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I'm running KVM on a centOS 7 server and it has been working fine, but occasionally the network connections to the virtual machines seams to drop and the virtual machines can't be reached until the main server is rebooted. I have 2 network connections running into the server. 1 is the primary ethernet on the motherboard while the other is a secondary card ethernet connection. I configured the 2nd connection to be used for a bridge network using these instruction

http://www.server-world.info/en/note?os=CentOS_7&p=kvm&f=1

Any ideas or suggestions would be appreciated.

Update:

Network interfaces

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: em1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT qlen 1000
link/ether d4:ae:52:d3:46:5a brd ff:ff:ff:ff:ff:ff
3: p4p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UP mode DEFAULT qlen 1000
link/ether 68:05:ca:1b:39:0a brd ff:ff:ff:ff:ff:ff
4: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT 
link/ether 68:05:ca:1b:39:0a brd ff:ff:ff:ff:ff:ff
5: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT 
link/ether 52:54:00:73:66:ae brd ff:ff:ff:ff:ff:ff
6: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast master virbr0 state DOWN mode DEFAULT qlen 500
link/ether 52:54:00:73:66:ae brd ff:ff:ff:ff:ff:ff
7: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN mode DEFAULT qlen 500
link/ether fe:54:00:8c:52:72 brd ff:ff:ff:ff:ff:ff
8: vnet1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN mode DEFAULT qlen 500
link/ether fe:54:00:09:d3:6b brd ff:ff:ff:ff:ff:ff
9: vnet2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN mode DEFAULT qlen 500
link/ether fe:54:00:48:f6:01 brd ff:ff:ff:ff:ff:ff
10: vnet3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN mode DEFAULT qlen 500
link/ether fe:54:00:4f:32:b9 brd ff:ff:ff:ff:ff:ff
17: vnet4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN mode DEFAULT qlen 500
link/ether fe:54:00:a8:8e:bf brd ff:ff:ff:ff:ff:ff

This is what /etc/sysconfig/network-scripts/ifcfg-br0 looks like

DEVICE=br0
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes

/em1 looks like

HWADDR="D4:AE:52:D3:46:5A"
TYPE="Ethernet"
BOOTPROTO="dhcp"
DEFROUTE="yes"
PEERDNS="yes"
PEERROUTES="yes"
IPV4_FAILURE_FATAL="no"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_PEERDNS="yes"
IPV6_PEERROUTES="yes"
IPV6_FAILURE_FATAL="no"
NAME="em1"
UUID="75be4f11-11ad-4ce2-a0d3-1601dfd2a7a6"
ONBOOT="yes"

secondary ethernet card p4p1

HWADDR=68:05:CA:1B:39:0A
TYPE=Ethernet
BOOTPROTO=dhcp
DEFROUTE=yes
PEERDNS=yes
PEERROUTES=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_PEERDNS=yes
IPV6_PEERROUTES=yes
IPV6_FAILURE_FATAL=no
NAME=p4p1
UUID=2c82ff3a-04c5-4531-940b-48533a2edbf5
ONBOOT=yes
BRIDGE=br0
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You may have a MAC address issue. The bridge will always take the highest MAC address of any device on it, and the nics on the VMs have lower mac addresses. So any time a VM goes on or offline, the bridge mac address will change, causing network traffic to pause. If the mac address of eth0 is the cause of your problem, workaround would be to use the stock NATed virbr0 for your VMs instead, as it won't be bridged with eth0. But that would be somewhat counterintuitive...

What does your /etc/network/interfaces look like?

If that MAC address isn't your issue, the issue might be a very old bug within KVM; you can find more details about it below:

https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/997978

  • I added some more detail. – buhogastip May 28 '15 at 19:38

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