
                   Linux Ethernet Bonding Driver mini-howto

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>

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

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


Table of Contents
=================
 
Installation
Bond Configuration
Module Parameters
Configuring Multiple Bonds
Switch Configuration
Verifying Bond Configuration
Frequently Asked Questions
High Availability
Promiscuous Sniffing notes
Limitations
Resources and Links


Installation
============

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

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

Build and install the new kernel and modules.

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

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

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


Bond Configuration
==================

You will need to add at least the following line to /etc/modules.conf
so the bonding driver will automatically load when the bond0 interface is 
configured. Refer to the modules.conf manual page for specific modules.conf 
syntax details. The Module Parameters section of this document describes each 
bonding driver parameter. 

	alias bond0 bonding

Use standard distribution techniques to define the bond0 network interface. For
example, on modern Red Hat distributions, create an ifcfg-bond0 file in
the /etc/sysconfig/network-scripts directory that resembles the following:

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

(use appropriate values for your network above)

All interfaces that are part of a bond should have SLAVE and MASTER
definitions. For example, in the case of Red Hat, if you wish to make eth0 and
eth1 a part of the bonding interface bond0, their config files (ifcfg-eth0 and
ifcfg-eth1) should resemble the following:

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

Use DEVICE=eth1 in the ifcfg-eth1 config file. If you configure a second bonding
interface (bond1), use MASTER=bond1 in the config file to make the network
interface be a slave of bond1.

Restart the networking subsystem or just bring up the bonding device if your
administration tools allow it. Otherwise, reboot. On Red Hat distros you can 
issue `ifup bond0' or `/etc/rc.d/init.d/network restart'.

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

    # /sbin/ifconfig bond0 192.168.1.1 netmask 255.255.255.0 \
      broadcast 192.168.1.255 up

    # /sbin/ifenslave bond0 eth0
    # /sbin/ifenslave bond0 eth1

(use appropriate values for your network above)

You can then create a script containing these commands and place it in the 
appropriate rc directory.

If you specifically need all network drivers loaded before the bonding driver,
adding the following line to modules.conf will cause the network driver for 
eth0 and eth1 to be loaded before the bonding driver.

probeall bond0 eth0 eth1 bonding

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

To have device characteristics (such as MTU size) propagate to slave devices, 
set the bond characteristics before enslaving the device.  The characteristics 
are propagated during the enslave process.

If running SNMP agents, the bonding driver should be loaded before any network 
drivers participating in a bond. This requirement is due to the 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.


Module Parameters
=================

Optional parameters for the bonding driver can be supplied as command line 
arguments to the insmod command. Typically, these parameters are specified in 
the file /etc/modules.conf (see the manual page for modules.conf). 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.

mode
        Specifies one of four bonding policies. The default is round-robin.
        Possible values are:
 
        0       Round-robin policy: Transmit in a sequential order from the
                first available slave through the last. This mode provides
                load balancing and fault tolerance.

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

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

miimon
 
        Specifies the frequency in milli-seconds that MII link monitoring will 
        occur. A value of zero disables MII link monitoring. A value of 
        100 is a good starting point. See High Availability section for 
        additional information. The default value is 0.

downdelay
 
        Specifies the delay time in milli-seconds to disable a link after a 
        link failure has been detected. This should be a multiple of miimon
        value, otherwise the value will be rounded. The default value is 0.

updelay
 
        Specifies the delay time in milli-seconds to enable a link after a 
        link up status has been detected. This should be a multiple of miimon
        value, otherwise the value will be rounded. The default value is 0.
 
arp_interval
 
        Specifies the ARP monitoring frequency in milli-seconds. 
        If ARP monitoring is used in a load-balancing mode (mode 0 or 2), the 
        switch should be configured in a mode that evenly distributes packets 
        across all links - such as round-robin. 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 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 adresses must be seperated by a comma. At least one ip
        address needs to be given for ARP monitoring to work. The maximum number
        of targets that can be specified is set at 16.

primary

        A string (eth0, eth2, etc) to equate to a primary device. If this 
        value is entered, and the device is on-line, it will be used first as 
        the output media. Only when this device is off-line, will alternate 
        devices be used. Otherwise, once a failover is detected and a new 
        default output is chosen, it will remain the output media until it too 
        fails. This is useful when one slave was preferred over another, i.e. 
        when one slave is 1000Mbps and another is 100Mbps. If the 1000Mbps 
        slave fails and is later restored, it may be preferred the faster slave
        gracefully become the active slave - without deliberately failing the
        100Mbps slave. Specifying a primary is only valid in active-backup mode.

multicast

        Integer value for the mode of operation for multicast support.
        Possible values are:

        0       Disabled (no multicast support)

        1       Enabled on active slave only, useful in active-backup mode

        2       Enabled on all slaves, this is the default


Configuring Multiple Bonds
==========================

If several bonding interfaces are required, the driver must be loaded
multiple times. For example, to configure two bonding interfaces with link 
monitoring performed every 100 milli-seconds, the /etc/conf.modules should
resemble the following:

alias bond0 bonding
alias bond1 bonding

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

Configuring Multiple ARP Targets
================================

While ARP monitoring can be done with just one target, it can be usefull
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) would increase the reliability of the ARP monitoring.
Multiple ARP targets must be seperated 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



Switch Configuration
====================

While the switch does not need to be configured when the active-backup
policy is used (mode=1), it does need to be configured for the round-robin, 
XOR, and broadcast policies (mode=0, mode=2, and mode=3). 


Verifying Bond Configuration
============================

1) Bonding information files
----------------------------
The bonding driver information files reside in the /proc/net/bond* directories. 

