UplinkFast

UplinkFast
UplinkFast is used to minimize network downtime by ensuring that network loops do not occur
when the network topology changes. STP convergence time is very time-consuming, so network
loops can occur temporarily when the convergence is taking place. Additionally, some hosts will
not be available for communication during the convergence time because STP has disabled ports
on a switch during convergence. The key to both problems is decreased convergence time, which
UplinkFast was developed to provide.
UplinkFast enables a blocked port on a switch to begin forwarding frames immediately when
a link failure is detected on the root port. For the switch to change a port from blocking to forwarding
mode, UplinkFast must have direct knowledge of the link failure; otherwise a loop
might occur.
To utilize UplinkFast, several criteria must be met. First, UplinkFast must obviously be enabled
on the switch. The switch must have at least one blocked port, and the failure must be on the root
port. If the failure is not on a root port, UplinkFast ignores it and normal STP functions will occur.
When a link fault occurs on the primary root link, UplinkFast transitions the blocked port
to a forwarding state. UplinkFast changes the port without passing through the listening and
learning phases, which enables the switch to skip the normal convergence time and start forwarding
in about 3 to 4 seconds instead of the usual 50 seconds.
PortFast and BPDUs
Some switches support an addition to PortFast called BPDUGuard. There is never a guarantee that
someone won’t add a switch at their desk. Then, for redundancy, they also connect that switch to
the LAN drop at their neighbor’s desk. Now assume that you enable PortFast on that port. Congratulations,
you now have a spanning tree loop!
BPDUGuard is a feature that can be set on many switches that enable PortFast. It monitors for
BPDUs on that port. If a BPDU arrives, the switch shuts down the port, placing it in the errdisable
state, and generates a status message.
530 Chapter 16  Using Spanning Tree with VLANs
UplinkFast was designed to work with access layer switches, not core switches, because the
switch running UplinkFast must not be the root bridge.