Ethernet card bonding means combing multiple network cards and bonding them as a single interface. It is very important and useful over the network for high availability and fault tolerance for storage servers and KVM servers.
Let's see the start of boding.

We have to create a bond0 config file in network-scripts dir

[root@storage ~]# vi /etc/sysconfig/network-scripts/ifcfg-bond0

DEVICE=bond0
ONBOOT=yes
NM_CONTROLLED=no
BOOTPROTO=static
IPADDR=192.168.85.125
NETMASK=255.255.255.0
GATEWAY=192.168.85.254

We bond two nic's in this example.  First edit ifcfg-eth0

[root@storage ~]# vi /etc/sysconfig/network-scripts/ifcfg-eth0http://www.kernel.org/doc/Documentation/networking/bonding.txt

DEVICE=eth0
ONBOOT=yes
NM_CONTROLLED=no
MASTER=bond0
SLAVE=yes

edit ifcfg-eth1

[root@storage ~]# vi /etc/sysconfig/network-scripts/ifcfg-eth1

DEVICE=eth1
ONBOOT=yes
NM_CONTROLLED=no
MASTER=bond0
SLAVE=yes

Create /etc/modprobe.d/bonding.conf


[root@storage ~]# vi /etc/modprobe.d/bonding.conf

alias bond0 bonding
options bond0 mode=0 miimon=100

miimon is the time in sec's to monitor the bond and mode is the type of bond and purpose of the bond. Check the types of bond to under stand which suits you best. Usually 0 is used for fault tolerance in the network.