Specifications

CHAPTER 3
Spanning Tree
UplinkFast
UplinkFast is for speeding convergence when a direct link to an upstream switch fails. The switch identifies backup ports
for the root port. (These are called an uplink group.) If the root port fails, one of the ports in the uplink group is
unblocked and transitions immediately to forwarding; it bypasses the listening and learning stages. It should be used in
wiring closet switches with at least one blocked port.
The command to enable uplinkfast is shown next. Please note that uplinkfast is enabled globally, so the command affects
all ports and all VLANs.
(config)# spanning-tree uplinkfast
BackboneFast
BackboneFast is used for speeding convergence when a link fails that is not directly connected to the switch. It helps the
switch detect indirect failures. If a switch running BackboneFast receives an inferior BPDU from its designated bridge, it
knows a link on the path to the root has failed. (An inferior BPDU is one that lists the same switch for the root bridge and
designated bridge.)
The switch then tries to find an alternate path to the root by sending a Root Link Query (RLQ) frame out all alternate
ports. The root then responds with an RLQ response, and the port receiving this response can transition to forwarding.
Alternate ports are determined in this way:
n If the inferior BPDU was received on a blocked port, the root port and any other blocked ports are considered alternates.
n If the inferior BPDU was received on the root port, all blocked ports are considered alternates.
n If the inferior BPDU was received on the root port and there are no blocked ports, the switch assumes it has lost
connectivity with the root and advertises itself as root.
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue