Tuesday, September 25, 2007

Linux Ethernet Bonding

What is bonding?
Bonding is the same as port trunking. In the following I will use the word bonding because practically we will bond interfaces as one.

But still...what is bonding?
Bonding allows you to aggregate multiple ports into a single group, effectively combining the bandwidth into a single connection. Bonding also allows you to create multi-gigabit pipes to transport traffic through the highest traffic areas of your network. For example, you can aggregate three megabits ports (1 mb each) into a three-megabits trunk port. That is equivalent with having one interface with three megabits speed.

Where should I use bonding?
You can use it wherever you need redundant links, fault tolerance or load balancing networks. It is the best way to have a high availability network segment. A very useful way to use bonding is to use it in connection with 802.1q VLAN support (your network equipment must have 802.1q protocol implemented).

Note :
The bonding driver originally came from Donald Becker's beowulf patches for
kernel 2.0. It has changed quite a bit since, and the original tools from
extreme-linux and beowulf sites will not work with this version of the driver.

For new versions of the driver, patches for older kernels and the updated
userspace tools, please follow the links at the end of this file.


1) Build kernel with the bonding driver
For the latest version of the bonding driver, use kernel 2.4.12 or above
(otherwise you will need to apply a patch).

Configure kernel with `make menuconfig/xconfig/config', and select
"Bonding driver support" in the "Network device support" section. It is
recommended to configure the driver as module since it is currently the only way
to pass parameters to the driver and configure more than one bonding device.

Build and install the new kernel and modules.

2) Get and install the userspace tools
This version of the bonding driver requires updated ifenslave program. The
original one from extreme-linux and beowulf will not work. Kernels 2.4.12
and above include the updated version of ifenslave.c in Documentation/network
directory. For older kernels, please follow the links at the end of this file.

IMPORTANT!!! If you are running on Redhat 7.1 or greater, you need
to be careful because /usr/include/linux is no longer a symbolic link
to /usr/src/linux/include/linux. If you build ifenslave while this is
true, ifenslave will appear to succeed but your bond won't work. The purpose
of the -I option on the ifenslave compile line is to make sure it uses
/usr/src/linux/include/linux/if_bonding.h instead of the version from

To install ifenslave.c, do:
# gcc -Wall -Wstrict-prototypes -O -I/usr/src/linux/include ifenslave.c -o ifenslave
# cp ifenslave /sbin/ifenslave

3) Configure your system
Also see the following section on the module parameters. You will need to add
at least the following line to /etc/conf.modules (or /etc/modules.conf):

alias bond0 bonding

Use standard distribution techniques to define bond0 network interface. For
example, on modern RedHat distributions, create ifcfg-bond0 file in
/etc/sysconfig/network-scripts directory that looks like this:


(put the appropriate values for you network instead of 192.168.1).

All interfaces that are part of the trunk, should have SLAVE and MASTER
definitions. For example, in the case of RedHat, if you wish to make eth0 and
eth1 (or other interfaces) a part of the bonding interface bond0, their config
files (ifcfg-eth0, ifcfg-eth1, etc.) should look like this:


(use DEVICE=eth1 for eth1 and MASTER=bond1 for bond1 if you have configured
second bonding interface).

Restart the networking subsystem or just bring up the bonding device if your
administration tools allow it. Otherwise, reboot. (For the case of RedHat
distros, you can do `ifup bond0' or `/etc/rc.d/init.d/network restart'.)

If the administration tools of your distribution do not support master/slave
notation in configuration of network interfaces, you will need to configure
the bonding device with the following commands manually:

# /sbin/ifconfig bond0 up
# /sbin/ifenslave bond0 eth0
# /sbin/ifenslave bond0 eth1

(substitute with your IP address and add custom network and custom
netmask to the arguments of ifconfig if required).

You can then create a script with these commands and put it into the appropriate
rc directory.

If you specifically need that all your network drivers are loaded before the
bonding driver, use one of modutils' powerful features : in your modules.conf,
tell that when asked for bond0, modprobe should first load all your interfaces :

probeall bond0 eth0 eth1 bonding

Be careful not to reference bond0 itself at the end of the line, or modprobe will
die in an endless recursive loop.

4) Module parameters.
The following module parameters can be passed:


Possible values are 0 (round robin policy, default) and 1 (active backup
policy), and 2 (XOR). See question 9 and the HA section for additional info.


Use integer value for the frequency (in ms) of MII link monitoring. Zero value
is default and means the link monitoring will be disabled. A good value is 100
if you wish to use link monitoring. See HA section for additional info.


Use integer value for delaying disabling a link by this number (in ms) after
the link failure has been detected. Must be a multiple of miimon. Default
value is zero. See HA section for additional info.


