on my Pi model 2b I connected a ethernet cable and an edimax n300 wifi usb dongle. I want to use the Pi over WLAN so I attached the WLAN dongle to the Pi and configured them with the following additional configuration in /etc/network/interfaces:
auto wlan0 iface wlan0 inet dhcp wpa-ap-scan 1 wpa-scan-ssid 1 wpa-ssid "RouterName" wpa-psk "***************"
After restarting the networking all worked fine. The new IP address that the Pi received from the router is pingable. Also in the console of the router I see the Pi with the additional IP and MAC address. If I disconnect the ethernet channel I cannot ping the new IP of the WLAN interface. Also SSH is not possible. When I connect the ethernet cable again to the Pi it works fine (ping,SSH,...).
I tested both: dhcp and static IP
Are there some other config steps necessary? Did someone also had this problems?