normal spanning-tree rules.
If the switch has alternate paths to the root switch, it uses these alternate paths to send a root link query
(RLQ) request. The switch sends the RLQ request on all alternate paths to the root switch and waits for
an RLQ reply from other switches in the network. If the switch determines that it still has an alternate
path to the root, it expires the maximum aging time on the port that received the inferior BPDU. If all
the alternate paths to the root switch indicate that the switch has lost connectivity to the root switch, the
switch expires the maximum aging time on the port that received the RLQ reply . If one or more alternate
paths can still connect to the root switch, the switch makes all ports on which it received an inferior
BPDU its designated ports and moves them from the blocking state (if they were in the blocking state),
through the listening and learning states, and into the forwarding state.
Figure 13-7 shows an example topology with no link failures. Switch A, the root switch, connects
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 interface on Switch C that
connects directly to Switch B is in the blocking state.
Figure 13-7 BackboneFast Example Before Indirect Link Failure
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Blocked port
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