Hangs down/up Intel NIC during creating vlan. Bug em driver ???

[Prokofiev S.P.]

Hi ALL !
I have several servers with FreeBSD 8.0-PRERELEASE on SuperMicro with 
different Intel NICs. When I create new vlan on em interface
( ifconfig vlan557 create vlandev em0 vlan 557),
it hangs down and then up and server becomes inaccessible by ssh, but 
reply on icmp request.

on amd64, custom kernel:

em0 at pci0:13:0:0:        class=0x020000 card=0x108c15d9 chip=0x108c8086 
rev=0x03 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Intel Corporation 82573E Gigabit Ethernet Controller 
(Copper) (82573E)'
    class      = network
    subclass   = ethernet
em1 at pci0:14:0:0:        class=0x020000 card=0x109a15d9 chip=0x109a8086 
rev=0x00 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Intel PRO/1000 PL Network Adaptor (82573L)'
    class      = network
    subclass   = ethernet

on i386, GENERIC kernel:

em0 at pci0:2:1:0: class=0x020000 card=0x11798086 chip=0x10798086 rev=0x03 
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Dual Port Gigabit Ethernet Controller (82546EB)'
    class      = network
    subclass   = ethernet
em1 at pci0:2:1:1: class=0x020000 card=0x11798086 chip=0x10798086 rev=0x03 
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Dual Port Gigabit Ethernet Controller (82546EB)'
    class      = network
    subclass   = ethernet

/var/log/messages:

Nov 23 11:35:49 freebsd kernel: em1: link state changed to DOWN
Nov 23 11:35:49 freebsd kernel: vlan557: link state changed to DOWN
Nov 23 11:35:52 freebsd kernel: em1: link state changed to UP
Nov 23 11:35:52 freebsd kernel: vlan557: link state changed to UP
Nov 23 11:36:09 freebsd kernel: em1: link state changed to DOWN
Nov 23 11:36:13 freebsd kernel: em1: link state changed to UP