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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Understanding Advanced STP Features
Understanding BackboneFast
BackboneFast is started 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 STP rules, the switch ignores inferior BPDUs for the configured maximum aging time
specified by the spanning-tree max-age global configuration command.
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 to expire, and becomes the root switch
according to normal STP rules.
If the switch has alternate paths to the root switch, it uses these alternate paths to send a new kind of
Protocol Data Unit (PDU) called the Root Link Query PDU. The switch sends the Root Link Query PDU
on all alternate paths to the root switch. 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 switch indicate that the switch has lost connectivity to the root switch,
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 switch, 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 10-10
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 10-10 BackboneFast Example Before Indirect Link Failure
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 switch over L1, it detects the failure, elects
itself the root, and begins sending BPDUs to Switch C, identifying itself as the root. When Switch C
receives the inferior BPDUs from Switch B, Switch C assumes 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 interface on Switch C to the forwarding state, providing a path from Switch B to Switch A. This
L1
L2
L3
Switch C
Switch A
(Root)
Switch B
Blocked port
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