Bonding network cards: or “How I learned to stop worrying and love 2Gb/s ethernet”.

Most servers ship with at least two network ports, often more.

And yet so often we plug one of them into a switch and ignore the other one. We’ve paid for an expensive server with very capable networking, and now we’re going to ignore half of its capabilities. Meanwhile, we’re asking it to do more and more. Sooner or later, that gigabit network port is the bottleneck.

Why not use both network ports simultaneously?

There’s various ways you can set this up. Some require special configuration of managed switches; some don’t. For this blog post, I’m going to concentrate on methods that don’t require special switch configuration because they’re a little bit easier and they’re somewhat less fragile – you don’t risk your network collapsing in a big heap just because someone plugged a network port into the wrong socket. (We also get the added bonus that if one of the two network ports in our server fails, it’ll still work, albeit more slowly. But I can’t remember the last time I saw a network port fail…)

These instructions are written purely for Debian Squeeze. You may need minor tweaks to use them in Ubuntu; you’ll almost certainly need significant changes to use them in other distributions.

First, install the ifenslave package:

apt-get install ifenslave

Configuration is just a few lines in /etc/network/interfaces:

# The primary network interface
auto bond0
iface bond0 inet static
    address 192.168.42.43
    broadcast 192.168.42.255
    netmask 255.255.255.0
    gateway 192.168.42.254
    network 192.168.42.0
    bond_mode balance-tlb
    bond_miimon 100
    bond_downdelay 200
    bond_updelay 200
    slaves eth0 eth1

Whatever you do, make sure you test everything on a non-production network first!

More information: Linux Kernel bonding documentation

About the Author: James Cort

James Cort is Managing Director of Bediwin Information Services, providing IT management and integration services in the South West of England.
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