[ubuntu] virtual machine in bridge mode

I created two KVM virtual machines VM1 and VM2 in bridge mode. The brctl and ifconfig info in the following shows the br0 and vnet0/1 interfaces.
But only vm2 respond to arp request but not vm1.
Any specific configuration for multiple virtual machines in bridge mode to work?

Thanks in advance.

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br0 Link encap:Ethernet HWaddr e0:db:55:93:76:40
inet addr:192.168.1.19 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::e2db:55ff:fe93:7640/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:858634 errors:0 dropped:0 overruns:0 frame:0
TX packets:384880 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1197115849 (1.1 GB) TX bytes:33889357 (33.8 MB)


eth0 Link encap:Ethernet HWaddr e0:db:55:93:76:40
inet6 addr: fe80::e2db:55ff:fe93:7640/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:866131 errors:0 dropped:0 overruns:0 frame:0
TX packets:384844 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1212256988 (1.2 GB) TX bytes:33965755 (33.9 MB)
Interrupt:41


lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:10165 errors:0 dropped:0 overruns:0 frame:0
TX packets:10165 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1131178 (1.1 MB) TX bytes:1131178 (1.1 MB)


vnet0 Link encap:Ethernet HWaddr fe:54:00:76:a7:2c
inet6 addr: fe80::fc54:ff:fe76:a72c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:1 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)


vnet1 Link encap:Ethernet HWaddr fe:54:00:9b:a1:97
inet6 addr: fe80::fc54:ff:fe9b:a197/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2040 errors:0 dropped:0 overruns:0 frame:0
TX packets:11642 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:172564 (172.5 KB) TX bytes:3927012 (3.9 MB)

# brctl
bridge name bridge id STP enabled interfaces
br-ex 0000.ca8db2a4c34a no qg-4437477c-a9
br-int 0000.be86bc44a140 no qr-e7d98d75-8c
tap1281d678-fd
br0 8000.e0db55937640 no eth0
vnet0
vnet1