What is PortFast ?

PortFast is a cisco proprietary STP enhancement. It can be used with STP or RSTP. Let’s first discuss STP or Spanning Tree Protocol.┬áSpanning Tree Protocol is used to quickly prevent and eliminate bridging loops. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops.

The time Spanning Tree Protocol (STP) takes to transition ports over to the Forwarding state can cause problems such as booting into WDS or a workstation getting an IP address to login to a domain. PortFast can be used to speed up this time on a port.

Turning on PortFast or using the Spanning-tree portfast trunk command should never be used on switch interconnects because with switch interconnects you want STP to make sure there is no loop before forwarding.

A loop is not formed by enabling portfast on trunk ports. If you do have a loop and your switch interconnects have portfast enabled then the switch will not be able to detect the loop in time.

Enabling portfast on a port does not disable STP but it allows the port to forward immediately.


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