Can't change MTU size

[Jay Cliburn]

What on earth am I doing wrong?

[root at osprey ~]# ifconfig eth0 mtu 1492
SIOCSIFMTU: Invalid argument
[root at osprey ~]# ifconfig eth0 inet mtu 1492
SIOCSIFMTU: Invalid argument

I tried stopping eth0 (ifconfig eth0 down), then executing the mtu size
change, but it didn't help.

I also tried adding MTU=1492
in /etc/sysconfig/network-scripts/ifcfg-eth0, but on network restart I
get the same SIOCSIFMTU error.

Background info:

Abit AV8 motherboard with Via Velocity 6122 Gbit onboard nic.

Autonegotiation is on.

>From dmesg:
VIA Networking Velocity Family Gigabit Ethernet Adapter Driver Ver. 1.13
Copyright (c) 2002, 2003 VIA Networking Technologies, Inc.
Copyright (c) 2004 Red Hat Inc.
GSI 17 sharing vector 0xB1 and IRQ 17
ACPI: PCI Interrupt 0000:00:0e.0[A] -> GSI 22 (level, low) -> IRQ 17
PCI: Via IRQ fixup for 0000:00:0e.0, from 10 to 1
eth0: VIA Networking Velocity Family Gigabit Ethernet Adapter
eth0: Ethernet Address: 00:50:8D:EF:90:69

[root at osprey ~]# uname -a;ifconfig eth0
Linux osprey 2.6.16-1.2111_FC5 #1 SMP Thu May 4 21:16:04 EDT 2006 x86_64
x86_64 x86_64 GNU/Linux
eth0      Link encap:Ethernet  HWaddr 00:50:8D:EF:90:69
          inet addr:192.168.1.3  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::250:8dff:feef:9069/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:6274 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6011 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:4922524 (4.6 MiB)  TX bytes:1218897 (1.1 MiB)
          Interrupt:17 Base address:0xd000