From HiPBX Wiki

Below is an example of how to set up a reliable and redundant cabling layout.

You'll notice that everything is redundant. The 'bond1' interface runs two VLAN's - one for DRBD, and one for Heartbeat. There are enough spare USB ports to cover a failure of a USB card, and the machine runs N+1 power supplies - that is, the server can run with only one power supply receiving power. This is extremely handy when you need to do UPS maintenance, or the power goes out. As long as the machines are receiving power from either of the power supplies, everything's still up.

There are two bonded interfaces (4 network cards). An important thing to note is that this is not the best way to do this. The better way to do this is to bond the interfaces across network cards. I did try that originally, but the cabling looks messier, and isn't as easy to understand. So, in real life, your cabling would be slightly different to this.

The two onboard NICs are eth0 and eth1, whilst the PCIe nics are eth2 and eth3. They're not