SIOCSIFMEDIA and fxp0 and 'Device not configured'

[Richard P. Williamson]

O fount of knowledge,

I'm stuck. 

I've got
gndv# ifconfig fxp0
fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        inet 172.x.x.188 netmask 0xffff0000 broadcast 172.255.255.255
        inet6 fe80::230:64ff:fe01:86ff%fxp0 prefixlen 64 scopeid 0x1
        ether 00:30:64:01:86:ff
        media: Ethernet manual

(gndv is a 'generic networking device', a custom built embedded 
system based on a pentium III board)

I'm trying
gndv# ifconfig fxp0 media 100baseTX mediaopt full-duplex

I'm being told
ifconfig: SIOCSIFMEDIA: Device not configured

while dmesg says
gndv# dmesg | grep fxp0
fxp0: <Intel Pro 10/100B/100+ Ethernet> port 0xd400-0xd43f mem 0xe7200000-0xe72fffff,0xe7302000-0xe7302fff irq 11 at device 8.0 on pci0
fxp0: Ethernet address 00:30:64:01:86:ff, 10Mbps

Any of the media/opt types give the same response.  This is identical
for each of the five fxp devices on the board (three on-board, two 
additional on a PCI card).  Am I misinterpreting the comment
'Device not configured'?

Because the behavior is identical with all five ethernet devices
on the gndv, I'm assuming the problem is my kernel config (4.8R), 
with the rc.conf setup, or something equally generic to the entire 
system.  I have tried, however, running the same kernel on a 
straight PC clone (also 4.8R) with three network ports (rl0, fxp0 
and fxp1) and was able to ifconfig the media and mediaopts on the 
fxp devices without any trouble.

I'd welcome some suggestions about where to look for a solution
to this issue.

TIA,
rip