/Documentation/networking/bonding.txt

		Linux Ethernet Bonding Driver HOWTO

		Latest update: 23 September 2009

Initial release : Thomas Davis <tadavis at lbl.gov>
Corrections, HA extensions : 2000/10/03-15 :
  - Willy Tarreau <willy at meta-x.org>
  - Constantine Gavrilov <const-g at xpert.com>
  - Chad N. Tindel <ctindel at ieee dot org>
  - Janice Girouard <girouard at us dot ibm dot com>
  - Jay Vosburgh <fubar at us dot ibm dot com>

Reorganized and updated Feb 2005 by Jay Vosburgh
Added Sysfs information: 2006/04/24
  - Mitch Williams <mitch.a.williams at intel.com>

Introduction
============

	The Linux bonding driver provides a method for aggregating
multiple network interfaces into a single logical "bonded" interface.
The behavior of the bonded interfaces depends upon the mode; generally
speaking, modes provide either hot standby or load balancing services.
Additionally, link integrity monitoring may be performed.
	
	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, updated userspace tools, and
who to ask for help, please follow the links at the end of this file.

Table of Contents
=================

1. Bonding Driver Installation

2. Bonding Driver Options

3. Configuring Bonding Devices
3.1	Configuration with Sysconfig Support
3.1.1		Using DHCP with Sysconfig
3.1.2		Configuring Multiple Bonds with Sysconfig
3.2	Configuration with Initscripts Support
3.2.1		Using DHCP with Initscripts
3.2.2		Configuring Multiple Bonds with Initscripts
3.3	Configuring Bonding Manually with Ifenslave
3.3.1		Configuring Multiple Bonds Manually
3.4	Configuring Bonding Manually via Sysfs

4. Querying Bonding Configuration
4.1	Bonding Configuration
4.2	Network Configuration

5. Switch Configuration

6. 802.1q VLAN Support

7. Link Monitoring
7.1	ARP Monitor Operation
7.2	Configuring Multiple ARP Targets
7.3	MII Monitor Operation

8. Potential Trouble Sources
8.1	Adventures in Routing
8.2	Ethernet Device Renaming
8.3	Painfully Slow Or No Failed Link Detection By Miimon

9. SNMP agents

10. Promiscuous mode

11. Configuring Bonding for High Availability
11.1	High Availability in a Single Switch Topology
11.2	High Availability in a Multiple Switch Topology
11.2.1		HA Bonding Mode Selection for Multiple Switch Topology
11.2.2		HA Link Monitoring for Multiple Switch Topology

12. Configuring Bonding for Maximum Throughput
12.1	Maximum Throughput in a Single Switch Topology
12.1.1		MT Bonding Mode Selection for Single Switch Topology
12.1.2		MT Link Monitoring for Single Switch Topology
12.2	Maximum Throughput in a Multiple Switch Topology
12.2.1		MT Bonding Mode Selection for Multiple Switch Topology
12.2.2		MT Link Monitoring for Multiple Switch Topology

13. Switch Behavior Issues
13.1	Link Establishment and Failover Delays
13.2	Duplicated Incoming Packets

14. Hardware Specific Considerations
14.1	IBM BladeCenter

15. Frequently Asked Questions

16. Resources and Links


1. Bonding Driver Installation
==============================

	Most popular distro kernels ship with the bonding driver
already available as a module and the ifenslave user level control
program installed and ready for use. If your distro does not, or you
have need to compile bonding from source (e.g., configuring and
installing a mainline kernel from kernel.org), you'll need to perform
the following steps:

1.1 Configure and build the kernel with bonding
-----------------------------------------------

	The current version of the bonding driver is available in the
drivers/net/bonding subdirectory of the most recent kernel source
(which is available on http://kernel.org).  Most users "rolling their
own" will want to use the most recent kernel from kernel.org.

	Configure kernel with "make menuconfig" (or "make xconfig" or
"make config"), then select "Bonding driver support" in the "Network
device support" section.  It is recommended that you configure the
driver as module since it is currently the only way to pass parameters
to the driver or configure more than one bonding device.

	Build and install the new kernel and modules, then continue
below to install ifenslave.

1.2 Install ifenslave Control Utility
-------------------------------------

	The ifenslave user level control program is included in the
kernel source tree, in the file Documentation/networking/ifenslave.c.
It is generally recommended that you use the ifenslave that
corresponds to the kernel that you are using (either from the same
source tree or supplied with the distro), however, ifenslave
executables from older kernels should function (but features newer
than the ifenslave release are not supported).  Running an ifenslave
that is newer than the kernel is not supported, and may or may not
work.

	To install ifenslave, do the following:

# gcc -Wall -O -I/usr/src/linux/include ifenslave.c -o ifenslave
# cp ifenslave /sbin/ifenslave

	If your kernel source is not in "/usr/src/linux," then replace
"/usr/src/linux/include" in the above with the location of your kernel
source include directory.

	You may wish to back up any existing /sbin/ifenslave, or, for
testing or informal use, tag the ifenslave to the kernel version
(e.g., name the ifenslave executable /sbin/ifenslave-2.6.10).

IMPORTANT NOTE:

	If you omit the "-I" or specify an incorrect directory, you
may end up with an ifenslave that is incompatible with the kernel
you're trying to build it for.  Some distros (e.g., Red Hat from 7.1
onwards) do not have /usr/include/linux symbolically linked to the
default kernel source include directory.

SECOND IMPORTANT NOTE:
	If you plan to configure bonding using sysfs, you do not need
to use ifenslave.

2. Bonding Driver Options
=========================

	Options for the bonding driver are supplied as parameters to the
bonding module at load time, or are specified via sysfs.

	Module options may be given as command line arguments to the
insmod or modprobe command, but are usually specified in either the
/etc/modules.conf or /etc/modprobe.conf configuration file, or in a
distro-specific configuration file (some of which are detailed in the next
section).

	Details on bonding support for sysfs is provided in the
"Configuring Bonding Manually via Sysfs" section, below.

	The available bonding driver parameters are listed below. If a
parameter is not specified the default value is used.  When initially
configuring a bond, it is recommended "tail -f /var/log/messages" be
run in a separate window to watch for bonding driver error messages.

	It is critical that either the miimon or arp_interval and
arp_ip_target parameters be specified, otherwise serious network
degradation will occur during link failures.  Very few devices do not
support at least miimon, so there is really no reason not to use it.

	Options with textual values will accept either the text name
or, for backwards compatibility, the option value.  E.g.,
"mode=802.3ad" and "mode=4" set the same mode.

	The parameters are as follows:

ad_select

	Specifies the 802.3ad aggregation selection logic to use.  The
	possible values and their effects are:

	stable or 0

		The active aggregator is chosen by largest aggregate
		bandwidth.

		Reselection of the active aggregator occurs only when all
		slaves of the active aggregator are down or the active
		aggregator has no slaves.

		This is the default value.

	bandwidth or 1

		The active aggregator is chosen by largest aggregate
		bandwidth.  Reselection occurs if:

		- A slave is added to or removed from the bond

		- Any slave's link state changes

		- Any slave's 802.3ad association state changes

		- The bond's administrative state changes to up

	count or 2

		The active aggregator is chosen by the largest number of
		ports (slaves).  Reselection occurs as described under the
		"bandwidth" setting, above.

	The bandwidth and count selection policies permit failover of
	802.3ad aggregations when partial failure of the active aggregator
	occurs.  This keeps the aggregator with the highest availability
	(either in bandwidth or in number of ports) active at all times.

	This option was added in bonding version 3.4.0.

arp_interval

	Specifies the ARP link monitoring frequency in milliseconds.

	The ARP monitor works by periodically checking the slave
	devices to determine whether they have sent or received
	traffic recently (the precise criteria depends upon the
	bonding mode, and the state of the slave).  Regular traffic is
	generated via ARP probes issued for the addresses specified by
	the arp_ip_target option.

	This behavior can be modified by the arp_validate option,
	below.

	If ARP monitoring is used in an etherchannel compatible mode
	(modes 0 and 2), the switch should be configured in a mode
	that evenly distributes packets across all links. If the
	switch is configured to distribute the packets in an XOR
	fashion, all replies from the ARP targets will be received on
	the same link which could cause the other team members to
	fail.  ARP monitoring should not be used in conjunction with
	miimon.  A value of 0 disables ARP monitoring.  The default
	value is 0.

arp_ip_target

	Specifies the IP addresses to use as ARP monitoring peers when
	arp_interval is > 0.  These are the targets of the ARP request
	sent to determine the health of the link to the targets.
	Specify these values in ddd.ddd.ddd.ddd format.  Multiple IP
	addresses must be separated by a comma.  At least one IP
	address must be given for ARP monitoring to function.  The
	maximum number of targets that can be specified is 16.  The
	default value is no IP addresses.

arp_validate

	Specifies whether or not ARP probes and replies should be
	validated in the active-backup mode.  This causes the ARP
	monitor to examine the incoming ARP requests and replies, and
	only consider a slave to be up if it is receiving the
	appropriate ARP traffic.

	Possible values are:

	none or 0

		No validation is performed.  This is the default.

	active or 1

		Validation is performed only for the active slave.

	backup or 2

		Validation is performed only for backup slaves.

	all or 3

		Validation is performed for all slaves.

	For the active slave, the validation checks ARP replies to
	confirm that they were generated by an arp_ip_target.  Since
	backup slaves do not typically receive these replies, the
	validation performed for backup slaves is on the ARP request
	sent out via the active slave.  It is possible that some
	switch or network configurations may result in situations
	wherein the backup slaves do not receive the ARP requests; in
	such a situation, validation of backup slaves must be
	disabled.

	This option is useful in network configurations in which
	multiple bonding hosts are concurrently issuing ARPs to one or
	more targets beyond a common switch.  Should the link between
	the switch and target fail (but not the switch itself), the
	probe traffic generated by the multiple bonding instances will
	fool the standard ARP monitor into considering the links as
	still up.  Use of the arp_validate option can resolve this, as
	the ARP monitor will only consider ARP requests and replies
	associated with its own instance of bonding.

