Wired IoT sensor network with PoE?
I think I am using a setup like you wish at my home:


PoE Switch (for example Cisco SG300-28P) 
Arduino Ethernet's with PoE Module (https://www.arduino.cc/en/Main/ArduinoBoardEthernet) I bought one for every room ;-)
Use DHCP-Server on raspberry PI
Put your sensors on your Arduino ethernet board
I am also using MQTT as protokoll for example http://pubsubclient.knolleary.net/
Use MQTT broker Mosquitto on raspberry pi


For the case that you want to use your switch also for internet connectivity, define a vlan (search wikipedia) for your arduino sensor network.

Hope that helps.