BackboneFast

BackboneFast
Sometimes a switch might receive a BPDU from another switch that identifies the second switch
as the root bridge when a root bridge already exists. This shouldn’t happen, except when a new
switch comes online and the BPDU is considered “inferior.”
BPDUs are considered inferior when a switch has lost its link to the root bridge. The switch transmits
the BPDUs with the information that it is now the root bridge as well as the designated bridge.
The receiving switch ignores the inferior BPDU for the max age time, to prevent spanning tree loops.
After receiving inferior BPDUs, the receiving switch tries to determine whether there is an
alternate path to the root bridge. If the port that the inferior BPDUs are received on is already
in blocking mode, then the root port and other blocked ports on the switch become alternate
paths to the root bridge. However, if the inferior BPDUs are received on a root port, then all
presently blocking ports become the alternate paths to the root bridge. Also, if the inferior
BPDUs are received on a root port and there are no other blocking ports on the switch, the
receiving switch assumes that the link to the root bridge is down and the max age time expires,
which turns the switch into the root switch.
If the switch finds an alternate path to the root bridge, it uses this new alternate path. This new
path, and any other alternate paths, will be used to send a Root Link Query BPDU. By turning on
BackboneFast, the Root Link Query BPDUs are sent out as soon as an inferior BPDU is received.
This can enable faster convergence in the event of a backbone link failure. To ensure proper operation,
BackboneFast should be enabled on all switches, including the root, if it is enabled at all.