cannot set mtu > 1500 on em0 optical Gig-E

[Jay Borkenhagen]

Hi,

For quite a while now it appears that mtu on my gigabit ethernet fiber
em0 cards cannot be configured higher than the default 1500 bytes.

Here's what I get when I attempt to set it lower and higher on a brand
new 8.2-RC3 box:

===
sandbox# uname -v
FreeBSD 8.2-RC3 #0: Sun Jan 30 06:52:51 UTC 2011     root at almeida.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC 
sandbox# 

sandbox# ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
     options=18<VLAN_MTU,VLAN_HWTAGGING>
     ether 00:d0:b7:7c:f8:01
     inet 10.1.2.3 netmask 0xffffff00 broadcast 10.1.2.255
     media: Ethernet autoselect
     status: no carrier
sandbox# ifconfig em0 mtu 1499
sandbox# ifconfig em0 | grep mtu
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1499
sandbox# ifconfig em0 mtu 1501
ifconfig: ioctl (set mtu): Invalid argument
sandbox# ifconfig em0 | grep mtu
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1499
sandbox# ifconfig em0 mtu 1500
sandbox# ifconfig em0 | grep mtu
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
sandbox# 

sandbox# pciconf -lv | sed -n '/em0@/,/em1/p'
em0 at pci0:0:2:0:	 class=0x020000 card=0x10008086 chip=0x10008086 rev=0x03 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82542 Gigabit Ethernet Controller'
    class      = network
    subclass   = ethernet
em1 at pci0:0:4:0:	 class=0x020000 card=0x10008086 chip=0x10008086 rev=0x03 hdr=0x00
sandbox# 

===

However, back in the 6.2 days it allowed it:

===

[jayb at cactus ~]$ uname -v
FreeBSD 6.2-PRERELEASE #0: Tue Dec 26 16:53:18 EST 2006     root at cactus.iplabs.att.com:/usr/obj/usr/src/sys/CACTUS 
[jayb at cactus ~]$ 

[jayb at cactus ~]$ ifconfig em0 
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
     options=8<VLAN_MTU>
     inet 10.0.1.152 netmask 0xfffffffc broadcast 10.0.1.155
     ether 00:90:27:e3:0a:67
     media: Ethernet autoselect
     status: no carrier
[jayb at cactus ~]$ 

[jayb at cactus ~]$ sudo ifconfig em0 mtu 9000
[jayb at cactus ~]$ ifconfig em0 | grep mtu
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 9000
[jayb at cactus ~]$ 

[jayb at cactus ~]$ pciconf -lv | sed -n '/em0@/,/none0@/p'
em0 at pci0:2:0:	 class=0x020000 card=0x10008086 chip=0x10008086 rev=0x02 hdr=0x00
    vendor   = 'Intel Corporation'
    device   = '82542 Gigabit Ethernet Controller'
    class    = network
    subclass = ethernet
none0 at pci0:4:0:	class=0x088000 card=0x0767101e chip=0x9063101e rev=0x04 hdr=0x00
[jayb at cactus ~]$ 

===

Can this be made to work again?

Thanks.

					Jay B.