A standard Ethernet switch allows control of an RM-series mixer from the CS18AI but routing audio to and from the CS18AI’s audio inputs and outputs to the RM-series mixer requires an AVB-compatible switch.
For the most current list of compatible AVB switches please refer to the following article - http://support.presonus.com/hc/en-us/articles/210194293-Supported-AVB-network-switches
The CS18AI rear panel includes two TRS Line Input jacks. These jacks provide additional line inputs for sources from front-of-house, such as 2-track or computer playback. On a StudioLive RM-series mixer connected to the CS18AI via AVB, these two inputs show up as digital returns 33 and 34.
See sections 2.3.5 - 2.4.1 of the CS18 manual for more information in this regard.