Good morning Marijus,
after my night shift yesterday, I would like to share the results of the investigations with you and the community.
But first of all, thank you very much for your patient support and all the information you gave me. I was able to learn a lot from the RUT955 and some details of network technology. And to clear up any doubts, the RUT955 works correctly!
My thanks also go to the gentleman from GNS3NETWORK, who shared a great video about the DORA process at “DORA Process in DHCP - Explained in detail - GNS3 Network”. He mentioned this abbreviated DORA process and its background; that’s when something “clicked” for me.
But now to the result.
Not only the DHCP server in the RUT955 maintains a DHCP lease time, but also the DHCP client, in this case in my mobile phone. This means that as long as my mobile phone was within the DHCP lease time, it always tried to connect to the DHCP server using a shortened DORA process, starting with a DORA Request with the stored IP address instead of a DORA Discover. And this IP address was from an “old” connection before I configured the IP-MAC binding. The other devices that I had configured with IP-MAC binding all came “from scratch” and therefore went through a complete DORA process starting with DORA Discover.
If you switch off the mobile phone or any other device once after configuration and start it again, it then goes through the complete DORA process and is correctly assigned the IP address configured in the RUT955.
For many of you this will be self-evident, but I didn’t want to withhold the information from everyone else.
We can consider the issue resolved; I’m therefore marking this answer as “solution”.
Thanks again to Marijus and best regards
Guenther