	This option was added in bonding version 3.1.0.

downdelay

	Specifies the time, in milliseconds, to wait before disabling
	a slave after a link failure has been detected.  This option
	is only valid for the miimon link monitor.  The downdelay
	value should be a multiple of the miimon value; if not, it
	will be rounded down to the nearest multiple.  The default
	value is 0.

fail_over_mac

	Specifies whether active-backup mode should set all slaves to
	the same MAC address at enslavement (the traditional
	behavior), or, when enabled, perform special handling of the
	bond's MAC address in accordance with the selected policy.

	Possible values are:

	none or 0

		This setting disables fail_over_mac, and causes
		bonding to set all slaves of an active-backup bond to
		the same MAC address at enslavement time.  This is the
		default.

	active or 1

		The "active" fail_over_mac policy indicates that the
		MAC address of the bond should always be the MAC
		address of the currently active slave.  The MAC
		address of the slaves is not changed; instead, the MAC
		address of the bond changes during a failover.

		This policy is useful for devices that cannot ever
		alter their MAC address, or for devices that refuse
		incoming broadcasts with their own source MAC (which
		interferes with the ARP monitor).

		The down side of this policy is that every device on
		the network must be updated via gratuitous ARP,
		vs. just updating a switch or set of switches (which
		often takes place for any traffic, not just ARP
		traffic, if the switch snoops incoming traffic to
		update its tables) for the traditional method.  If the
		gratuitous ARP is lost, communication may be
		disrupted.

		When this policy is used in conjuction with the mii
		monitor, devices which assert link up prior to being
		able to actually transmit and receive are particularly
		susceptible to loss of the gratuitous ARP, and an
		appropriate updelay setting may be required.

	follow or 2

		The "follow" fail_over_mac policy causes the MAC
		address of the bond to be selected normally (normally
		the MAC address of the first slave added to the bond).
		However, the second and subsequent slaves are not set
		to this MAC address while they are in a backup role; a
		slave is programmed with the bond's MAC address at
		failover time (and the formerly active slave receives
		the newly active slave's MAC address).

		This policy is useful for multiport devices that
		either become confused or incur a performance penalty
		when multiple ports are programmed with the same MAC
		address.


	The default policy is none, unless the first slave cannot
	change its MAC address, in which case the active policy is
	selected by default.

	This option may be modified via sysfs only when no slaves are
	present in the bond.

	This option was added in bonding version 3.2.0.  The "follow"
	policy was added in bonding version 3.3.0.

lacp_rate

	Option specifying the rate in which we'll ask our link partner
	to transmit LACPDU packets in 802.3ad mode.  Possible values
	are:

	slow or 0
		Request partner to transmit LACPDUs every 30 seconds

	fast or 1
		Request partner to transmit LACPDUs every 1 second

	The default is slow.

max_bonds

	Specifies the number of bonding devices to create for this
	instance of the bonding driver.  E.g., if max_bonds is 3, and
	the bonding driver is not already loaded, then bond0, bond1
	and bond2 will be created.  The default value is 1.  Specifying
	a value of 0 will load bonding, but will not create any devices.

miimon

	Specifies the MII link monitoring frequency in milliseconds.
	This determines how often the link state of each slave is
	inspected for link failures.  A value of zero disables MII
	link monitoring.  A value of 100 is a good starting point.
	The use_carrier option, below, affects how the link state is
	determined.  See the High Availability section for additional
	information.  The default value is 0.

mode

	Specifies one of the bonding policies. The default is
	balance-rr (round robin).  Possible values are:

	balance-rr or 0

		Round-robin policy: Transmit packets in sequential
		order from the first available slave through the
		last.  This mode provides load balancing and fault
		tolerance.

	active-backup or 1

		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.

	balance-xor or 2

		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.

	broadcast or 3

		Broadcast policy: transmits everything on all slave
		interfaces.  This mode provides fault tolerance.

	802.3ad or 4

		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 xmit_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.

	balance-tlb or 5

		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.

	balance-alb or 6

		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.

		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.

num_grat_arp

	Specifies the number of gratuitous ARPs to be issued after a
	failover event.  One gratuitous ARP is issued immediately after
	the failover, subsequent ARPs are sent at a rate of one per link
	monitor interval (arp_interval or miimon, whichever is active).

	The valid range is 0 - 255; the default value is 1.  This option
	affects only the active-backup mode.  This option was added for
	bonding version 3.3.0.

num_unsol_na

	Specifies the number of unsolicited IPv6 Neighbor Advertisements
	to be issued after a failover event.  One unsolicited NA is issued
	immediately after the failover.

	The valid range is 0 - 255; the default value is 1.  This option
	affects only the active-backup mode.  This option was added for
	bonding version 3.4.0.

primary

	A string (eth0, eth2, etc) specifying which slave is the
	primary device.  The specified device will always be the
	active slave while it is available.  Only when the primary is
	off-line will alternate devices be used.  This is useful when
	one slave is preferred over another, e.g., when one slave has
	higher throughput than another.

	The primary option is only valid for active-backup mode.

primary_reselect

	Specifies the reselection policy for the primary slave.  This
	affects how the primary slave is chosen to become the active slave
	when failure of the active slave or recovery of the primary slave
	occurs.  This option is designed to prevent flip-flopping between
	the primary slave and other slaves.  Possible values are:

	always or 0 (default)

		The primary slave becomes the active slave whenever it
		comes back up.

	better or 1

		The primary slave becomes the active slave when it comes
		back up, if the speed and duplex of the primary slave is
		better than the speed and duplex of the current active
		slave.

	failure or 2

		The primary slave becomes the active slave only if the
		current active slave fails and the primary slave is up.

	The primary_reselect setting is ignored in two cases:

		If no slaves are active, the first slave to recover is
		made the active slave.

		When initially enslaved, the primary slave is always made
		the active slave.

	Changing the primary_reselect policy via sysfs will cause an
	immediate selection of the best active slave according to the new
	policy.  This may or may not result in a change of the active
	slave, depending upon the circumstances.

	This option was added for bonding version 3.6.0.

updelay

	Specifies the time, in milliseconds, to wait before enabling a
	slave after a link recovery has been detected.  This option is
	only valid for the miimon link monitor.  The updelay value
	should be a multiple of the miimon value; if not, it will be
	rounded down to the nearest multiple.  The default value is 0.

use_carrier

	Specifies whether or not miimon should use MII or ETHTOOL
	ioctls vs. netif_carrier_ok() to determine the link
	status. The MII or ETHTOOL ioctls are less efficient and
	utilize a deprecated calling sequence within the kernel.  The
	netif_carrier_ok() relies on the device driver to maintain its
	state with netif_carrier_on/off; at this writing, most, but
	not all, device drivers support this facility.

	If bonding insists that the link is up when it should not be,
	it may be that your network device driver does not support
	netif_carrier_on/off.  The default state for netif_carrier is
	"carrier on," so if a driver does not support netif_carrier,
	it will appear as if the link is always up.  In this case,
	setting use_carrier to 0 will cause bonding to revert to the
	MII / ETHTOOL ioctl method to determine the link state.

	A value of 1 enables the use of netif_carrier_ok(), a value of
	0 will use the deprecated MII / ETHTOOL ioctls.  The default
	value is 1.

xmit_hash_policy

	Selects the transmit hash policy to use for slave selection in
	balance-xor and 802.3ad modes.  Possible values are:

	layer2

		Uses XOR of hardware MAC addresses to generate the
		hash.  The formula is

		(source MAC XOR destination MAC) modulo slave count

		This algorithm will place all traffic to a particular
		network peer on the same slave.

		This algorithm is 802.3ad compliant.

	layer2+3

		This policy uses a combination of layer2 and layer3
		protocol information to generate the hash.

		Uses XOR of hardware MAC addresses and IP addresses to
		generate the hash.  The formula is

		(((source IP XOR dest IP) AND 0xffff) XOR
			( source MAC XOR destination MAC ))
				modulo slave count

		This algorithm will place all traffic to a particular
		network peer on the same slave.  For non-IP traffic,
		the formula is the same as for the layer2 transmit
		hash policy.

		This policy is intended to provide a more balanced
		distribution of traffic than layer2 alone, especially
		in environments where a layer3 gateway device is
		required to reach most destinations.

		This algorithm is 802.3ad compliant.

	layer3+4

		This policy uses upper layer protocol information,
		when available, to generate the hash.  This allows for
		traffic to a particular network peer to span multiple
		slaves, although a single connection will not span
		multiple slaves.

		The formula for unfragmented TCP and UDP packets is

		((source port XOR dest port) XOR
			 ((source IP XOR dest IP) AND 0xffff)
				modulo slave count

		For fragmented TCP or UDP packets and all other IP
		protocol traffic, the source and destination port
		information is omitted.  For non-IP traffic, the
		formula is the same as for the layer2 transmit hash
		policy.

		This policy is intended to mimic the behavior of
		certain switches, notably Cisco switches with PFC2 as
		well as some Foundry and IBM products.

		This algorithm is not fully 802.3ad compliant.  A
		single TCP or UDP conversation containing both
		fragmented and unfragmented packets will see packets
		striped across two interfaces.  This may result in out
		of order delivery.  Most traffic types will not meet
		this criteria, as TCP rarely fragments traffic, and
		most UDP traffic is not involved in extended
		conversations.  Other implementations of 802.3ad may
		or may not tolerate this noncompliance.

	The default value is layer2.  This option was added in bonding
	version 2.6.3.  In earlier versions of bonding, this parameter
	does not exist, and the layer2 policy is the only policy.  The
	layer2+3 value was added for bonding version 3.2.2.


