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Redundant Links with STP

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BackboneFast

Sometimes a switch may receive from a designated switch a BPDU that iden-
tifies the root bridge and the designated bridge as the same switch. This
shouldn't happen, and the BPDU is then considered inferior.

BPDUs are considered inferior when a link from the designated switch has

lost its link to the root bridge. The designated switch transmits the BPDUs
with the information that it is now the root bridge as well as the designated
bridge. The receiving switch will ignore the inferior BPDU for the maxage time.

After receiving inferior BPDUs, the receiving switch will try to determine

if 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 infe-
rior 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 maximum aging time expires, which turns the switch into
the root switch.

If the switch finds an alternate path to the root bridge, it will use 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 basi-
cally can enable faster convergence in the event of a backbone link failure.

Configuring and Verifying BackboneFast

Configuring BackboneFast is pretty easy, but it sounds difficult, which is the
cool part about this command. You turn it on with the set spantree
backbonefast

command. Here is an example of this command being

enabled:

Todd5000> (enable) set spantree backbonefast

Usage: set spantree backbonefast <enable|disable>

Todd5000> (enable) set spantree backbonefast enable

Backbonefast enabled for all VLANs

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