This seems one of those situations where you want Wireshark to see what happens on the wire and see who is at fault.When you said "They just simply stop responding and then magically come back" , when it is on the broken state, does the switchport st...

If using HSRP, another consideration is to make sure you are running rapid-pvst on both devices and make sure the STP root matches with the HSRP active Switch for each Vlan. If SW#1 is the HSRP active for vlans 10,20,30 then make SW#1 the STP root fo...

"is this configuration designed in my schema is correct for performance and redundancy if one node of fw or c3850 is down"Yes, the way you have placed the Portchannels on your diagram will cover the redundancy part, however that depends on the Firewa...

Understood. No problem. Then on the C3850s do you have a C3850-NM-4-1G for the Uplink connections ? Or perhaps you are planning on using some of the 12 1G fiber ports on those couple of C3850s to connect that Switch Stack to the Firewalls? On both of...

Now that you have the sub-interface on the Router with the "dot1q 10 native" ,  make both trunks on the Switch (Fa0/1 and Fa0/2) to have the native Vlan also set to Vlan 10.