3. Configuring Bonding Devices
==============================

	You can configure bonding using either your distro's network
initialization scripts, or manually using either ifenslave or the
sysfs interface.  Distros generally use one of two packages for the
network initialization scripts: initscripts or sysconfig.  Recent
versions of these packages have support for bonding, while older
versions do not.

	We will first describe the options for configuring bonding for
distros using versions of initscripts and sysconfig with full or
partial support for bonding, then provide information on enabling
bonding without support from the network initialization scripts (i.e.,
older versions of initscripts or sysconfig).

	If you're unsure whether your distro uses sysconfig or
initscripts, or don't know if it's new enough, have no fear.
Determining this is fairly straightforward.

	First, issue the command:

$ rpm -qf /sbin/ifup

	It will respond with a line of text starting with either
"initscripts" or "sysconfig," followed by some numbers.  This is the
package that provides your network initialization scripts.

	Next, to determine if your installation supports bonding,
issue the command:

$ grep ifenslave /sbin/ifup

	If this returns any matches, then your initscripts or
sysconfig has support for bonding.

3.1 Configuration with Sysconfig Support
----------------------------------------

	This section applies to distros using a version of sysconfig
with bonding support, for example, SuSE Linux Enterprise Server 9.

	SuSE SLES 9's networking configuration system does support
bonding, however, at this writing, the YaST system configuration
front end does not provide any means to work with bonding devices.
Bonding devices can be managed by hand, however, as follows.

	First, if they have not already been configured, configure the
slave devices.  On SLES 9, this is most easily done by running the
yast2 sysconfig configuration utility.  The goal is for to create an
ifcfg-id file for each slave device.  The simplest way to accomplish
this is to configure the devices for DHCP (this is only to get the
file ifcfg-id file created; see below for some issues with DHCP).  The
name of the configuration file for each device will be of the form:

ifcfg-id-xx:xx:xx:xx:xx:xx

	Where the "xx" portion will be replaced with the digits from
the device's permanent MAC address.

	Once the set of ifcfg-id-xx:xx:xx:xx:xx:xx files has been
created, it is necessary to edit the configuration files for the slave
devices (the MAC addresses correspond to those of the slave devices).
Before editing, the file will contain multiple lines, and will look
something like this:

BOOTPROTO='dhcp'
STARTMODE='on'
USERCTL='no'
UNIQUE='XNzu.WeZGOGF+4wE'
_nm_name='bus-pci-0001:61:01.0'

	Change the BOOTPROTO and STARTMODE lines to the following:

BOOTPROTO='none'
STARTMODE='off'

	Do not alter the UNIQUE or _nm_name lines.  Remove any other
lines (USERCTL, etc).

	Once the ifcfg-id-xx:xx:xx:xx:xx:xx files have been modified,
it's time to create the configuration file for the bonding device
itself.  This file is named ifcfg-bondX, where X is the number of the
bonding device to create, starting at 0.  The first such file is
ifcfg-bond0, the second is ifcfg-bond1, and so on.  The sysconfig
network configuration system will correctly start multiple instances
of bonding.

	The contents of the ifcfg-bondX file is as follows:

BOOTPROTO="static"
BROADCAST="10.0.2.255"
IPADDR="10.0.2.10"
NETMASK="255.255.0.0"
NETWORK="10.0.2.0"
REMOTE_IPADDR=""
STARTMODE="onboot"
BONDING_MASTER="yes"
BONDING_MODULE_OPTS="mode=active-backup miimon=100"
BONDING_SLAVE0="eth0"
BONDING_SLAVE1="bus-pci-0000:06:08.1"

	Replace the sample BROADCAST, IPADDR, NETMASK and NETWORK
values with the appropriate values for your network.

	The STARTMODE specifies when the device is brought online.
The possible values are:

	onboot:	 The device is started at boot time.  If you're not
		 sure, this is probably what you want.

	manual:	 The device is started only when ifup is called
		 manually.  Bonding devices may be configured this
		 way if you do not wish them to start automatically
		 at boot for some reason.

	hotplug: The device is started by a hotplug event.  This is not
		 a valid choice for a bonding device.

	off or ignore: The device configuration is ignored.

	The line BONDING_MASTER='yes' indicates that the device is a
bonding master device.  The only useful value is "yes."

	The contents of BONDING_MODULE_OPTS are supplied to the
instance of the bonding module for this device.  Specify the options
for the bonding mode, link monitoring, and so on here.  Do not include
the max_bonds bonding parameter; this will confuse the configuration
system if you have multiple bonding devices.

	Finally, supply one BONDING_SLAVEn="slave device" for each
slave.  where "n" is an increasing value, one for each slave.  The
"slave device" is either an interface name, e.g., "eth0", or a device
specifier for the network device.  The interface name is easier to
find, but the ethN names are subject to change at boot time if, e.g.,
a device early in the sequence has failed.  The device specifiers
(bus-pci-0000:06:08.1 in the example above) specify the physical
network device, and will not change unless the device's bus location
changes (for example, it is moved from one PCI slot to another).  The
example above uses one of each type for demonstration purposes; most
configurations will choose one or the other for all slave devices.

	When all configuration files have been modified or created,
networking must be restarted for the configuration changes to take
effect.  This can be accomplished via the following:

# /etc/init.d/network restart

	Note that the network control script (/sbin/ifdown) will
remove the bonding module as part of the network shutdown processing,
so it is not necessary to remove the module by hand if, e.g., the
module parameters have changed.

	Also, at this writing, YaST/YaST2 will not manage bonding
devices (they do not show bonding interfaces on its list of network
devices).  It is necessary to edit the configuration file by hand to
change the bonding configuration.

	Additional general options and details of the ifcfg file
format can be found in an example ifcfg template file:

/etc/sysconfig/network/ifcfg.template

	Note that the template does not document the various BONDING_
settings described above, but does describe many of the other options.

3.1.1 Using DHCP with Sysconfig
-------------------------------

	Under sysconfig, configuring a device with BOOTPROTO='dhcp'
will cause it to query DHCP for its IP address information.  At this
writing, this does not function for bonding devices; the scripts
attempt to obtain the device address from DHCP prior to adding any of
the slave devices.  Without active slaves, the DHCP requests are not
sent to the network.

3.1.2 Configuring Multiple Bonds with Sysconfig
-----------------------------------------------

	The sysconfig network initialization system is capable of
handling multiple bonding devices.  All that is necessary is for each
bonding instance to have an appropriately configured ifcfg-bondX file
(as described above).  Do not specify the "max_bonds" parameter to any
instance of bonding, as this will confuse sysconfig.  If you require
multiple bonding devices with identical parameters, create multiple
ifcfg-bondX files.

	Because the sysconfig scripts supply the bonding module
options in the ifcfg-bondX file, it is not necessary to add them to
the system /etc/modules.conf or /etc/modprobe.conf configuration file.

3.2 Configuration with Initscripts Support
------------------------------------------

	This section applies to distros using a recent version of
initscripts with bonding support, for example, Red Hat Enterprise Linux
version 3 or later, Fedora, etc.  On these systems, the network
initialization scripts have knowledge of bonding, and can be configured to
control bonding devices.  Note that older versions of the initscripts
package have lower levels of support for bonding; this will be noted where
applicable.

	These distros will not automatically load the network adapter
driver unless the ethX device is configured with an IP address.
Because of this constraint, users must manually configure a
network-script file for all physical adapters that will be members of
a bondX link.  Network script files are located in the directory:

/etc/sysconfig/network-scripts

	The file name must be prefixed with "ifcfg-eth" and suffixed
with the adapter's physical adapter number.  For example, the script
for eth0 would be named /etc/sysconfig/network-scripts/ifcfg-eth0.
Place the following text in the file:

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

	The DEVICE= line will be different for every ethX device and
must correspond with the name of the file, i.e., ifcfg-eth1 must have
a device line of DEVICE=eth1.  The setting of the MASTER= line will
also depend on the final bonding interface name chosen for your bond.
As with other network devices, these typically start at 0, and go up
one for each device, i.e., the first bonding instance is bond0, the
second is bond1, and so on.

	Next, create a bond network script.  The file name for this
script will be /etc/sysconfig/network-scripts/ifcfg-bondX where X is
the number of the bond.  For bond0 the file is named "ifcfg-bond0",
for bond1 it is named "ifcfg-bond1", and so on.  Within that file,
place the following text:

DEVICE=bond0
IPADDR=192.168.1.1
NETMASK=255.255.255.0
NETWORK=192.168.1.0
BROADCAST=192.168.1.255
ONBOOT=yes
BOOTPROTO=none
USERCTL=no

	Be sure to change the networking specific lines (IPADDR,
NETMASK, NETWORK and BROADCAST) to match your network configuration.

	For later versions of initscripts, such as that found with Fedora
7 (or later) and Red Hat Enterprise Linux version 5 (or later), it is possible,
and, indeed, preferable, to specify the bonding options in the ifcfg-bond0
file, e.g. a line of the format:

BONDING_OPTS="mode=active-backup arp_interval=60 arp_ip_target=192.168.1.254"

	will configure the bond with the specified options.  The options
specified in BONDING_OPTS are identical to the bonding module parameters
except for the arp_ip_target field when using versions of initscripts older
than and 8.57 (Fedora 8) and 8.45.19 (Red Hat Enterprise Linux 5.2).  When
using older versions each target should be included as a separate option and
should be preceded by a '+' to indicate it should be added to the list of
queried targets, e.g.,

	arp_ip_target=+192.168.1.1 arp_ip_target=+192.168.1.2

	is the proper syntax to specify multiple targets.  When specifying
options via BONDING_OPTS, it is not necessary to edit /etc/modules.conf or
/etc/modprobe.conf.

	For even older versions of initscripts that do not support
BONDING_OPTS, it is necessary to edit /etc/modules.conf (or
/etc/modprobe.conf, depending upon your distro) to load the bonding module
with your desired options when the bond0 interface is brought up.  The
following lines in /etc/modules.conf (or modprobe.conf) will load the
bonding module, and select its options:

alias bond0 bonding
options bond0 mode=balance-alb miimon=100

	Replace the sample parameters with the appropriate set of
options for your configuration.

