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Spanning tree portfast

Hello, If I have port configure as spanning tree portfast and I plugged another switch instead of computer what will happened can it create loop or shutdown the port?
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Reza Sharifi
Hall of Fame Expert Hall of Fame Expert
Hall of Fame Expert

hi,

You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops.  

 

link:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html

HTH

Hello horacio27,

You can use PortFast on access switch ports  or trunk ports that are connected to a single workstation, server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state.

You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops.

To Prevent loops, in network  the most secure implementation of PortFast is to enable it only on ports that connect end stations to switches. Because PortFast can be enabled on nontrunking ports connecting two switches, spanning tree loops can occur because BPDUs are still being transmitted and received on those ports.

PortFast with BPDU guard prevents loops by moving non trunking port to err-disable state.

 

 

paul driver
VIP Expert VIP Expert
VIP Expert
Hello Yes its a possibility as portfast puts the port directly into a stp forwarding state Thus telling the switch that the device attached is a end host so if you plug a hub/switch to that port and those devices also has multiple connections to other switches stp still isn negotiated and is continuously in a forwarding state thus a loop could occur Res Paul


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Paul
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