libvirt users - Sep 2013 - windows guest network kept down automatically when several windows guest running in one KVM host,

Hi all:
I have some kvm host(rhel 6.4, 2.6.32-358.0.1.el6.x86_64), I ran several windows
guest on it(more than 10 guest on one host and the guest os are
win7-32/win7-64/win2k8), but the guest network kept down automatically, lost
package. I tried virtio drive and e1000 drive,but it didn't work. However,
when I run cmd.exe ping some other subnet ip it worked.
The host and guest are connected by bridge.
Here is some configure infomation of my guest:
    <interface type='bridge'>
      <mac address='aa:aa:aa:49:39:71'/>
      <source bridge='kvmbr3'/>
      <model type='e1000'/>
      <address type='pci' domain='0x0000' bus='0x00'
slot='0x05' function='0x0'/>
    </interface>
brctl show as follow:
kvmbr28000.b8ca3a5cbea2noeth2
vnet0
vnet11
vnet13
vnet15
vnet17
vnet19
vnet2
vnet21
vnet23
vnet25
vnet27
vnet29
vnet5
vnet7
vnet9
The /etc/sysctl.conf configure as follow:
# Kernel sysctl configuration file for Red Hat Linux
#
# For binary values, 0 is disabled, 1 is enabled.  See sysctl(8) and
# sysctl.conf(5) for more details.


# Controls IP packet forwarding
net.ipv4.ip_forward = 0
#net.ipv4.ip_forward = 1


# Controls source route verification
net.ipv4.conf.default.rp_filter = 1


# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0


# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0


# Controls whether core dumps will append the PID to the core filename.
# Useful for debugging multi-threaded applications.
kernel.core_uses_pid = 1


# Controls the use of TCP syncookies
net.ipv4.tcp_syncookies = 1


# Disable netfilter on bridges.


# Controls the default maxmimum size of a mesage queue
kernel.msgmnb = 65536


# Controls the maximum size of a message, in bytes
kernel.msgmax = 65536


# Controls the maximum shared segment size, in bytes
kernel.shmmax = 68719476736


# Controls the maximum number of shared memory segments, in pages
kernel.shmall = 4294967296
########### add by operation V1.0 begin ############
net.ipv4.ip_local_port_range = 32768 65000
net.core.rmem_max = 8388608
net.core.wmem_max = 8388608
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_wmem = 4096 65536 8388608
net.ipv4.tcp_max_syn_backlog = 8192
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_sack = 0
net.ipv4.tcp_timestamps = 0
kernel.panic = 5
vm.swappiness = 51
########### add by operation V1.0 end ############


net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0




#add by wdh
#net.ipv4.conf.all.proxy_arp = 1


Best Regards,
cochen