Bonding for link aggregation must be supported by both endpoints. Two linux machines connected via crossover cables can take advantage of link aggregation. A single machine connected with two physical cables to a switch which supports port trunking can use link aggregation to the switch. Any conventional switch will become ineffably confused by a hardware address appearing on multiple ports simultaneously.

[root@real-server root]# modprobe  bonding
[root@real-server root]# ip addr add 192.168.100.33/24 brd + dev bond0
[root@real-server root]# ip link set dev bond0 up
[root@real-server root]# ifenslave  bond0 eth2 eth3
master has no hw address assigned; getting one from slave!
The interface eth2 is up, shutting it down it to enslave it.
The interface eth3 is up, shutting it down it to enslave it.
[root@real-server root]# ifenslave  bond0 eth2 eth3
[root@real-server root]# cat /proc/net/bond0/info
Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth2
MII Status: up
Link Failure Count: 0

Slave Interface: eth3
MII Status: up
Link Failure Count: 0
        

FIXME; Need an experiment here....maybe a tcpdump to show how the management frames appear on the wire.

This Beowulf software page describes in a bit more detail the rationale and a practical application of linux channel bonding (for link aggregation).

Bonding support under linux is part of a high availability solution. For an entry point into the complexity of high availability in conjunction with linux, see the linux-ha.org site. To guard against layer two (switch) and layer one (cable) failure, a machine can be configured with multiple physical connections to separate switch devices while presenting a single logical interface to userspace.

The name of the interface can be specified by the user. It is commonly bond0 or something similar. As a logical interface, it can be used in routing tables and by tcpdump.

The bond interface, when created, has no link layer address. In the example below, an address is manually added to the interface. See Example 2.12, “Link aggregation bonding” for an example of the bonding driver reporting setting the link layer address when the first device is enslaved to the bond (doesn't that sound cruel!).

[root@real-server root]# modprobe bonding mode=1 miimon=100 downdelay=200 updelay=200
[root@real-server root]# ip link set dev bond0 addr 00:80:c8:e7:ab:5c
[root@real-server root]# ip addr add 192.168.100.33/24 brd + dev bond0
[root@real-server root]# ip link set dev bond0 up
[root@real-server root]# ifenslave  bond0 eth2 eth3
The interface eth2 is up, shutting it down it to enslave it.
The interface eth3 is up, shutting it down it to enslave it.
[root@real-server root]# ip link show eth2 ; ip link show eth3 ; ip link show bond0
4: eth2: <BROADCAST,MULTICAST,SLAVE,UP> mtu 1500 qdisc pfifo_fast master bond0 qlen 100
  link/ether 00:80:c8:e7:ab:5c brd ff:ff:ff:ff:ff:ff
5: eth3: <BROADCAST,MULTICAST,NOARP,SLAVE,DEBUG,AUTOMEDIA,PORTSEL,NOTRAILERS,UP> mtu 1500 qdisc pfifo_fast master bond0 qlen 100
  link/ether 00:80:c8:e7:ab:5c brd ff:ff:ff:ff:ff:ff
58: bond0: <BROADCAST,MULTICAST,MASTER,UP> mtu 1500 qdisc noqueue
  link/ether 00:80:c8:e7:ab:5c brd ff:ff:ff:ff:ff:ff
        

Immediately noticeable, there is a new flag in the ip link show output. The MASTER and SLAVE flags clearly report the nature of the relationship between the interfaces. Also, the Ethernet interfaces indicate the master interface via the keywords master bond0.

Note also, that all three of the interfaces share the same link layer address, 00:80:c8:e7:ab:5c.

FIXME; What doe DEBUG,AUTOMEDIA,PORTSEL,NOTRAILERS mean?