Disable portfast on switch-to-switch connection

Question

Hi,
From http://www.cisco.com/en/US/customer/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml, it says, "do not use portfast when you have switch-to-switch connection. In this case, the command can result in a loop."
For e.g. when someone connects a switch port(with portfast enabled) to another switch port(with portfast enabled)
1) can this scenario cause a loop even though the connection between these switch is only a single link?
2) If it does not cause a loop, what will happen? STP recalculation which causes a short network outage?
Thanks.
Christina

Answer 1

Hi Christina,
1) Only if there is a redundant physical path between the switches, such as through another switch. If the only physical path between the two switches is the switch-to-switch link in question, then no loop can occur.
2) Portfast simply ignores the standard STP state transitions and immediately enters the forwarding state, so assuming no loop, a switch-to-switch link that has portfast configured will skip the 50 second STP convergence time and immediately forward traffic.
HTH,
Bobby
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