	Finally run "/etc/rc.d/init.d/network restart" as root.  This
will restart the networking subsystem and your bond link should be now
up and running.

3.2.1 Using DHCP with Initscripts
---------------------------------

	Recent versions of initscripts (the versions supplied with Fedora
Core 3 and Red Hat Enterprise Linux 4, or later versions, are reported to
work) have support for assigning IP information to bonding devices via
DHCP.

	To configure bonding for DHCP, configure it as described
above, except replace the line "BOOTPROTO=none" with "BOOTPROTO=dhcp"
and add a line consisting of "TYPE=Bonding".  Note that the TYPE value
is case sensitive.

3.2.2 Configuring Multiple Bonds with Initscripts
-------------------------------------------------

	Initscripts packages that are included with Fedora 7 and Red Hat
Enterprise Linux 5 support multiple bonding interfaces by simply
specifying the appropriate BONDING_OPTS= in ifcfg-bondX where X is the
number of the bond.  This support requires sysfs support in the kernel,
and a bonding driver of version 3.0.0 or later.  Other configurations may
not support this method for specifying multiple bonding interfaces; for
those instances, see the "Configuring Multiple Bonds Manually" section,
below.

3.3 Configuring Bonding Manually with Ifenslave
-----------------------------------------------

	This section applies to distros whose network initialization
scripts (the sysconfig or initscripts package) do not have specific
knowledge of bonding.  One such distro is SuSE Linux Enterprise Server
version 8.

	The general method for these systems is to place the bonding
module parameters into /etc/modules.conf or /etc/modprobe.conf (as
appropriate for the installed distro), then add modprobe and/or
ifenslave commands to the system's global init script.  The name of
the global init script differs; for sysconfig, it is
/etc/init.d/boot.local and for initscripts it is /etc/rc.d/rc.local.

	For example, if you wanted to make a simple bond of two e100
devices (presumed to be eth0 and eth1), and have it persist across
reboots, edit the appropriate file (/etc/init.d/boot.local or
/etc/rc.d/rc.local), and add the following:

modprobe bonding mode=balance-alb miimon=100
modprobe e100
ifconfig bond0 192.168.1.1 netmask 255.255.255.0 up
ifenslave bond0 eth0
ifenslave bond0 eth1

	Replace the example bonding module parameters and bond0
network configuration (IP address, netmask, etc) with the appropriate
values for your configuration.

	Unfortunately, this method will not provide support for the
ifup and ifdown scripts on the bond devices.  To reload the bonding
configuration, it is necessary to run the initialization script, e.g.,

# /etc/init.d/boot.local

	or

# /etc/rc.d/rc.local

	It may be desirable in such a case to create a separate script
which only initializes the bonding configuration, then call that
separate script from within boot.local.  This allows for bonding to be
enabled without re-running the entire global init script.

	To shut down the bonding devices, it is necessary to first
mark the bonding device itself as being down, then remove the
appropriate device driver modules.  For our example above, you can do
the following:

# ifconfig bond0 down
# rmmod bonding
# rmmod e100

	Again, for convenience, it may be desirable to create a script
with these commands.


3.3.1 Configuring Multiple Bonds Manually
-----------------------------------------

	This section contains information on configuring multiple
bonding devices with differing options for those systems whose network
initialization scripts lack support for configuring multiple bonds.

	If you require multiple bonding devices, but all with the same
options, you may wish to use the "max_bonds" module parameter,
documented above.

	To create multiple bonding devices with differing options, it is
preferrable to use bonding parameters exported by sysfs, documented in the
section below.

	For versions of bonding without sysfs support, the only means to
provide multiple instances of bonding with differing options is to load
the bonding driver multiple times.  Note that current versions of the
sysconfig network initialization scripts handle this automatically; if
your distro uses these scripts, no special action is needed.  See the
section Configuring Bonding Devices, above, if you're not sure about your
network initialization scripts.

	To load multiple instances of the module, it is necessary to
specify a different name for each instance (the module loading system
requires that every loaded module, even multiple instances of the same
module, have a unique name).  This is accomplished by supplying multiple
sets of bonding options in /etc/modprobe.conf, for example:

alias bond0 bonding
options bond0 -o bond0 mode=balance-rr miimon=100

alias bond1 bonding
options bond1 -o bond1 mode=balance-alb miimon=50

	will load the bonding module two times.  The first instance is
named "bond0" and creates the bond0 device in balance-rr mode with an
miimon of 100.  The second instance is named "bond1" and creates the
bond1 device in balance-alb mode with an miimon of 50.

	In some circumstances (typically with older distributions),
the above does not work, and the second bonding instance never sees
its options.  In that case, the second options line can be substituted
as follows:

install bond1 /sbin/modprobe --ignore-install bonding -o bond1 \
	mode=balance-alb miimon=50

	This may be repeated any number of times, specifying a new and
unique name in place of bond1 for each subsequent instance.

	It has been observed that some Red Hat supplied kernels are unable
to rename modules at load time (the "-o bond1" part).  Attempts to pass
that option to modprobe will produce an "Operation not permitted" error.
This has been reported on some Fedora Core kernels, and has been seen on
RHEL 4 as well.  On kernels exhibiting this problem, it will be impossible
to configure multiple bonds with differing parameters (as they are older
kernels, and also lack sysfs support).

3.4 Configuring Bonding Manually via Sysfs
------------------------------------------

	Starting with version 3.0.0, Channel Bonding may be configured
via the sysfs interface.  This interface allows dynamic configuration
of all bonds in the system without unloading the module.  It also
allows for adding and removing bonds at runtime.  Ifenslave is no
longer required, though it is still supported.

	Use of the sysfs interface allows you to use multiple bonds
with different configurations without having to reload the module.
It also allows you to use multiple, differently configured bonds when
bonding is compiled into the kernel.

	You must have the sysfs filesystem mounted to configure
bonding this way.  The examples in this document assume that you
are using the standard mount point for sysfs, e.g. /sys.  If your
sysfs filesystem is mounted elsewhere, you will need to adjust the
example paths accordingly.

Creating and Destroying Bonds
-----------------------------
To add a new bond foo:
# echo +foo > /sys/class/net/bonding_masters

To remove an existing bond bar:
# echo -bar > /sys/class/net/bonding_masters

To show all existing bonds:
# cat /sys/class/net/bonding_masters

NOTE: due to 4K size limitation of sysfs files, this list may be
truncated if you have more than a few hundred bonds.  This is unlikely
to occur under normal operating conditions.

Adding and Removing Slaves
--------------------------
	Interfaces may be enslaved to a bond using the file
/sys/class/net/<bond>/bonding/slaves.  The semantics for this file
are the same as for the bonding_masters file.

To enslave interface eth0 to bond bond0:
# ifconfig bond0 up
# echo +eth0 > /sys/class/net/bond0/bonding/slaves

To free slave eth0 from bond bond0:
# echo -eth0 > /sys/class/net/bond0/bonding/slaves

	When an interface is enslaved to a bond, symlinks between the
two are created in the sysfs filesystem.  In this case, you would get
/sys/class/net/bond0/slave_eth0 pointing to /sys/class/net/eth0, and
/sys/class/net/eth0/master pointing to /sys/class/net/bond0.

	This means that you can tell quickly whether or not an
interface is enslaved by looking for the master symlink.  Thus:
# echo -eth0 > /sys/class/net/eth0/master/bonding/slaves
will free eth0 from whatever bond it is enslaved to, regardless of
the name of the bond interface.

Changing a Bond's Configuration
-------------------------------
	Each bond may be configured individually by manipulating the
files located in /sys/class/net/<bond name>/bonding

	The names of these files correspond directly with the command-
line parameters described elsewhere in this file, and, with the
exception of arp_ip_target, they accept the same values.  To see the
current setting, simply cat the appropriate file.

	A few examples will be given here; for specific usage
guidelines for each parameter, see the appropriate section in this
document.

To configure bond0 for balance-alb mode:
# ifconfig bond0 down
# echo 6 > /sys/class/net/bond0/bonding/mode
 - or -
# echo balance-alb > /sys/class/net/bond0/bonding/mode
	NOTE: The bond interface must be down before the mode can be
changed.

To enable MII monitoring on bond0 with a 1 second interval:
# echo 1000 > /sys/class/net/bond0/bonding/miimon
	NOTE: If ARP monitoring is enabled, it will disabled when MII
monitoring is enabled, and vice-versa.

To add ARP targets:
# echo +192.168.0.100 > /sys/class/net/bond0/bonding/arp_ip_target
# echo +192.168.0.101 > /sys/class/net/bond0/bonding/arp_ip_target
	NOTE:  up to 16 target addresses may be specified.

To remove an ARP target:
# echo -192.168.0.100 > /sys/class/net/bond0/bonding/arp_ip_target

Example Configuration
---------------------
	We begin with the same example that is shown in section 3.3,
executed with sysfs, and without using ifenslave.

	To make a simple bond of two e100 devices (presumed to be eth0
and eth1), and have it persist across reboots, edit the appropriate
file (/etc/init.d/boot.local or /etc/rc.d/rc.local), and add the
following:

modprobe bonding
modprobe e100
echo balance-alb > /sys/class/net/bond0/bonding/mode
ifconfig bond0 192.168.1.1 netmask 255.255.255.0 up
echo 100 > /sys/class/net/bond0/bonding/miimon
echo +eth0 > /sys/class/net/bond0/bonding/slaves
echo +eth1 > /sys/class/net/bond0/bonding/slaves

	To add a second bond, with two e1000 interfaces in
active-backup mode, using ARP monitoring, add the following lines to
your init script:

modprobe e1000
echo +bond1 > /sys/class/net/bonding_masters
echo active-backup > /sys/class/net/bond1/bonding/mode
ifconfig bond1 192.168.2.1 netmask 255.255.255.0 up
echo +192.168.2.100 /sys/class/net/bond1/bonding/arp_ip_target
echo 2000 > /sys/class/net/bond1/bonding/arp_interval
echo +eth2 > /sys/class/net/bond1/bonding/slaves
echo +eth3 > /sys/class/net/bond1/bonding/slaves


4. Querying Bonding Configuration 
=================================

4.1 Bonding Configuration
-------------------------

	Each bonding device has a read-only file residing in the
/proc/net/bonding directory.  The file contents include information
about the bonding configuration, options and state of each slave.

