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is configuring Port fast same with disabling STP?

hi mates

What happens when you connect another switch (which causes loop) to PORT FAST configured port?

Does STP work after some time?

or acts same way as we disabled STP and never prevents any loop?

thanks for helping :)

2 REPLIES

Re: is configuring Port fast same with disabling STP?

Hi,

Portfast is basically used at the edge ports and it transitions the port from blocking state to forwarding state of STP. It skips the learning and listening state of STP.

No using portfast is not the same as disabling STP on the port.Connecting a switch on the portfast port will result into a network loop and might bring your whole network down. It is not recommended to connect a switch or a bridge on a portfast port.

HTH, Please rate if it does.

-amit singh

Re: is configuring Port fast same with disabling STP?

Yes, STP should work "after some time". However it is rather dangerous to open a loop, even temporarily, in a layer 2 network.

-A bridging loop will basically take the whole bandwidth of the links involved. The switches are supposed to prioritize BPDUs, however, it may become harder to successfully transmit them in that scenario. Without BPDU transmission, the loop will not be broken.

-Also, some of the packets that are looping wire rate might also be forwarded to the CPU of the switches involved. This generally result in the switches not being able to run STP properly and, again, prevent them from stopping the loop.

-At last, temporary bridging loops can result in inconsistencies in the cam table that can lead to loss of connectivity for some time (worst case 5 minutes).

So basically, if everything goes fine, your network *should* go back to stability after a while if you introduce a temporary loop. However, you cannot guarantee when or if it's going to stabilize;-)

Failures can be catastrophic and hard to recover at layer 2. That's why there is much more pressure on the STP to not fail than on regular layer 3 protocols (and why many people consider in an unfair way that STP is not reliable).

Regards,

Francois

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