Use integer value for delaying enabling a link by this number (in ms) after
the "link up" status has been detected. Must be a multiple of miimon. Default
value is zero. See HA section for additional info.


Use integer value for the frequency (in ms) of arp monitoring. Zero value
is default and means the arp monitoring will be disabled. See HA section
for additional info. This field is value in active_backup mode only.


An ip address to use when arp_interval is > 0. This is the target of the
arp request sent to determine the health of the link to the target.
Specify this value in ddd.ddd.ddd.ddd format.

If you need to configure several bonding devices, the driver must be loaded
several times. I.e. for two bonding devices, your /etc/conf.modules must look
like this:

alias bond0 bonding
alias bond1 bonding

options bond0 miimon=100
options bond1 -o bonding1 miimon=100

5) Testing configuration
You can test the configuration and transmit policy with ifconfig. For example,
for round robin policy, you should get something like this:

[root]# /sbin/ifconfig
bond0 Link encap:Ethernet HWaddr 00:C0:F0:1F:37:B4
inet addr:XXX.XXX.XXX.YYY Bcast:XXX.XXX.XXX.255 Mask:
RX packets:7224794 errors:0 dropped:0 overruns:0 frame:0
TX packets:3286647 errors:1 dropped:0 overruns:1 carrier:0
collisions:0 txqueuelen:0

eth0 Link encap:Ethernet HWaddr 00:C0:F0:1F:37:B4
inet addr:XXX.XXX.XXX.YYY Bcast:XXX.XXX.XXX.255 Mask:
RX packets:3573025 errors:0 dropped:0 overruns:0 frame:0
TX packets:1643167 errors:1 dropped:0 overruns:1 carrier:0
collisions:0 txqueuelen:100
Interrupt:10 Base address:0x1080

eth1 Link encap:Ethernet HWaddr 00:C0:F0:1F:37:B4
inet addr:XXX.XXX.XXX.YYY Bcast:XXX.XXX.XXX.255 Mask:
RX packets:3651769 errors:0 dropped:0 overruns:0 frame:0
TX packets:1643480 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
Interrupt:9 Base address:0x1400

Questions :

1. Is it SMP safe?

Yes. The old 2.0.xx channel bonding patch was not SMP safe.
The new driver was designed to be SMP safe from the start.

2. What type of cards will work with it?

Any Ethernet type cards (you can even mix cards - a Intel
EtherExpress PRO/100 and a 3com 3c905b, for example).
You can even bond together Gigabit Ethernet cards!

3. How many bonding devices can I have?

One for each module you load. See section on module parameters for how
to accomplish this.

4. How many slaves can a bonding device have?

Limited by the number of network interfaces Linux supports and the
number of cards you can place in your system.

5. What happens when a slave link dies?

If your ethernet cards support MII status monitoring and the MII
monitoring has been enabled in the driver (see description of module
parameters), there will be no adverse consequences. This release
of the bonding driver knows how to get the MII information and
enables or disables its slaves according to their link status.
See section on HA for additional information.

For ethernet cards not supporting MII status, or if you wish to
verify that packets have been both send and received, you may
configure the arp_interval and arp_ip_target. If packets have
not been sent or received during this interval, an arp request
is sent to the target to generate send and receive traffic.
If after this interval, either the successful send and/or
receive count has not incremented, the next slave in the sequence
will become the active slave.

If neither mii_monitor and arp_interval is configured, the bonding
driver will not handle this situation very well. The driver will
continue to send packets but some packets will be lost. Retransmits
will cause serious degradation of performance (in the case when one
of two slave links fails, 50% packets will be lost, which is a serious
problem for both TCP and UDP).

6. Can bonding be used for High Availability?

Yes, if you use MII monitoring and ALL your cards support MII link
status reporting. See section on HA for more information.

7. Which switches/systems does it work with?

In round-robin mode, it works with systems that support trunking:

* Cisco 5500 series (look for EtherChannel support).
* SunTrunking software.
* Alteon AceDirector switches / WebOS (use Trunks).
* BayStack Switches (trunks must be explicitly configured). Stackable
models (450) can define trunks between ports on different physical
* Linux bonding, of course !

In Active-backup mode, it should work with any Layer-II switches.

8. Where does a bonding device get its MAC address from?

If not explicitly configured with ifconfig, the MAC address of the
bonding device is taken from its first slave device. This MAC address
is then passed to all following slaves and remains persistent (even if
the the first slave is removed) until the bonding device is brought
down or reconfigured.

If you wish to change the MAC address, you can set it with ifconfig:

# ifconfig bond0 ha ether 00:11:22:33:44:55

The MAC address can be also changed by bringing down/up the device
and then changing its slaves (or their order):

# ifconfig bond0 down ; modprobe -r bonding
# ifconfig bond0 .... up
# ifenslave bond0 eth...

This method will automatically take the address from the next slave
that will be added.