	For example, the contents of /proc/net/bonding/bond0 after the
driver is loaded with parameters of mode=0 and miimon=1000 is
generally as follows:

	Ethernet Channel Bonding Driver: 2.6.1 (October 29, 2004)
        Bonding Mode: load balancing (round-robin)
        Currently Active Slave: eth0
        MII Status: up
        MII Polling Interval (ms): 1000
        Up Delay (ms): 0
        Down Delay (ms): 0

        Slave Interface: eth1
        MII Status: up
        Link Failure Count: 1

        Slave Interface: eth0
        MII Status: up
        Link Failure Count: 1

	The precise format and contents will change depending upon the
bonding configuration, state, and version of the bonding driver.

4.2 Network configuration
-------------------------

	The network configuration can be inspected using the ifconfig
command.  Bonding devices will have the MASTER flag set; Bonding slave
devices will have the SLAVE flag set.  The ifconfig output does not
contain information on which slaves are associated with which masters.

	In the example below, the bond0 interface is the master
(MASTER) while eth0 and eth1 are slaves (SLAVE). Notice all slaves of
bond0 have the same MAC address (HWaddr) as bond0 for all modes except
TLB and ALB that require a unique MAC address for each slave.

# /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:255.255.252.0
          UP BROADCAST RUNNING MASTER MULTICAST  MTU:1500  Metric:1
          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
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          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
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          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

5. Switch Configuration
=======================

	For this section, "switch" refers to whatever system the
bonded devices are directly connected to (i.e., where the other end of
the cable plugs into).  This may be an actual dedicated switch device,
or it may be another regular system (e.g., another computer running
Linux),

	The active-backup, balance-tlb and balance-alb modes do not
require any specific configuration of the switch.

	The 802.3ad mode requires that the switch have the appropriate
ports configured as an 802.3ad aggregation.  The precise method used
to configure this varies from switch to switch, but, for example, a
Cisco 3550 series switch requires that the appropriate ports first be
grouped together in a single etherchannel instance, then that
etherchannel is set to mode "lacp" to enable 802.3ad (instead of
standard EtherChannel).

	The balance-rr, balance-xor and broadcast modes generally
require that the switch have the appropriate ports grouped together.
The nomenclature for such a group differs between switches, it may be
called an "etherchannel" (as in the Cisco example, above), a "trunk
group" or some other similar variation.  For these modes, each switch
will also have its own configuration options for the switch's transmit
policy to the bond.  Typical choices include XOR of either the MAC or
IP addresses.  The transmit policy of the two peers does not need to
match.  For these three modes, the bonding mode really selects a
transmit policy for an EtherChannel group; all three will interoperate
with another EtherChannel group.


6. 802.1q VLAN Support
======================

	It is possible to configure VLAN devices over a bond interface
using the 8021q driver.  However, only packets coming from the 8021q
driver and passing through bonding will be tagged by default.  Self
generated packets, for example, bonding's learning packets or ARP
packets generated by either ALB mode or the ARP monitor mechanism, are
tagged internally by bonding itself.  As a result, bonding must
"learn" the VLAN IDs configured above it, and use those IDs to tag
self generated packets.

	For reasons of simplicity, and to support the use of adapters
that can do VLAN hardware acceleration offloading, the bonding
interface declares itself as fully hardware offloading capable, it gets
the add_vid/kill_vid notifications to gather the necessary
information, and it propagates those actions to the slaves.  In case
of mixed adapter types, hardware accelerated tagged packets that
should go through an adapter that is not offloading capable are
"un-accelerated" by the bonding driver so the VLAN tag sits in the
regular location.

	VLAN interfaces *must* be added on top of a bonding interface
only after enslaving at least one slave.  The bonding interface has a
hardware address of 00:00:00:00:00:00 until the first slave is added.
If the VLAN interface is created prior to the first enslavement, it
would pick up the all-zeroes hardware address.  Once the first slave
is attached to the bond, the bond device itself will pick up the
slave's hardware address, which is then available for the VLAN device.

	Also, be aware that a similar problem can occur if all slaves
are released from a bond that still has one or more VLAN interfaces on
top of it.  When a new slave is added, the bonding interface will
obtain its hardware address from the first slave, which might not
match the hardware address of the VLAN interfaces (which was
ultimately copied from an earlier slave).

	There are two methods to insure that the VLAN device operates
with the correct hardware address if all slaves are removed from a
bond interface:

	1. Remove all VLAN interfaces then recreate them

	2. Set the bonding interface's hardware address so that it
matches the hardware address of the VLAN interfaces.

	Note that changing a VLAN interface's HW address would set the
underlying device -- i.e. the bonding interface -- to promiscuous
mode, which might not be what you want.


7. Link Monitoring
==================

	The bonding driver at present supports two schemes for
monitoring a slave device's link state: the ARP monitor and the MII
monitor.

	At the present time, due to implementation restrictions in the
bonding driver itself, it is not possible to enable both ARP and MII
monitoring simultaneously.

7.1 ARP Monitor Operation
-------------------------

	The ARP monitor operates as its name suggests: it sends ARP
queries to one or more designated peer systems on the network, and
uses the response as an indication that the link is operating.  This
gives some assurance that traffic is actually flowing to and from one
or more peers on the local network.

	The ARP monitor relies on the device driver itself to verify
that traffic is flowing.  In particular, the driver must keep up to
date the last receive time, dev->last_rx, and transmit start time,
dev->trans_start.  If these are not updated by the driver, then the
ARP monitor will immediately fail any slaves using that driver, and
those slaves will stay down.  If networking monitoring (tcpdump, etc)
shows the ARP requests and replies on the network, then it may be that
your device driver is not updating last_rx and trans_start.

7.2 Configuring Multiple ARP Targets
------------------------------------

	While ARP monitoring can be done with just one target, it can
be useful in a High Availability setup to have several targets to
monitor.  In the case of just one target, the target itself may go
down or have a problem making it unresponsive to ARP requests.  Having
an additional target (or several) increases the reliability of the ARP
monitoring.

	Multiple ARP targets must be separated by commas as follows:

# example options for ARP monitoring with three targets
alias bond0 bonding
options bond0 arp_interval=60 arp_ip_target=192.168.0.1,192.168.0.3,192.168.0.9

	For just a single target the options would resemble:

# example options for ARP monitoring with one target
alias bond0 bonding
options bond0 arp_interval=60 arp_ip_target=192.168.0.100


7.3 MII Monitor Operation
-------------------------

	The MII monitor monitors only the carrier state of the local
network interface.  It accomplishes this in one of three ways: by
depending upon the device driver to maintain its carrier state, by
querying the device's MII registers, or by making an ethtool query to
the device.

	If the use_carrier module parameter is 1 (the default value),
then the MII monitor will rely on the driver for carrier state
information (via the netif_carrier subsystem).  As explained in the
use_carrier parameter information, above, if the MII monitor fails to
detect carrier loss on the device (e.g., when the cable is physically
disconnected), it may be that the driver does not support
netif_carrier.

	If use_carrier is 0, then the MII monitor will first query the
device's (via ioctl) MII registers and check the link state.  If that
request fails (not just that it returns carrier down), then the MII
monitor will make an ethtool ETHOOL_GLINK request to attempt to obtain
the same information.  If both methods fail (i.e., the driver either
does not support or had some error in processing both the MII register
and ethtool requests), then the MII monitor will assume the link is
up.

8. Potential Sources of Trouble
===============================

8.1 Adventures in Routing
-------------------------

	When bonding is configured, it is important that the slave
devices not have routes that supersede routes of the master (or,
generally, not have routes at all).  For example, suppose the bonding
device bond0 has two slaves, eth0 and eth1, and the routing table is
as follows:

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
10.0.0.0        0.0.0.0         255.255.0.0     U        40 0          0 eth0
10.0.0.0        0.0.0.0         255.255.0.0     U        40 0          0 eth1
10.0.0.0        0.0.0.0         255.255.0.0     U        40 0          0 bond0
127.0.0.0       0.0.0.0         255.0.0.0       U        40 0          0 lo

	This routing configuration will likely still update the
receive/transmit times in the driver (needed by the ARP monitor), but
may bypass the bonding driver (because outgoing traffic to, in this
case, another host on network 10 would use eth0 or eth1 before bond0).

	The ARP monitor (and ARP itself) may become confused by this
configuration, because ARP requests (generated by the ARP monitor)
will be sent on one interface (bond0), but the corresponding reply
will arrive on a different interface (eth0).  This reply looks to ARP
as an unsolicited ARP reply (because ARP matches replies on an
interface basis), and is discarded.  The MII monitor is not affected
by the state of the routing table.

	The solution here is simply to insure that slaves do not have
routes of their own, and if for some reason they must, those routes do
not supersede routes of their master.  This should generally be the
case, but unusual configurations or errant manual or automatic static
route additions may cause trouble.

8.2 Ethernet Device Renaming
----------------------------

	On systems with network configuration scripts that do not
associate physical devices directly with network interface names (so
that the same physical device always has the same "ethX" name), it may
be necessary to add some special logic to either /etc/modules.conf or
/etc/modprobe.conf (depending upon which is installed on the system).

