STP disable vs portfast

[wingu]

Hello, All.

What's the difference from STP disabling and portfast?

I know portfast didn't meant STP disable, just but change state to forwarding immediately.

But portfast can be a cause of network loop.

I think this is same to STP disable(for edge port).

What's the difference?

[milan.kulik]

Hi,

portfast could cause some L2 loops in theory (especially when used on trunks).

But the loops would disappear when the first STP BPDUs are received/sent to the neighboring switches and correct MAC addresses learnt from the LAN.

Disabling STP could create L2 loops (if someone by a mistake interconnects two switches by ports with STP disabled, e.g.) which would stay until fixed manually by shutting some ports down by the LAN admin.

BR,

Milan

[Akhileshpk_2]

Hi Wingu,

STP will disable the port to avoid the looping at Layer 2. This disabled port technically called as Blocked state. In case any one of the other links in a looped Switch goes down (Link connecting Switch to Switch), the Blocked port will go to the Forwarding state.

In a normal state, if you connect a PC to a Switch port, it takes around 30Sec (Listening State and Learning State - 15Sec + 15 Sec) to come to Forwarding state (Till the first 30Sec the LED status will be Amber). This is to make sure that, there is no looping because of the new connection.

Loooping will come only when you connect a Switch to another Switch and will not come if you connect a PC to a switch port. So if you are very sure that, the port on the switch will be used only for connecting PCs (No Switch is going to connect on this port), you can enable the Portfast on the port. If you enable portfast on a port and connect a PC, it immediatly goes to Forwarding state without waiting for 30Sec and your link will come up immediatly (Goes to Green Immediatly).

Hope this will clarify your doubts !!!

Regards,

Akhil