To restore your slaves' MAC addresses, you need to detach them
from the bond (`ifenslave -d bond0 eth0'), set them down
(`ifconfig eth0 down'), unload the drivers (`rmmod 3c59x', for
example) and reload them to get the MAC addresses from their
eeproms. If the driver is shared by several devices, you need
to turn them all down. Another solution is to look for the MAC
address at boot time (dmesg or tail /var/log/messages) and to
reset it by hand with ifconfig :

# ifconfig eth0 down
# ifconfig eth0 hw ether 00:20:40:60:80:A0

9. Which transmit polices can be used?

Round robin, based on the order of enslaving, the output device
is selected base on the next available slave. Regardless of
the source and/or destination of the packet.

XOR, based on (src hw addr XOR dst hw addr) % slave cnt. This
selects the same slave for each destination hw address.

Active-backup policy that ensures that one and only one device will
transmit at any given moment. Active-backup policy is useful for
implementing high availability solutions using two hubs (see
section on HA).

High availability

To implement high availability using the bonding driver, you need to
compile the driver as module because currently it is the only way to pass
parameters to the driver. This may change in the future.

High availability is achieved by using MII status reporting. You need to
verify that all your interfaces support MII link status reporting. On Linux
kernel 2.2.17, all the 100 Mbps capable drivers and yellowfin gigabit driver
support it. If your system has an interface that does not support MII status
reporting, a failure of its link will not be detected!

The bonding driver can regularly check all its slaves links by checking the
MII status registers. The check interval is specified by the module argument
"miimon" (MII monitoring). It takes an integer that represents the
checking time in milliseconds. It should not come to close to (1000/HZ)
(10 ms on i386) because it may then reduce the system interactivity. 100 ms
seems to be a good value. It means that a dead link will be detected at most
100 ms after it goes down.


# modprobe bonding miimon=100

Or, put in your /etc/modules.conf :

alias bond0 bonding
options bond0 miimon=100

There are currently two policies for high availability, depending on whether
a) hosts are connected to a single host or switch that support trunking
b) hosts are connected to several different switches or a single switch that
does not support trunking.

1) HA on a single switch or host - load balancing
It is the easiest to set up and to understand. Simply configure the
remote equipment (host or switch) to aggregate traffic over several
ports (Trunk, EtherChannel, etc.) and configure the bonding interfaces.
If the module has been loaded with the proper MII option, it will work
automatically. You can then try to remove and restore different links
and see in your logs what the driver detects. When testing, you may
encounter problems on some buggy switches that disable the trunk for a
long time if all ports in a trunk go down. This is not Linux, but really
the switch (reboot it to ensure).

Example 1 : host to host at double speed

+----------+ +----------+
| |eth0 eth0| |
| Host A +--------------------------+ Host B |
| +--------------------------+ |
| |eth1 eth1| |
+----------+ +----------+

On each host :
# modprobe bonding miimon=100
# ifconfig bond0 addr
# ifenslave bond0 eth0 eth1

Example 2 : host to switch at double speed

+----------+ +----------+
| |eth0 port1| |
| Host A +--------------------------+ switch |
| +--------------------------+ |
| |eth1 port2| |
+----------+ +----------+

On host A : On the switch :
# modprobe bonding miimon=100 # set up a trunk on port1
# ifconfig bond0 addr and port2
# ifenslave bond0 eth0 eth1

2) HA on two or more switches (or a single switch without trunking support)
This mode is more problematic because it relies on the fact that there
are multiple ports and the host's MAC address should be visible on one
port only to avoid confusing the switches.

If you need to know which interface is the active one, and which ones are
backup, use ifconfig. All backup interfaces have the NOARP flag set.

To use this mode, pass "mode=1" to the module at load time :

# modprobe bonding miimon=100 mode=1

Or, put in your /etc/modules.conf :

alias bond0 bonding
options bond0 miimon=100 mode=1

Example 1: Using multiple host and multiple switches to build a "no single
point of failure" solution.

| |
|port3 port3|
+-----+----+ +-----+----+
| |port7 ISL port7| |
| switch A +--------------------------+ switch B |
| +--------------------------+ |
| |port8 port8| |
+----++----+ +-----++---+
port2||port1 port1||port2
|| +-------+ ||
|+-------------+ host1 +---------------+|
| eth0 +-------+ eth1 |
| |
| +-------+ |
+--------------+ host2 +----------------+
eth0 +-------+ eth1

In this configuration, there are an ISL - Inter Switch Link (could be a trunk),
several servers (host1, host2 ...) attached to both switches each, and one or
more ports to the outside world (port3...). One an only one slave on each host
is active at a time, while all links are still monitored (the system can
detect a failure of active and backup links).