	For example, given a modules.conf containing the following:

alias bond0 bonding
options bond0 mode=some-mode miimon=50
alias eth0 tg3
alias eth1 tg3
alias eth2 e1000
alias eth3 e1000

	If neither eth0 and eth1 are slaves to bond0, then when the
bond0 interface comes up, the devices may end up reordered.  This
happens because bonding is loaded first, then its slave device's
drivers are loaded next.  Since no other drivers have been loaded,
when the e1000 driver loads, it will receive eth0 and eth1 for its
devices, but the bonding configuration tries to enslave eth2 and eth3
(which may later be assigned to the tg3 devices).

	Adding the following:

add above bonding e1000 tg3

	causes modprobe to load e1000 then tg3, in that order, when
bonding is loaded.  This command is fully documented in the
modules.conf manual page.

	On systems utilizing modprobe.conf (or modprobe.conf.local),
an equivalent problem can occur.  In this case, the following can be
added to modprobe.conf (or modprobe.conf.local, as appropriate), as
follows (all on one line; it has been split here for clarity):

install bonding /sbin/modprobe tg3; /sbin/modprobe e1000;
	/sbin/modprobe --ignore-install bonding

	This will, when loading the bonding module, rather than
performing the normal action, instead execute the provided command.
This command loads the device drivers in the order needed, then calls
modprobe with --ignore-install to cause the normal action to then take
place.  Full documentation on this can be found in the modprobe.conf
and modprobe manual pages.

8.3. Painfully Slow Or No Failed Link Detection By Miimon
---------------------------------------------------------

	By default, bonding enables the use_carrier option, which
instructs bonding to trust the driver to maintain carrier state.

	As discussed in the options section, above, some drivers do
not support the netif_carrier_on/_off link state tracking system.
With use_carrier enabled, bonding will always see these links as up,
regardless of their actual state.

	Additionally, other drivers do support netif_carrier, but do
not maintain it in real time, e.g., only polling the link state at
some fixed interval.  In this case, miimon will detect failures, but
only after some long period of time has expired.  If it appears that
miimon is very slow in detecting link failures, try specifying
use_carrier=0 to see if that improves the failure detection time.  If
it does, then it may be that the driver checks the carrier state at a
fixed interval, but does not cache the MII register values (so the
use_carrier=0 method of querying the registers directly works).  If
use_carrier=0 does not improve the failover, then the driver may cache
the registers, or the problem may be elsewhere.

	Also, remember that miimon only checks for the device's
carrier state.  It has no way to determine the state of devices on or
beyond other ports of a switch, or if a switch is refusing to pass
traffic while still maintaining carrier on.

9. SNMP agents
===============

	If running SNMP agents, the bonding driver should be loaded
before any network drivers participating in a bond.  This requirement
is due to the interface index (ipAdEntIfIndex) being associated to
the first interface found with a given IP address.  That is, there is
only one ipAdEntIfIndex for each IP address.  For example, if eth0 and
eth1 are slaves of bond0 and the driver for eth0 is loaded before the
bonding driver, the interface for the IP address will be associated
with the eth0 interface.  This configuration is shown below, the IP
address 192.168.1.1 has an interface index of 2 which indexes to eth0
in the ifDescr table (ifDescr.2).

     interfaces.ifTable.ifEntry.ifDescr.1 = lo
     interfaces.ifTable.ifEntry.ifDescr.2 = eth0
     interfaces.ifTable.ifEntry.ifDescr.3 = eth1
     interfaces.ifTable.ifEntry.ifDescr.4 = eth2
     interfaces.ifTable.ifEntry.ifDescr.5 = eth3
     interfaces.ifTable.ifEntry.ifDescr.6 = bond0
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.10.10.10.10 = 5
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.192.168.1.1 = 2
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.10.74.20.94 = 4
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.127.0.0.1 = 1

	This problem is avoided by loading the bonding driver before
any network drivers participating in a bond.  Below is an example of
loading the bonding driver first, the IP address 192.168.1.1 is
correctly associated with ifDescr.2.

     interfaces.ifTable.ifEntry.ifDescr.1 = lo
     interfaces.ifTable.ifEntry.ifDescr.2 = bond0
     interfaces.ifTable.ifEntry.ifDescr.3 = eth0
     interfaces.ifTable.ifEntry.ifDescr.4 = eth1
     interfaces.ifTable.ifEntry.ifDescr.5 = eth2
     interfaces.ifTable.ifEntry.ifDescr.6 = eth3
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.10.10.10.10 = 6
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.192.168.1.1 = 2
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.10.74.20.94 = 5
     ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.127.0.0.1 = 1

	While some distributions may not report the interface name in
ifDescr, the association between the IP address and IfIndex remains
and SNMP functions such as Interface_Scan_Next will report that
association.

10. Promiscuous mode
====================

	When running network monitoring tools, e.g., tcpdump, it is
common to enable promiscuous mode on the device, so that all traffic
is seen (instead of seeing only traffic destined for the local host).
The bonding driver handles promiscuous mode changes to the bonding
master device (e.g., bond0), and propagates the setting to the slave
devices.

	For the balance-rr, balance-xor, broadcast, and 802.3ad modes,
the promiscuous mode setting is propagated to all slaves.

	For the active-backup, balance-tlb and balance-alb modes, the
promiscuous mode setting is propagated only to the active slave.

	For balance-tlb mode, the active slave is the slave currently
receiving inbound traffic.

	For balance-alb mode, the active slave is the slave used as a
"primary."  This slave is used for mode-specific control traffic, for
sending to peers that are unassigned or if the load is unbalanced.

	For the active-backup, balance-tlb and balance-alb modes, when
the active slave changes (e.g., due to a link failure), the
promiscuous setting will be propagated to the new active slave.

11. Configuring Bonding for High Availability
=============================================

	High Availability refers to configurations that provide
maximum network availability by having redundant or backup devices,
links or switches between the host and the rest of the world.  The
goal is to provide the maximum availability of network connectivity
(i.e., the network always works), even though other configurations
could provide higher throughput.

11.1 High Availability in a Single Switch Topology
--------------------------------------------------

	If two hosts (or a host and a single switch) are directly
connected via multiple physical links, then there is no availability
penalty to optimizing for maximum bandwidth.  In this case, there is
only one switch (or peer), so if it fails, there is no alternative
access to fail over to.  Additionally, the bonding load balance modes
support link monitoring of their members, so if individual links fail,
the load will be rebalanced across the remaining devices.

	See Section 13, "Configuring Bonding for Maximum Throughput"
for information on configuring bonding with one peer device.

11.2 High Availability in a Multiple Switch Topology
----------------------------------------------------

	With multiple switches, the configuration of bonding and the
network changes dramatically.  In multiple switch topologies, there is
a trade off between network availability and usable bandwidth.

	Below is a sample network, configured to maximize the
availability of the network:

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

	In this configuration, there is a link between the two
switches (ISL, or inter switch link), and multiple ports connecting to
the outside world ("port3" on each switch).  There is no technical
reason that this could not be extended to a third switch.

11.2.1 HA Bonding Mode Selection for Multiple Switch Topology
-------------------------------------------------------------

	In a topology such as the example above, the active-backup and
broadcast modes are the only useful bonding modes when optimizing for
availability; the other modes require all links to terminate on the
same peer for them to behave rationally.

active-backup: This is generally the preferred mode, particularly if
	the switches have an ISL and play together well.  If the
	network configuration is such that one switch is specifically
	a backup switch (e.g., has lower capacity, higher cost, etc),
	then the primary option can be used to insure that the
	preferred link is always used when it is available.

broadcast: This mode is really a special purpose mode, and is suitable
	only for very specific needs.  For example, if the two
	switches are not connected (no ISL), and the networks beyond
	them are totally independent.  In this case, if it is
	necessary for some specific one-way traffic to reach both
	independent networks, then the broadcast mode may be suitable.

11.2.2 HA Link Monitoring Selection for Multiple Switch Topology
----------------------------------------------------------------

	The choice of link monitoring ultimately depends upon your
switch.  If the switch can reliably fail ports in response to other
failures, then either the MII or ARP monitors should work.  For
example, in the above example, if the "port3" link fails at the remote
end, the MII monitor has no direct means to detect this.  The ARP
monitor could be configured with a target at the remote end of port3,
thus detecting that failure without switch support.

	In general, however, in a multiple switch topology, the ARP
monitor can provide a higher level of reliability in detecting end to
end connectivity failures (which may be caused by the failure of any
individual component to pass traffic for any reason).  Additionally,
the ARP monitor should be configured with multiple targets (at least
one for each switch in the network).  This will insure that,
regardless of which switch is active, the ARP monitor has a suitable
target to query.

	Note, also, that of late many switches now support a functionality
generally referred to as "trunk failover."  This is a feature of the
switch that causes the link state of a particular switch port to be set
down (or up) when the state of another switch port goes down (or up).
Its purpose is to propagate link failures from logically "exterior" ports
to the logically "interior" ports that bonding is able to monitor via
miimon.  Availability and configuration for trunk failover varies by
switch, but this can be a viable alternative to the ARP monitor when using
suitable switches.

12. Configuring Bonding for Maximum Throughput
==============================================

12.1 Maximizing Throughput in a Single Switch Topology
------------------------------------------------------

	In a single switch configuration, the best method to maximize
throughput depends upon the application and network environment.  The
various load balancing modes each have strengths and weaknesses in
different environments, as detailed below.

	For this discussion, we will break down the topologies into
two categories.  Depending upon the destination of most traffic, we
categorize them into either "gatewayed" or "local" configurations.

	In a gatewayed configuration, the "switch" is acting primarily
as a router, and the majority of traffic passes through this router to
other networks.  An example would be the following:


     +----------+                     +----------+
     |          |eth0            port1|          | to other networks
     | Host A   +---------------------+ router   +------------------->
     |          +---------------------+          | Hosts B and C are out
     |          |eth1            port2|          | here somewhere
     +----------+                     +----------+

	The router may be a dedicated router device, or another host
acting as a gateway.  For our discussion, the important point is that
the majority of traffic from Host A will pass through the router to
some other network before reaching its final destination.

