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disable portfast on switch-to-switch connection

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

2 REPLIES
Cisco Employee

Re: disable portfast on switch-to-switch connection

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.

Re: disable portfast on switch-to-switch connection

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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