No response time on the network after doing a loop back

​ On my network I have 12 PC. ​

​ I have a central point with a PC5324 and several link with 53 24 or 33 24 or 3348. ​

​ Each link are independent like a star. ​

​ I configure the switches with RSTP and be carreful about the fastlink (no fastlink on an port if STP enable) ​

​ When I want to join two independents links to do a backup route, the response time on all the networtk down. ​

​ If I loop back a switch on itself, there is no problem. ​

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​ Someone have a idea about this trouble ? ​

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​ Thanks ​

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