To connect the CS1 (60212) with CS2 (60213/60214/60215) the CS1 (60212) must have the Hardware-Update, which Marklin built into the CS1 for the Customer's back in about 2006-2007.
The CS1 must also have Software-version 2.0.3 or higher (After the Hardware-Update supplied by Marklin the CS1 was returned with Software-version 2.0.3, when i got mine back again)
If the CS1 have label on the bottom side which show "v2.0" the CS1 has been Updated with the Hardware by Marklin
You dont need the Marklin 60121 Unit.
The connection between CS1 and CS2 can be build with "straight" Network-cables.
On Marklin's Website :
Software und Service zu Märklin Systemsyou will find the Software-version 2.0.4 for the CS 1 (60212), which is the Last Software-Update Marklin made for the CS 1 (60212)
Connect your CS1 and CS2 with network-cables as shown in "Eksempel 2" or "Eksempel 3" (use "straight" network-cables).
(Netgear Router is only used as an example here, you can use your own Router.)

Setup the IP-Address for the CS2 (use DHCP)
Setup the IP-Address for the CS1 (use DHCP, if any problems with DHCP set it manually)
When you are finished with connecting the CS1 and CS2 with Network-cables, you should Install the Software-Update Version 2.0.4 for CS 1
with the Help of the Built-in Web-Interface for the CS 1.
Enter this line in your Browser (Internet Explorer / Firefox or whatever browser you are using :
http://xxx.xxx.xxx.xxx (where xxx.xxx.xxx.xxx is replaced with the IP-Address that was assigned to the CS1) and press enter
More help can be found in the Userguide for the CS1.
Please also remember that the CS 2 must be updated to minimum Software-version 1.3.0 or higher for this to work.
(the latest Update for the CS2 is Software-version :
2.0.1) which you can download from the Marklin Website or
perform an Update straight from your CS2 when it is connected to the Internet over your Router.
Also Remember that if you choose to install the ESU Software-Update 3.0.0 on your CS1 you "cant" make a Network-connection with the CS2
after the ESU-Update has been installed on your CS1.
If you need more help, just ask.