BackboneFast addresses the situation where an indirect failure causes a
topology change and therefore a switch must find an alternative path through an
intermediate switch.
BackboneFast is
initiated when a root port or blocked port on a switch receives inferior BPDUs
from its designated bridge. Under normal spanning-tree rules, the switch
ignores inferior BPDUs for the configured maximum aging time, as specified by
the agingtime variable of the set spantree maxage
command.
Example: BackboneFast Operation
BackboneFast
operation is best illustrated by the failure of the link between the root and
the backup root bridge.
The
backup root bridge does not assume that the root bridge is still available. The
backup switch will immediately block all previously forwarding ports and then
transmit configuration BPDUs claiming root responsibility.
When the
access switch receives the BPDU of the backup root bridge, the access switch
views the BPDU as inferior because its own root port is still active, and the
last indication it has is that the backup root bridge is the designated root
bridge. If configured for BackboneFast, the access switch then transmits a
special root query message to explicitly determine if the root bridge is still
active. Upon receipt of a response to the root query message, the access switch
sends a BPDU using its known root bridge parameters to the backup root bridge
and cycles the port connected to the backup root bridge through the listening
and learning states.
This differs from standard 802.1D spanning tree
operation in that normally the blocked port does not process the received BPDUs
until the maxage interval has expired. By using the BackboneFast feature, the
network recovers from an indirect failure in two times the forward delay time,
which is 30 seconds by default, rather than max_age plus two times forward
delay time, which is 50 seconds by default.