Portfast
The portfast feature skips through various stages of
spanning-tree states, and it immediately brings a port from blocking to
forwarding state. There are two reasons behind portfast. First, the delay
incurred through normal spanning-tree states could cause startup problems. For
example, a host machine's user login screen times out because the port is still
transitioning through its spanning-tree states. It takes 30 seconds for a port
to transition to forwarding state. While the port is transitioning, the Windows
software is attempting to log on to the server and subsequently will fail
because the host machine does not have full network connectivity. This problem
was ubiquitous with users that had Novell Clients.
The second reason behind portfast is that Topology Change
Notifications (TCNs) are not generated when a host machine joins or leaves a
port. This reason is significant. If portfast is disabled on a host port,
anytime a user does a restart or a shutdown on his machine, a TCN is generated
by the switch and forwarded on the bridge Root Port. (See Figure 10-1.)
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