Why do you need to enable PortFast on access ports?

Brief Explanation

PortFast is a good feature to put switch interfaces in forwarding state immediately. Also, it helps reducing of STP re-calculations on switches and relatively providing network performance improvements.

Without PortFast

If you don’t use it under an interface configuration, when the link state changes to UP, switch firstly put interface in blocking state and starts sending BPDUs to other switches. It will take between 30-50 seconds putting an interface in forwarding state. Also, switch sends Topology Change Notifications to other switches. When the other switches got TCN packet, they erase their MAC tables and re-populate it.

Do We Need to Re-Calculate STP Topology for a Client?

Answer is simple, no. Because clients do not join to STP topology. For instance; A client never be a STP Root for a VLAN. Because of that, there is no need to run STP calculations again for access devices.

Where You Need To Use?

You need to enable it on your Access Layer switches’ client facing ports. Your client facing ports will be used by clients. If you don’t enable PortFast on these ports, when each client connects its computer to the network via ethernet cable, STP will be re-calculated for related VLAN on switches and client computer will wait until the switchport state changes to forwarding.

Also, you need to enable it for server access switches’ access ports. The reason behind of this is that if a server lose its connectivity to the network because of a link problem, when the link comes UP again, there is no reason to waiting a server until the completion of STP re-calculation process. Furthermore, server needs to be available for clients as soon as possible.

What About Trunk Ports?

If switchports connected to a virtual switch that doesn’t support STP, like ESXi, it is possible to enable PortFast on these interfaces with “portfast trunk” command. You shouldn’t use this feature on interfaces that are connected to a different physical switches or STP supported devices. Also, never disable STP on a switchport. If you disable STP BPDUs on a switchport, switch cannot detect and avoid a layer 2 traffic loop.

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