STP backup fonctionnality

Unanswered Question
Jun 27th, 2008

Hello, I have a question concerning the functionnement of the spanning tree protocol. I want to use in order to have 2 redundant network links.

I have a fiber channel (1gb) which is the main link and a microwave link (100 Mb). These 2 links are connected using 2 Cisco commutators (3750G). I would like to play with the port-priority attribut to define which link is the main one.

In normal fonctionnement, the 2 switchports where will be connected the FC will be in FWD STP mode whereas the 2 where will be connected the microwave will be in BLK STP mode.

If there are some transmission problems with the secondary lin, will the primary link would be affected.

I hope this description is clear.

Thanks in advance for your helps.


I have this problem too.
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iptlearner Fri, 06/27/2008 - 02:29

In short, no.

If you have your secondary link as an STP Blocked link and there are problems with it, it should not interfere with your primary link.

Also, since you have two different link speeds, ie: 1gb (fiber) and 100mb (microwave), the priority command may not necessarily required.

In this case, the path costs are different for the two links. The 1gb link has a lower cost and should become the forwarding port.

Please see the following link for more Spanning Tree priority information:

Much more STP info at this link:

jmariette Fri, 06/27/2008 - 03:23

Thanks for your reply,

For the redundance, I think too that the secondary could not interfere with the primary one, but someone tell me the contrary...

For the priority command, effectively, STP is able to define a priority with the link speed, but I prefer to use this command, because it is a very crucial connection for my enterprise.

If someone has a different point of vue concerning the redundancy, i would be happy to see his opinion.

Thanks again !


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