How to setup Centos 6.8 network on Proxmox KVM

Edit the file /etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
BOOTPROTO=none
ONBOOT=yes
USERCTL=no
IPV6INIT=no
PEERDNS=yes
TYPE=Ethernet
NETMASK=255.255.255.255
IPADDR=IP-FAILOVER
GATEWAY=GATEWAY-IP
DNS1=8.8.8.8
DNS2=8.8.4.4
ARP=yes
HWADDR=VIRTUAL-MAC

Edit the file or create /etc/sysconfig/network-scripts/route-eth0

GATEWAY-IP dev eth0 
default via GATEWAY-IP dev eth0

service network restart

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Source https://documentation.online.net/en/dedicated-server/network/ip-failover/virtual-machine