Sample contents of /proc/net/bond0/info after the driver is loaded with 
parameters of mode=0 and miimon=1000 is shown below.
 
        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

2) Network verification
-----------------------
The network configuration can be verified using the ifconfig command. 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.

[root]# /sbin/ifconfig
bond0     Link encap:Ethernet  HWaddr 00:C0:F0:1F:37:B4  
          inet addr:XXX.XXX.XXX.YYY  Bcast:XXX.XXX.XXX.255  Mask: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  
          inet addr:XXX.XXX.XXX.YYY  Bcast:XXX.XXX.XXX.255  Mask:255.255.252.0
          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  
          inet addr:XXX.XXX.XXX.YYY  Bcast:XXX.XXX.XXX.255  Mask:255.255.252.0
          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 


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).
	You can even bond together Gigabit Ethernet cards!

3.  How many bonding devices can I have?

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

4.  How many slaves can a bonding device have?

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

5.  What happens when a slave link dies?

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

	For ethernet cards not supporting MII status, the arp_interval and
        arp_ip_target parameters must be specified for bonding to work
        correctly. If packets have not been sent or received during the
        specified arp_interval durration, an ARP request is sent to the targets 
        to generate send and receive traffic. If after this interval, either 
        the successful send and/or receive count has not incremented, the next 
        slave in the sequence will become the active slave.

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

6.  Can bonding be used for High Availability?

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

7.  Which switches/systems does it work with?

	In round-robin and XOR mode, it works with systems that support 
	trunking:
	
	* Cisco 5500 series (look for EtherChannel support).
	* SunTrunking software.
	* Alteon AceDirector switches / WebOS (use Trunks).
	* BayStack Switches (trunks must be explicitly configured). Stackable
	  models (450) can define trunks between ports on different physical
	  units.
	* Linux bonding, of course !
	
	In active-backup mode, it should work with any Layer-II switche.


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

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

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

	The MAC address can be also changed by bringing down/up the device
	and then changing its slaves (or their order):
	
	  # ifconfig bond0 down ; modprobe -r bonding
	  # ifconfig bond0 .... up
	  # ifenslave bond0 eth...

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

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

9.  Which transmit polices can be used?

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

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

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

	Broadcast policy transmits everything on all slave interfaces.


High Availability
=================

To implement high availability using the bonding driver, the driver needs to be
compiled as a module, because currently it is the only way to pass parameters 
to the driver. This may change in the future.

High availability is achieved by using MII or ETHTOOL status reporting. You 
need to verify that all your interfaces support MII or ETHTOOL link status 
reporting.  On Linux kernel 2.2.17, all the 100 Mbps capable drivers and 
yellowfin gigabit driver support MII. To determine if ETHTOOL link reporting 
is available for interface eth0, type "ethtool eth0" and the "Link detected:" 
line should contain the correct link status. If your system has an interface 
that does not support MII or ETHTOOL status reporting, a failure of its link 
will not be detected! A message indicating MII and ETHTOOL is not supported by 
a network driver is logged when the bonding driver is loaded with a non-zero 
miimon value.

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

Example:

   # modprobe bonding miimon=100

Or, put the following lines in /etc/modules.conf:

   alias bond0 bonding
   options bond0 miimon=100

There are currently two policies for high availability. They are dependent on 
whether:

   a) hosts are connected to a single host or switch that support trunking

   b) hosts are connected to several different switches or a single switch that
      does not support trunking


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

Example 1 : host to host at twice the speed

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

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

Example 2 : host to switch at twice the speed

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

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


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

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

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

    # modprobe bonding miimon=100 mode=1

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

    alias bond0 bonding
    options bond0 miimon=100 mode=1

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


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

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

Each time a host changes its active interface, it sticks to the new one until
it goes down. In this example, the hosts are negligibly affected by the
expiration time of the switches' forwarding tables.

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

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


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

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

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


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

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

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

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

Examples :

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


Promiscuous Sniffing notes
==========================

If you wish to bond channels together for a network sniffing
application --- you wish to run tcpdump, or ethereal, or an IDS like
snort, with its input aggregated from multiple interfaces using the
bonding driver --- then you need to handle the Promiscuous interface
setting by hand. Specifically, when you "ifconfing bond0 up" you
must add the promisc flag there; it will be propagated down to the
slave interfaces at ifenslave time; a full example might look like:

   grep bond0 /etc/modules.conf || echo alias bond0 bonding >/etc/modules.conf
   ifconfig bond0 promisc up
   for if in eth1 eth2 ...;do
       ifconfig $if up
       ifenslave bond0 $if
   done
   snort ... -i bond0 ...

Ifenslave also wants to propagate addresses from interface to
interface, appropriately for its design functions in HA and channel
capacity aggregating; but it works fine for unnumbered interfaces;
just ignore all the warnings it emits.


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

  - A Transmit Load Balancing policy is not currently available. This mode 
    allows every slave in the bond to transmit while only one receives. If 
    the "receiving" slave fails, another slave takes over the MAC address of 
    the failed receiving slave.


Resources and Links
===================

Current development on this driver is posted to:
 - http://www.sourceforge.net/projects/bonding/

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.

For new versions of the driver, patches for older kernels and the updated
userspace tools, take a look at Willy Tarreau's site :
 - http://wtarreau.free.fr/pub/bonding/
 - http://www-miaif.lip6.fr/willy/pub/bonding/

To get latest informations about Linux Kernel development, please consult
the Linux Kernel Mailing List Archives at :
   http://boudicca.tux.org/hypermail/linux-kernel/latest/

-- END --
