STP applications

Unanswered Question
Jun 22nd, 2007

Just studying for the BCMSN and have questions regarding STP.

1) other than backward compatibility, why would you want to use STP when RSTP converges much faster. RSTP has been around long enough that I am suprised it is not a default.

2) The next issue is with portfast. When configuring portfast do you ever notice the little message that IOS gives regarding that it will only be in affect on access ports? With that being the case and assuming you do not lock your ports as access or trunk ports, if you were to plug in another switch that is configured to form a trunk, and it forms a trunk with a switch that has portfast enabled on the port that is now in trunking mode. Would portfast not be enabled now?

I have this problem too.
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dgoodridge Mon, 06/25/2007 - 03:19


1) You are correct, the only reasons to use STP as opposed to RSTP is because of backwards compatibility or when matching in-house skill sets.

2) If portfast is enabled on a port that hasn't been tied down to a specific function then it will still be active. However, if that port receives a BPDU then it will enter the error disabled state because it shouldn't receive STP info on an access port.

The recommended configuration is to hard-code ports into either an access or a trunk state.

I hope that helps.

jlhainy Mon, 06/25/2007 - 04:37

It does and I understand the recommendations. So you are saying that if I have a port that is configured as dynamic auto or desirable and it has portfast enabled, if the port dynamically configures to an access port then all is well. If it dynamically configures to a trunk then that port would go to the errdisable state instead of just disabling portfast? Either way, you are still loop free.

srue Mon, 06/25/2007 - 18:03

sidenote: switches that support PVST have that as the default spanning tree mode , not normal STP


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