04-27-2006 05:04 PM - edited 03-03-2019 02:59 AM
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
04-27-2006 05:07 PM
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
*Please rate helpful posts.
04-27-2006 06:23 PM
hello,
as stated you cant have a loop with a single connection between 2 switches.
STP wont recalculate, as port fast is configured to prevent this calculation.
When you enable PortFast on a port, the port is immediately and permanently transitioned to the spanning-tree forwarding state to FWD and stays there.
This is why usually you configured BPDU guard , to prevent loops that STP wont catch.
Hope this help,
if it does I'd appreciate if you'd rate this post.
Vlad
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