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What I have done many times and what works quite well is the following scenario:

1 NIC connected to a bridge and then N VMs connected to the network via that bridge (on Ubuntu 10.4).

My newest server now has 2 NICs and I thought I do the same and in addition simply add the second NIC to the same physical LAN and to the bridge (using bridge_ports eth0 eth1); hopefully getting redundancy and higher throughput. This does not seem to work very well. Network traffic is extremely unstable irrespective of network load. Even my ssh session keeps dropping out and reconnecting all the time.

Now my questions:

  • Should my approach basically work or am I doing something wrong? E.g. fundamentally misunderstanding what bridges can do with 2 NICs?
  • I read that there is also NIC bonding that can combine two NICs. Does this have a higher chance of working? And if yes would I then still add a bridge to the bond and the VMs to the bridge?

For reference my /etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface   
auto lo
iface lo inet loopback

# The primary network interface
auto br0
iface br0 inet dhcp
      bridge_ports eth0 eth1

Note: This question is the same as this one on server fault. Since it got no answers and I am using Ubuntu I hope to have more luck here.

Thanks, Carsten

share|improve this question
What switch do your NICs connect to? In Cisco terms, you'll need to set up a PortChannel, specifying the two ports. Even then, Cisco only really supports per Mac load balancing, so if the majority of your traffic is to a single client, you'll only use a single NIC. Still very useful for resiliency however and multiple clients will generate hits to each connection. Not sure how to set up the bond on Linux, however (although a quick Google on "ubuntu bond NIC portchannel) generated a few possibilities. – Scaine Dec 8 '10 at 12:36
up vote 1 down vote accepted

From networking point of view - bridging of both physical interfaces caused the problem. If you want to make some kind of speed improvement you should go bonding route. Other possibility is to do load balancing over these 2 interfaces using ECMP and ip-tables.

Sadly enough i cannot provide correct syntax on how to do this as i only can do this on some proprietary system.

For ECMP - ip-tables provides connection counters (at least i think they do), use those to select odd and even connections and send them through one or another connection. Be vary if there are some NAT in the middle as that can cause problems with establishing related connections. Also, services with several connections may fail.

About boding - that would also work, but you have to make bonding on both ends of the link, other way it will not work.

share|improve this answer
Thanks for your reply. Just to clarify I understand your answer: Having 2 bridged NICs on the same switch, or on 2 switches in the same LAN will confuse the network and is never a good idea? – user7054 Dec 10 '10 at 1:09
if you have dedicated hardware, then it is called switch, same job can be done in software using bridge, so it is not clear for me what exactly you want. If your bridge support STP or RSTP protocol then that should be fine and no network storms should bring your network down if by change some loop appear. – Osis Jan 28 '11 at 10:26

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