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BackboneFast is initiated 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 indicates 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 bridge). Under normal
Spanning-Tree rules, the switch ignores inferior BPDUs for the
configured maximum aging time.
The switch tries to determine if it
has an alternate path to the root bridge. 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 bridge. If the
inferior BPDU arrives on the root port, all blocked ports become
alternate paths to the root bridge. 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 bridge, causes the maximum
aging time on the root to expire, and becomes the root switch
according to normal Spanning-Tree rules.
If the switch has alternate paths to
the root bridge, the switch uses these alternate paths to transmit a
new kind of BPDU called the root link query protocol data unit (PDU).
The switch sends 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 the switch 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 timers on the ports on which the switch 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 the designated ports and moves these ports
out of the blocking state (if the ports were in the blocking state),
through the listening and learning states, and into the forwarding
state.
Use the following commands to configure and verify BackboneFast in the switch block. The switch
software also provides commands that allow configuring the
operation of BackboneFast. This feature enables faster convergence
in case of a backbone link failure.
Enable BackboneFast on a set
command-based switch by entering the following command:
Switch> (enable) set spantree
backbonefast
Verify that BackboneFast is enabled
by entering the following command:
Switch> (enable) show spantree
backbonefast
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Lab
Activity |
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In this lab
activity, you will learn how to use PortFast, UplinkFast, and
BackboneFast to improve the time it takes for the spanning-tree
algorithm to configure the mode of the port. |
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