FreeBSD 10.1-REL - network unaccessible after high traffic

[Cs]

Hi all,

I have two FreeBSd 10.1-RELEASE servers connected to each other. They 
were connected via cross link, but they are connected to a cisco switch 
now. When transferring huge files (50-500GB backup files) via Gigabit 
(it is important!) the network randomly dies. The backup runs every 
day/week and sometimes the connection is ok for months sometimes it 
happens twice a week. When the network dies I can log in to the server 
via IPMI and use the console everything is OK, but can't send anything 
out on the network. ifconfig em0 down/up doesn't help nor netif restart. 
The problem never occured when I used 100Mbit connection between them, 
but it was 3com NIC (xl), gigabit adapter is Intel (em0). When I limit 
the transfer rate (rsync bandwith limit or ipfw pipe) the problem is 
much more rare.

I tried to set these tuning parameters on both servers with different 
buffer size but nothing helped:

# cat /etc/sysctl.conf
security.bsd.see_other_uids=0
net.inet.tcp.recvspace=512000
net.route.netisr_maxqlen=2048
kern.ipc.nmbclusters=1310720
net.inet.tcp.sendbuf_max=16777216
net.inet.tcp.recvbuf_max=16777216
kern.ipc.soacceptqueue=32768

# cat /boot/loader.conf
geom_mirror_load="YES" # RAID1 disk driver (see gmirror(8))
ipfw_load="YES"
net.inet.ip.fw.default_to_accept=1
kern.maxusers=4096
accf_data_load="YES"

Any ideas? Thanks guys!