Gigabit Ethernet adapter problem

[Ernie Luzar]

em - Intel(R) PRO/1000 Gigabit Ethernet adapter driver

The boot messages show the Gigabit Ethernet adapter is correctly 
identified at boot time as shown below.

em0: <Intel(R) PRO/1000 Network Connection 7.6.1-k> port 0xf080-0xf09f
      mem 0xf7d00000-0xf7d1ffff,0xf7d3c000-0xf7d3cfff irq 20 at
      device 25.0 on pci0
em0: Using an MSI interrupt
em0: Ethernet address: d0:50:99:93:75:98
em0: netmap queues/slots: TX 1/1024, RX 1/1024


But after the system is up and running ifconfig em0 shows

em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST>
         metric 0 mtu 1500
	options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,
         TSO4,VLAN_HWTSO>
	ether d0:50:99:93:75:98
	hwaddr d0:50:99:93:75:98
	inet 10.0.10.2 netmask 0xff000000 broadcast 10.255.255.255
	nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
	media: Ethernet autoselect (100baseTX <full-duplex>)
	status: active

and we see that (100baseTX <full-duplex>) has been selected as default 
and it should be (1000baseTX <full-duplex>).

How do I force the Ethernet adapter into Gigabit mode?