Each time a host changes its active interface, it sticks to the new one until
it goes down. In this example, the hosts are not too much affected by the
expiration time of the switches' forwarding tables.

If host1 and host2 have the same functionality and are used in load balancing
by another external mechanism, it is good to have host1's active interface
connected to one switch and host2's to the other. Such system will survive
a failure of a single host, cable, or switch. The worst thing that may happen
in the case of a switch failure is that half of the hosts will be temporarily
unreachable until the other switch expires its tables.

Example 2: Using multiple ethernet cards connected to a switch to configure
NIC failover (switch is not required to support trunking).

+----------+ +----------+
| |eth0 port1| |
| Host A +--------------------------+ switch |
| +--------------------------+ |
| |eth1 port2| |
+----------+ +----------+

On host A : On the switch :
# modprobe bonding miimon=100 mode=1 # (optional) minimize the time
# ifconfig bond0 addr # for table expiration
# ifenslave bond0 eth0 eth1

Each time the host changes its active interface, it sticks to the new one until
it goes down. In this example, the host is strongly affected by the expiration
time of the switch forwarding table.

3) Adapting to your switches' timing
If your switches take a long time to go into backup mode, it may be
desirable not to activate a backup interface immediately after a link goes
down. It is possible to delay the moment at which a link will be
completely disabled by passing the module parameter "downdelay" (in
milliseconds, must be a multiple of miimon).

When a switch reboots, it is possible that its ports report "link up" status
before they become usable. This could fool a bond device by causing it to
use some ports that are not ready yet. It is possible to delay the moment at
which an active link will be reused by passing the module parameter "updelay"
(in milliseconds, must be a multiple of miimon).

A similar situation can occur when a host re-negotiates a lost link with the
switch (a case of cable replacement).

A special case is when a bonding interface has lost all slave links. Then the
driver will immediately reuse the first link that goes up, even if updelay
parameter was specified. (If there are slave interfaces in the "updelay" state,
the interface that first went into that state will be immediately reused.) This
allows to reduce down-time if the value of updelay has been overestimated.

Examples :

# modprobe bonding miimon=100 mode=1 downdelay=2000 updelay=5000
# modprobe bonding miimon=100 mode=0 downdelay=0 updelay=5000

4) Limitations
The main limitations are :
- only the link status is monitored. If the switch on the other side is
partially down (e.g. doesn't forward anymore, but the link is OK), the link
won't be disabled. Another way to check for a dead link could be to count
incoming frames on a heavily loaded host. This is not applicable to small
servers, but may be useful when the front switches send multicast
information on their links (e.g. VRRP), or even health-check the servers.
Use the arp_interval/arp_ip_target parameters to count incoming/outgoing


The following script (the gray area) will configure a bond interface (bond0) using two ethernet interface (eth0 and eth1). You can place it onto your on file and run it at boot time..


modprobe bonding mode=0 miimon=100 # load bonding module

ifconfig eth0 down # putting down the eth0 interface
ifconfig eth1 down # putting down the eth1 interface

ifconfig bond0 hw ether 00:11:22:33:44:55 # changing the MAC address of the bond0 interface
ifconfig bond0 up # to set ethX interfaces as slave the bond0 must have an ip.

ifenslave bond0 eth0 # putting the eth0 interface in the slave mod for bond0
ifenslave bond0 eth1 # putting the eth1 interface in the slave mod for bond0

You can set up your bond interface according to your needs. Changing one parameters (mode=X) you can have the following bonding types:

mode=0 (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.

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. This mode provides fault tolerance. The primary option affects the behavior of this mode.

mode=2 (balance-xor)
XOR policy: Transmit based on [(source MAC address XOR'd with destination MAC address) modulo slave count]. This selects the same slave for each destination MAC address. This mode provides load balancing and fault tolerance.

mode=3 (broadcast)
Broadcast policy: transmits everything on all slave interfaces. This mode provides fault tolerance.

mode=4 (802.3ad)
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.
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
Most switches will require some type of configuration
to enable 802.3ad mode.
mode=5 (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.
Ethtool support in the base drivers for retrieving the
speed of each slave.
mode=6 (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 hardware 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.

The most used are the first four mode types...

Also you can use multiple bond interface but for that you must load the bonding module as many as you need.
Presuming that you want two bond interface you must configure the /etc/modules.conf as follow:
 alias bond0 bonding
options bond0 -o bond0 mode=0 miimon=100
alias bond1 bonding
options bond1 -o bond1 mode=1 miimon=100

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