There was a strange problem at my costumers site where out of the sudden the Internet speeds were inconsistent. Let me clarify, the main Internet link was 50/50Mbps (provider optics) and speed in the network was sometimes 2/50Mbps and sometimes 45/8Mbps.
After checking every network device in the local network I have decided to check the ASA interfaces speed settings. Since the provider had a Cisco as their active terminal equipment and the costumer had also a Cisco ASA everything should be fine, right?
I have manually chancged the uplink interface (link between provider Cisco and my ASA). In my case it was the speed 100Mbps
For some reason they would not negotiate when interface speed was set to auto.
Also, when I have put a Cisco or MikroTik switch between the provider Cisco and my ASA, everything was also OK.
Just play around with it for a bit and everything will be ok.