	In a gatewayed network configuration, although Host A may
communicate with many other systems, all of its traffic will be sent
and received via one other peer on the local network, the router.

	Note that the case of two systems connected directly via
multiple physical links is, for purposes of configuring bonding, the
same as a gatewayed configuration.  In that case, it happens that all
traffic is destined for the "gateway" itself, not some other network
beyond the gateway.

	In a local configuration, the "switch" is acting primarily as
a switch, and the majority of traffic passes through this switch to
reach other stations on the same network.  An example would be the
following:

    +----------+            +----------+       +--------+
    |          |eth0   port1|          +-------+ Host B |
    |  Host A  +------------+  switch  |port3  +--------+
    |          +------------+          |                  +--------+
    |          |eth1   port2|          +------------------+ Host C |
    +----------+            +----------+port4             +--------+


	Again, the switch may be a dedicated switch device, or another
host acting as a gateway.  For our discussion, the important point is
that the majority of traffic from Host A is destined for other hosts
on the same local network (Hosts B and C in the above example).

	In summary, in a gatewayed configuration, traffic to and from
the bonded device will be to the same MAC level peer on the network
(the gateway itself, i.e., the router), regardless of its final
destination.  In a local configuration, traffic flows directly to and
from the final destinations, thus, each destination (Host B, Host C)
will be addressed directly by their individual MAC addresses.

	This distinction between a gatewayed and a local network
configuration is important because many of the load balancing modes
available use the MAC addresses of the local network source and
destination to make load balancing decisions.  The behavior of each
mode is described below.


12.1.1 MT Bonding Mode Selection for Single Switch Topology
-----------------------------------------------------------

	This configuration is the easiest to set up and to understand,
although you will have to decide which bonding mode best suits your
needs.  The trade offs for each mode are detailed below:

balance-rr: This mode is the only mode that will permit a single
	TCP/IP connection to stripe traffic across multiple
	interfaces. It is therefore the only mode that will allow a
	single TCP/IP stream to utilize more than one interface's
	worth of throughput.  This comes at a cost, however: the
	striping generally results in peer systems receiving packets out
	of order, causing TCP/IP's congestion control system to kick
	in, often by retransmitting segments.

	It is possible to adjust TCP/IP's congestion limits by
	altering the net.ipv4.tcp_reordering sysctl parameter.  The
	usual default value is 3, and the maximum useful value is 127.
	For a four interface balance-rr bond, expect that a single
	TCP/IP stream will utilize no more than approximately 2.3
	interface's worth of throughput, even after adjusting
	tcp_reordering.

	Note that the fraction of packets that will be delivered out of
	order is highly variable, and is unlikely to be zero.  The level
	of reordering depends upon a variety of factors, including the
	networking interfaces, the switch, and the topology of the
	configuration.  Speaking in general terms, higher speed network
	cards produce more reordering (due to factors such as packet
	coalescing), and a "many to many" topology will reorder at a
	higher rate than a "many slow to one fast" configuration.

	Many switches do not support any modes that stripe traffic
	(instead choosing a port based upon IP or MAC level addresses);
	for those devices, traffic for a particular connection flowing
	through the switch to a balance-rr bond will not utilize greater
	than one interface's worth of bandwidth.

	If you are utilizing protocols other than TCP/IP, UDP for
	example, and your application can tolerate out of order
	delivery, then this mode can allow for single stream datagram
	performance that scales near linearly as interfaces are added
	to the bond.

	This mode requires the switch to have the appropriate ports
	configured for "etherchannel" or "trunking."

active-backup: There is not much advantage in this network topology to
	the active-backup mode, as the inactive backup devices are all
	connected to the same peer as the primary.  In this case, a
	load balancing mode (with link monitoring) will provide the
	same level of network availability, but with increased
	available bandwidth.  On the plus side, active-backup mode
	does not require any configuration of the switch, so it may
	have value if the hardware available does not support any of
	the load balance modes.

balance-xor: This mode will limit traffic such that packets destined
	for specific peers will always be sent over the same
	interface.  Since the destination is determined by the MAC
	addresses involved, this mode works best in a "local" network
	configuration (as described above), with destinations all on
	the same local network.  This mode is likely to be suboptimal
	if all your traffic is passed through a single router (i.e., a
	"gatewayed" network configuration, as described above).

	As with balance-rr, the switch ports need to be configured for
	"etherchannel" or "trunking."

broadcast: Like active-backup, there is not much advantage to this
	mode in this type of network topology.

802.3ad: This mode can be a good choice for this type of network
	topology.  The 802.3ad mode is an IEEE standard, so all peers
	that implement 802.3ad should interoperate well.  The 802.3ad
	protocol includes automatic configuration of the aggregates,
	so minimal manual configuration of the switch is needed
	(typically only to designate that some set of devices is
	available for 802.3ad).  The 802.3ad standard also mandates
	that frames be delivered in order (within certain limits), so
	in general single connections will not see misordering of
	packets.  The 802.3ad mode does have some drawbacks: the
	standard mandates that all devices in the aggregate operate at
	the same speed and duplex.  Also, as with all bonding load
	balance modes other than balance-rr, no single connection will
	be able to utilize more than a single interface's worth of
	bandwidth.  

	Additionally, the linux bonding 802.3ad implementation
	distributes traffic by peer (using an XOR of MAC addresses),
	so in a "gatewayed" configuration, all outgoing traffic will
	generally use the same device.  Incoming traffic may also end
	up on a single device, but that is dependent upon the
	balancing policy of the peer's 8023.ad implementation.  In a
	"local" configuration, traffic will be distributed across the
	devices in the bond.

	Finally, the 802.3ad mode mandates the use of the MII monitor,
	therefore, the ARP monitor is not available in this mode.

balance-tlb: The balance-tlb mode balances outgoing traffic by peer.
	Since the balancing is done according to MAC address, in a
	"gatewayed" configuration (as described above), this mode will
	send all traffic across a single device.  However, in a
	"local" network configuration, this mode balances multiple
	local network peers across devices in a vaguely intelligent
	manner (not a simple XOR as in balance-xor or 802.3ad mode),
	so that mathematically unlucky MAC addresses (i.e., ones that
	XOR to the same value) will not all "bunch up" on a single
	interface.

	Unlike 802.3ad, interfaces may be of differing speeds, and no
	special switch configuration is required.  On the down side,
	in this mode all incoming traffic arrives over a single
	interface, this mode requires certain ethtool support in the
	network device driver of the slave interfaces, and the ARP
	monitor is not available.

balance-alb: This mode is everything that balance-tlb is, and more.
	It has all of the features (and restrictions) of balance-tlb,
	and will also balance incoming traffic from local network
	peers (as described in the Bonding Module Options section,
	above).

	The only additional down side to this mode is that the network
	device driver must support changing the hardware address while
	the device is open.

12.1.2 MT Link Monitoring for Single Switch Topology
----------------------------------------------------

	The choice of link monitoring may largely depend upon which
mode you choose to use.  The more advanced load balancing modes do not
support the use of the ARP monitor, and are thus restricted to using
the MII monitor (which does not provide as high a level of end to end
assurance as the ARP monitor).

12.2 Maximum Throughput in a Multiple Switch Topology
-----------------------------------------------------

	Multiple switches may be utilized to optimize for throughput
when they are configured in parallel as part of an isolated network
between two or more systems, for example:

                       +-----------+
                       |  Host A   | 
                       +-+---+---+-+
                         |   |   |
                +--------+   |   +---------+
                |            |             |
         +------+---+  +-----+----+  +-----+----+
         | Switch A |  | Switch B |  | Switch C |
         +------+---+  +-----+----+  +-----+----+
                |            |             |
                +--------+   |   +---------+
                         |   |   |
                       +-+---+---+-+
                       |  Host B   | 
                       +-----------+

	In this configuration, the switches are isolated from one
another.  One reason to employ a topology such as this is for an
isolated network with many hosts (a cluster configured for high
performance, for example), using multiple smaller switches can be more
cost effective than a single larger switch, e.g., on a network with 24
hosts, three 24 port switches can be significantly less expensive than
a single 72 port switch.

	If access beyond the network is required, an individual host
can be equipped with an additional network device connected to an
external network; this host then additionally acts as a gateway.

12.2.1 MT Bonding Mode Selection for Multiple Switch Topology
-------------------------------------------------------------

	In actual practice, the bonding mode typically employed in
configurations of this type is balance-rr.  Historically, in this
network configuration, the usual caveats about out of order packet
delivery are mitigated by the use of network adapters that do not do
any kind of packet coalescing (via the use of NAPI, or because the
device itself does not generate interrupts until some number of
packets has arrived).  When employed in this fashion, the balance-rr
mode allows individual connections between two hosts to effectively
utilize greater than one interface's bandwidth.

12.2.2 MT Link Monitoring for Multiple Switch Topology
------------------------------------------------------

	Again, in actual practice, the MII monitor is most often used
in this configuration, as performance is given preference over
availability.  The ARP monitor will function in this topology, but its
advantages over the MII monitor are mitigated by the volume of probes
needed as the number of systems involved grows (remember that each
host in the network is configured with bonding).

13. Switch Behavior Issues
==========================

13.1 Link Establishment and Failover Delays
-------------------------------------------

	Some switches exhibit undesirable behavior with regard to the
timing of link up and down reporting by the switch.

	First, when a link comes up, some switches may indicate that
the link is up (carrier available), but not pass traffic over the
interface for some period of time.  This delay is typically due to
some type of autonegotiation or routing protocol, but may also occur
during switch initialization (e.g., during recovery after a switch
failure).  If you find this to be a problem, specify an appropriate
value to the updelay bonding module option to delay the use of the
relevant interface(s).

