Using Multiple interfaces with KVM and Xen

I’ve been meaning to write a post on ethernet bridges and how they can easily be used to accommodate virtual machines with their “own” physical NIC(Network Interface Card).  I see a ton of post online about people struggling to get multiple ethernet cards to work in Xen and other hypervisors like KVM.  A common complaint is that when both NIC cards are plugged into the LAN they lose connectivity from all machines including the host.

Many sites make an attempt to explain the problem of multiple interfaces on the same network by walking you through a Xen custom configuration.  However they fail to identify the concept of bridges, layer 2 loops and why Spanning Tree Protocol is your friend!  So many virtualization nuts(like myself) spend hours trying to find a problem with Xen, Vmware, KVM, whatever… when the problem may just be how the interfaces are configured.

The goals of this post:

* define ethernet bridging

* explain ethernet loops

* discuss how this relates to VM’s and the hypervisor

* LAB: set up two ethernet cards for guest VM and my Fedora 10 KVM Server