Thanks everyone for your comments and advice.
It seemed I had commented out the HWADDR and UUID for some strange reason. I have now added them back in and all is back to normal.
Now I know the importance of those values.
Cheers
Bill
 
-----Original message-----
> From:~Stack~ <[log in to unmask]>
> Sent: Monday 27th February 2017 1:58
> To: [log in to unmask]
> Subject: Re: Strange network device name chages on reboot of SL7 kvm guests
> 
> On 02/25/2017 10:18 PM, Bill Maidment wrote:
> > Hi again
> Greetings!
> 
> > I have recently rebooted KVM guests with two virtual NICs (e.g. /dev/ens4 and /dev/eth0) only to find that the device name of the eth0 changes to eth1 and so the ifcfg-eth0 doesn't match.
> > So I fix the ifcfg file and restart network - all OK.
> > On a later reboot eth1 changes back to eth0.
> > What is going on? Anyone else observe this phenomena?
> 
> Yes. It is part of the goofy new naming structure. It's nice when it
> works, but it seems to not-work more often then it does-work. Then it is
> infuriating.
> <Insert your preferred choice of systemd-udev anger rant here>
> 
> Anyway...
> 
> Here's the short version.
> en: ethernet prefix
> o: onboard
> s: slot
> p: physical location of connector.
> 
> If the device is "unknown" then it gets "eth". And of course the numbers
> increment for each new device.
> 
> So what your device names tell me is that you have one card in slot 4
> and a second card that the OS can't figure out where it goes.
> 
> There are two ways of "fixing" this.
> 1)
> *If* you have your HWADDR= assigned in your
> /etc/sysconfig/network-scripts/ifcfg-eth0 then it *should* always get
> that interface regardless of what the OS detects it to be on boot. I
> have found this isn't always the case with some specialty cards that
> take FOREVER to initialize. I tend to throw in the UUID as well and that
> seems to resolve the problems for those cards.
> 
> 2)
> Now, here is the odd part because this next problem is also on my big
> KVM host. None of the above worked. I never found a good answer to why
> and I am wondering if it is related to KVM...but at this point my sample
> size is two so probably not something to make a solid educated guess on...
> 
> Anyway..
> 
> Read this:
> https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/ch-Consistent_Network_Device_Naming.html
> 
> There are some good tips throughout chapter 8 on how to tweak and/or
> disable udev from scanning devices. I had to rewrite the udev rules on
> my KVM box right after 7.1 released because *every* *single* reboot
> broke my networking. However, after I adjusted the udev rules based on
> the RH documentation I haven't had a problem with reboots since.
> 
> I _really_ hope that #1 fixes your issues as it is by far the easiest to
> do and manage. If not, the udev rules should do the trick.
> 
> One last thought. In that documentation there is something called
> "biosdevname"; I'm pretty sure that wasn't in the docs when I was having
> a problem with 7.1 as I don't recall seeing it before, but it looks
> interesting. You might just want to go through that chapter and give
> those suggestions a go.
> 
> Hope that helps!
> 
> ~Stack~
> 
>