What Does Spanning Tree PortFast Really Do?

As we know, STP generates TCNs every time a link changes (up/down) and changes the state of the port from blocking to forwarding. This state changing mechanism is a slow process which is not needed for edge ports where we don't plug switches. PortFast feature give solution to this problem..

The 2 things PortFast enabled interfaces do are

1. Go to forwarding state immediately

2. Does not generate TCNs

But remember, PortFast will not remove the interface from STP process. So you should not connect any hubs, bridges, switches etc. Only single hosts should be allowed to connect to those ports..

Let's take a 2960 access switch and see what is really happening when there is no PortFast and when there is PortFast..

The switch is not connected to any thing.

I have turned on debug spanning-tree events

When a PC is connected to Fa0/23 of the switch..

You can see it went through the STP states and became a Designated Forwarding port. This operation took 50 seconds by default..

Once it the Fa0/23 is disconnected, you will see the following output..
(click on the images to view in full size)

You can see a TC trap is generated..

Now I am configuring PortFast on the switch..
It can be done globally for all the access ports or for particular interfaces only. Affect is same..

SW-A(config)#spanning-tree portfast default

Now when I connect the PC to Fa0/23 it will say "jump to forwarding from blocking" and it will happen instantly..

Now you can see the port type is shown as P2p Edge instead of P2p..

When the link is removed it will only down the port without generating a TC..