PortFast & network loop creation

[codone76]

Hi to all!

because if I connect a layer 2 switch to a trunk port with portfast active, can I create a loop?
In other posts I read that the loop cmq could be a few seconds.

thanks to all in advance

[Giuseppe Larosa]

Hello @codone76 ,

>> because if I connect a layer 2 switch to a trunk port with portfast active, can I create a loop?

The problem arises when you connect  a second link between the two switches. If you have portfast on both links the bridging loop can form. At that point if broadcast frames are present in the network they get multiplied and you have a broadcast storm that can fill all the available bandwidth. The network might be able to recover by itself or not and this why you should not use portfast edge between switches.

Connections between switches should not use portfast / edge feature for the potential issues that are described above.

Hope to help

Giuseppe

[codone76]

Hi Giuseppe and thanks for the explanation.

loop creation is allowed because port in portfast cannot enter in listening mode & not partecipate to STP process !?

[Giuseppe Larosa · Accepted solution]

Hello @codone76 ,

>> loop creation is allowed because port in portfast cannot enter in listening mode & not partecipate to STP process !?

yes, your understanding is correct user traffic starts to flow before STP takes decisions on what to do on both sides of the link and this exposes the network to the risk of bridging loops.

Modern STP  Rapid STP 802.1w use the handshake between the two switches , but the idea is still the same STP has to act first and only later user traffic may be allowed to flow on the link.

Hope to help

Giuseppe