unable to set mtu 9000 on Intel x520-t2 adapter

Hello,

We are testing FreeBSD 9.0-RELEASE on a pair of machines with 10GBASE-T 
Intel x520-t2 adapters interconnected with a straight-through cat6 rj45 
cable. With mtu 1500, we are achieving aproximately 4 gigabits/s both 
directions as demonstrated with iperf. We wanted to test with mtu 9000 
to see whether there would be any difference, but we encountered a 
problem setting it. We ran the following commands to alter the MTU to 
9000 on the two machines (fw3 and fw4):

fw3# ifconfig ix1 192.168.1.100 mtu 9000
fw4# ifconfig ix1 192.168.1.200 mtu 9000

However, instead of getting mtu set to 9000, what we see is the 
following message in the kernel output of both machines:

ix1: Could not setup receive structures
ix1: Could not setup receive structures

and no traffic getting through. The moment we run:

fw3# ifconfig ix1 192.168.1.100 mtu 1500
fw4# ifconfig ix1 192.168.1.200 mtu 1500

we see:

ix1: link state changed to DOWN
ix1: link state changed to UP

and traffic starts getting through as before. More information about our 
hardware is:

Motherboard: Supermicro H8DGU-F
CPU:         AMD 6128 8-core
Memory:      16GB DDR3
NICs:        Intel x520-t2 10GbaseT adapters

An ifconfig ix1 output for one of the machines:

fw4# ifconfig ix1
ix1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500

options=401bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
         ether 00:1b:21:d6:19:51
         inet 192.168.1.200 netmask 0xffffff00 broadcast 192.168.1.255
         nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
         media: Ethernet autoselect (autoselect <full-duplex>)
         status: active

Many thanks for any help/action on this!

Sincerely,

Iordan Iordanov
CDF System Administrator
University of Toronto
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