	Second, some switches may "bounce" the link state one or more
times while a link is changing state.  This occurs most commonly while
the switch is initializing.  Again, an appropriate updelay value may
help.

	Note that when a bonding interface has no active links, the
driver will immediately reuse the first link that goes up, even if the
updelay parameter has been specified (the updelay is ignored in this
case).  If there are slave interfaces waiting for the updelay timeout
to expire, the interface that first went into that state will be
immediately reused.  This reduces down time of the network if the
value of updelay has been overestimated, and since this occurs only in
cases with no connectivity, there is no additional penalty for
ignoring the updelay.

	In addition to the concerns about switch timings, if your
switches take a long time to go into backup mode, it may be desirable
to not activate a backup interface immediately after a link goes down.
Failover may be delayed via the downdelay bonding module option.

13.2 Duplicated Incoming Packets
--------------------------------

	NOTE: Starting with version 3.0.2, the bonding driver has logic to
suppress duplicate packets, which should largely eliminate this problem.
The following description is kept for reference.

	It is not uncommon to observe a short burst of duplicated
traffic when the bonding device is first used, or after it has been
idle for some period of time.  This is most easily observed by issuing
a "ping" to some other host on the network, and noticing that the
output from ping flags duplicates (typically one per slave).

	For example, on a bond in active-backup mode with five slaves
all connected to one switch, the output may appear as follows:

# ping -n 10.0.4.2
PING 10.0.4.2 (10.0.4.2) from 10.0.3.10 : 56(84) bytes of data.
64 bytes from 10.0.4.2: icmp_seq=1 ttl=64 time=13.7 ms
64 bytes from 10.0.4.2: icmp_seq=1 ttl=64 time=13.8 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=1 ttl=64 time=13.8 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=1 ttl=64 time=13.8 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=1 ttl=64 time=13.8 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=2 ttl=64 time=0.216 ms
64 bytes from 10.0.4.2: icmp_seq=3 ttl=64 time=0.267 ms
64 bytes from 10.0.4.2: icmp_seq=4 ttl=64 time=0.222 ms

	This is not due to an error in the bonding driver, rather, it
is a side effect of how many switches update their MAC forwarding
tables.  Initially, the switch does not associate the MAC address in
the packet with a particular switch port, and so it may send the
traffic to all ports until its MAC forwarding table is updated.  Since
the interfaces attached to the bond may occupy multiple ports on a
single switch, when the switch (temporarily) floods the traffic to all
ports, the bond device receives multiple copies of the same packet
(one per slave device).

	The duplicated packet behavior is switch dependent, some
switches exhibit this, and some do not.  On switches that display this
behavior, it can be induced by clearing the MAC forwarding table (on
most Cisco switches, the privileged command "clear mac address-table
dynamic" will accomplish this).

14. Hardware Specific Considerations
====================================

	This section contains additional information for configuring
bonding on specific hardware platforms, or for interfacing bonding
with particular switches or other devices.

14.1 IBM BladeCenter
--------------------

	This applies to the JS20 and similar systems.

	On the JS20 blades, the bonding driver supports only
balance-rr, active-backup, balance-tlb and balance-alb modes.  This is
largely due to the network topology inside the BladeCenter, detailed
below.

JS20 network adapter information
--------------------------------

	All JS20s come with two Broadcom Gigabit Ethernet ports
integrated on the planar (that's "motherboard" in IBM-speak).  In the
BladeCenter chassis, the eth0 port of all JS20 blades is hard wired to
I/O Module #1; similarly, all eth1 ports are wired to I/O Module #2.
An add-on Broadcom daughter card can be installed on a JS20 to provide
two more Gigabit Ethernet ports.  These ports, eth2 and eth3, are
wired to I/O Modules 3 and 4, respectively.

	Each I/O Module may contain either a switch or a passthrough
module (which allows ports to be directly connected to an external
switch).  Some bonding modes require a specific BladeCenter internal
network topology in order to function; these are detailed below.

	Additional BladeCenter-specific networking information can be
found in two IBM Redbooks (www.ibm.com/redbooks):

"IBM eServer BladeCenter Networking Options"
"IBM eServer BladeCenter Layer 2-7 Network Switching"

BladeCenter networking configuration
------------------------------------

	Because a BladeCenter can be configured in a very large number
of ways, this discussion will be confined to describing basic
configurations.

	Normally, Ethernet Switch Modules (ESMs) are used in I/O
modules 1 and 2.  In this configuration, the eth0 and eth1 ports of a
JS20 will be connected to different internal switches (in the
respective I/O modules).

	A passthrough module (OPM or CPM, optical or copper,
passthrough module) connects the I/O module directly to an external
switch.  By using PMs in I/O module #1 and #2, the eth0 and eth1
interfaces of a JS20 can be redirected to the outside world and
connected to a common external switch.

	Depending upon the mix of ESMs and PMs, the network will
appear to bonding as either a single switch topology (all PMs) or as a
multiple switch topology (one or more ESMs, zero or more PMs).  It is
also possible to connect ESMs together, resulting in a configuration
much like the example in "High Availability in a Multiple Switch
Topology," above.

Requirements for specific modes
-------------------------------

	The balance-rr mode requires the use of passthrough modules
for devices in the bond, all connected to an common external switch.
That switch must be configured for "etherchannel" or "trunking" on the
appropriate ports, as is usual for balance-rr.

	The balance-alb and balance-tlb modes will function with
either switch modules or passthrough modules (or a mix).  The only
specific requirement for these modes is that all network interfaces
must be able to reach all destinations for traffic sent over the
bonding device (i.e., the network must converge at some point outside
the BladeCenter).

	The active-backup mode has no additional requirements.

Link monitoring issues
----------------------

	When an Ethernet Switch Module is in place, only the ARP
monitor will reliably detect link loss to an external switch.  This is
nothing unusual, but examination of the BladeCenter cabinet would
suggest that the "external" network ports are the ethernet ports for
the system, when it fact there is a switch between these "external"
ports and the devices on the JS20 system itself.  The MII monitor is
only able to detect link failures between the ESM and the JS20 system.

	When a passthrough module is in place, the MII monitor does
detect failures to the "external" port, which is then directly
connected to the JS20 system.

Other concerns
--------------

	The Serial Over LAN (SoL) link is established over the primary
ethernet (eth0) only, therefore, any loss of link to eth0 will result
in losing your SoL connection.  It will not fail over with other
network traffic, as the SoL system is beyond the control of the
bonding driver.

	It may be desirable to disable spanning tree on the switch
(either the internal Ethernet Switch Module, or an external switch) to
avoid fail-over delay issues when using bonding.

	
15. Frequently Asked 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).  For most modes,
devices need not be of the same speed.

	Starting with version 3.2.1, bonding also supports Infiniband
slaves in active-backup mode.

3.  How many bonding devices can I have?

	There is no limit.

4.  How many slaves can a bonding device have?

	This is limited only by the number of network interfaces Linux
supports and/or the number of network cards you can place in your
system.

5.  What happens when a slave link dies?

	If link monitoring is enabled, then the failing device will be
disabled.  The active-backup mode will fail over to a backup link, and
other modes will ignore the failed link.  The link will continue to be
monitored, and should it recover, it will rejoin the bond (in whatever
manner is appropriate for the mode). See the sections on High
Availability and the documentation for each mode for additional
information.
	
	Link monitoring can be enabled via either the miimon or
arp_interval parameters (described in the module parameters section,
above).  In general, miimon monitors the carrier state as sensed by
the underlying network device, and the arp monitor (arp_interval)
monitors connectivity to another host on the local network.

	If no link monitoring is configured, the bonding driver will
be unable to detect link failures, and will assume that all links are
always available.  This will likely result in lost packets, and a
resulting degradation of performance.  The precise performance loss
depends upon the bonding mode and network configuration.

6.  Can bonding be used for High Availability?

	Yes.  See the section on High Availability for details.

7.  Which switches/systems does it work with?

	The full answer to this depends upon the desired mode.

	In the basic balance modes (balance-rr and balance-xor), it
works with any system that supports etherchannel (also called
trunking).  Most managed switches currently available have such
support, and many unmanaged switches as well.

	The advanced balance modes (balance-tlb and balance-alb) do
not have special switch requirements, but do need device drivers that
support specific features (described in the appropriate section under
module parameters, above).

	In 802.3ad mode, it works with systems that support IEEE
802.3ad Dynamic Link Aggregation.  Most managed and many unmanaged
switches currently available support 802.3ad.

        The active-backup mode should work with any Layer-II switch.

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

	When using slave devices that have fixed MAC addresses, or when
the fail_over_mac option is enabled, the bonding device's MAC address is
the MAC address of the active slave.

	For other configurations, if not explicitly configured (with
ifconfig or ip link), 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 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 or ip link:

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

# ip link set bond0 address 66:77:88:99:aa:bb

	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 is added.

	To restore your slaves' MAC addresses, you need to detach them
from the bond (`ifenslave -d bond0 eth0'). The bonding driver will
then restore the MAC addresses that the slaves had before they were
enslaved.

16. Resources and Links
=======================

The latest version of the bonding driver can be found in the latest
version of the linux kernel, found on http://kernel.org

The latest version of this document can be found in either the latest
kernel source (named Documentation/networking/bonding.txt), or on the
bonding sourceforge site:

http://www.sourceforge.net/projects/bonding

Discussions regarding the bonding driver take place primarily on the
bonding-devel mailing list, hosted at sourceforge.net.  If you have
questions or problems, post them to the list.  The list address is:

bonding-devel@lists.sourceforge.net

	The administrative interface (to subscribe or unsubscribe) can
be found at:

https://lists.sourceforge.net/lists/listinfo/bonding-devel

Donald Becker's Ethernet Drivers and diag programs may be found at :
 - http://www.scyld.com/network/

You will also find a lot of information regarding Ethernet, NWay, MII,
etc. at www.scyld.com.

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