I'm using the following to setup the Ethernet interface:

    #define CS 6
    Ethernet.init(CS);
    while(!Ethernet.begin(mac)) {
       Serial.println("Ethernet.begin failed");
    }
    if (Ethernet.hardwareStatus() == EthernetNoHardware)
    {
        Serial.println(
            "Ethernet shield was not found.  Sorry, can't run without "
            "hardware. :(");
        while (true)
        {
            delay(1); // do nothing, no point running without Ethernet hardware
        }
    }
    if (Ethernet.linkStatus() == LinkOFF)
    {
        Serial.println("Ethernet cable is not connected.");
    }

With POE enabled, the code get stucked at the fisrt loop.

Is there anything missing?