This Blog entry is the extension  of Linux Network Bonding or Trunking on CentOS 5.x. RHEL derivatives have 7 modes (0-6) of possible bonding modes. One of the best information can be found at Linux Channel Bonding Project . Much information are taken from there.

1. Mode 0: Balance Round-Robin (balance-rr)
   Round-robin policy: Transmit packets in sequential order from the first available slave through the
   last.  This mode provides load balancing and fault tolerance.
2. Mode 1: Active-Backup
   Active-backup policy: Only one slave in the bond is active.  A different slave becomes active if, and only if, the active slave fails.  The bond’s MAC address is externally visible on only one port (network adapter) to avoid confusing the switch.In bonding version 2.6.2 or later, when a failover occurs in active-backup mode, bonding will issue one or more gratuitous ARPs on the newly active slave. One gratuitous ARP is issued for the bonding master interface and each VLAN interfaces configured above
   it, provided that the interface has at least one IP address configured.  Gratuitous ARPs issued for VLAN interfaces are tagged with the appropriate VLAN id.This mode provides fault tolerance.  The primary option, documented below, affects the behavior of this mode.
3. Mode 2: Balance-xor
   XOR policy: Transmit based on the selected transmit hash policy.  The default policy is a simple [(source MAC address XOR’d with destination MAC address) modulo
   slave count].  Alternate transmit policies may be selected via the xmit_hash_policy option, described below.This mode provides load balancing and fault tolerance.
4. Mode 3: Broadcast
   Broadcast policy: transmits everything on all slave interfaces.  This mode provides fault tolerance.
5. Mode 4: 802.3ad (Dynamic link aggregation)
   IEEE 802.3ad Dynamic link aggregation.  Creates aggregation groups that share the same speed and duplex settings.  Utilizes all slaves in the active aggregator according to the 802.3ad specification.Slave selection for outgoing traffic is done according to the transmit hash policy, which may be changed from the default simple XOR policy via the mit_hash_policy option, documented below.  Note that not all transmit policies may be 802.3ad compliant, particularly in regards to the packet mis-ordering requirements of section 43.2.4 of the 802.3ad standard.  Differing peer implementations will have varying tolerances for noncompliance.Prerequisites:
   1. Ethtool support in the base drivers for retrieving the speed and duplex of each slave.
   2. A switch that supports IEEE 802.3ad Dynamic link aggregation. Most switches will require some type of configuration to enable 802.3ad mode.
6. Mode 5 – Adaptive transmit load balancing (balance-tlb)
   Adaptive transmit load balancing: channel bonding that does not require any special switch support.  The outgoing traffic is distributed according to the current load (computed relative to the speed) on each slave.  Incoming traffic is received by the current slave.  If the receiving slave fails, another slave takes over the MAC address of the failed receiving slave.Prerequisite:
   Ethtool support in the base drivers for retrieving the speed of each slave.
7. Mode 6: Adaptive load balancing (balance-alb)
   Adaptive load balancing: includes balance-tlb plus receive load balancing (rlb) for IPV4 traffic, and does not require any special switch support.  The receive load balancing is achieved by ARP negotiation. The bonding driver intercepts the ARP Replies sent by the local system on their way out and overwrites the source ardware address with the unique hardware address of one of the slaves in the bond such that different peers use different hardware addresses for the server.Receive traffic from connections created by the server is also balanced.  When the local system sends an ARP Request the bonding driver copies and saves the peer’s
   IP information from the ARP packet.  When the ARP Reply arrives from the peer, its hardware address is retrieved and the bonding driver initiates an ARP reply to this peer assigning it to one of the slaves in the bond.  A problematic outcome of using ARP negotiation for balancing is that each time that an ARP request is broadcast it uses the hardware address of the bond.  Hence, peers learn the hardware address
   of the bond and the balancing of receive traffic collapses to the current slave.  This is handled by sending updates (ARP Replies) to all the peers with their individually assigned hardware address such that the traffic is redistributed.  Receive traffic is also redistributed when a new slave is added to the bond and when an inactive slave is re-activated.  The receive load is distributed sequentially (round robin) among the group of highest speed slaves in the bond.When a link is reconnected or a new slave joins the bond the receive traffic is redistributed among all active slaves in the bond by initiating ARP Replies with the selected MAC address to each of the clients. The updelay parameter (detailed below) must be set to a value equal or greater than the switch’s forwarding delay so that the ARP Replies sent to the peers will not be blocked by the switch.Prerequisites:
   1. Ethtool support in the base drivers for retrieving the speed of each slave.
   2. Base driver support for setting the hardware address of a device while it is open.  This is required so that there will always be one slave in the team using the bond hardware address (the curr_active_slave) while having a unique hardware address for each slave in the bond.  If the curr_active_slave fails its hardware address is swapped with the new curr_active_slave that was chosen.

Network Bonding or Trunking refers to the aggregation of multiple network ports into a single aggregated group, effectively aggregating the bandwidth of multiple interfaces into a single connection pipe. Bonding is used to provide network load balancing and fault tolerance

We are assuming that you are using CentOS 5.x

Step 1: Create a bond0 config file at /etc/sysconfig/network-scripts/ directory

# touch /etc/sysconfig/network-scripts/ifcfg-bond0
# vim  /etc/sysconfig/network-scripts/ifcfg-bond0

DEVICE=bond0
ONBOOT=yes
BOOTPROTO=none
IPADDR=192.168.10.1
NETWORK=192.168.10.0
NETMASK=255.255.255.0
USERCTL=no

(Where IP is the Actual Address of the Bonded IP Address shared by the 2 physical NIC)

Step 2: Edit the configuration files of the 2 physical Network Cards (eth0 & eth1) that you wish to bond

# vim /etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
ONBOOT=yes
BOOTPROTO=none
MASTER=bond0
SLAVE=yes
USERCTL=no

# vim /etc/sysconfig/network-scripts/ifcfg-eth1

DEVICE=eth1
ONBOOT=yes
BOOTPROTO=none
MASTER=bond0
SLAVE=yes
USERCTL=no

Step 3: Load the Bond Module

/etc/modprobe.conf

alias bond0 bonding
options bond0 mode=0 miimon=100

Note:

1. RHEL bonding supports 7 possible “modes” for bonded interfaces. See  Linux Bonding Modes website for more details on the modes.
2. miimonspecifies (in milliseconds) how often MII link monitoring occurs. This is useful if high availability is required because MII is used to verify that the NIC is active.

Step 4: Load the bond driver module from the command prompt.

modprobe bonding

Step 5: Restart the network

Restart the network

Step 6: Verify the /proc setting.

less /proc/net/bonding/bond0

Ethernet Channel Bonding Driver: v3.2.4 (January 28, 2008)

Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth0
MII Status: up
Link Failure Count: 0
Permanent HW addr: xx:xx:xx:xx:xx:xx

Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: yy:yy:yy:yy:yy:yy
Step 7: Final Tests

 Do a ifconfig to check everything is alright.
ping from remote station to bond0 ipaddress