Help with pxe guest VM install

[Phil Meyer]

On 11/07/2012 11:35 AM, William Murray wrote:
>   Dear list,
>       I have been trying to setup a PXE guess installation on Fedora 
> 17 with no success.
> I build a bridge, and connect my VM to it but DHCP times out in the VM 
> install.
> I don't know how to debug this bridge. I guess I could connect a 
> 'live' machine to it,
> but what then?
>
> I follow:
>
> http://docs.fedoraproject.org/en-US/Fedora/13/html/Virtualization_Guide/sect-Virtualization-Network_Configuration-Bridged_networking_with_libvirt.html 
>
> and
> https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/sect-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Guest_Installation-Installing_guests_with_PXE.html 
>
>
> These guides almost work. Two differences show up:
>  brctl show
> bridge name    bridge id        STP enabled    interfaces
> br0        8000.14feb5b90f56    no        p5p1
> virbr0        8000.525400da475b    yes        virbr0-nic

First make sure that ifcfg-br0 contains:
     DELAY=0
     as well as:
     STP=on
     which you have already shown is on.

When a new interface comes up on a switch or router, care needs to be 
taken so that the interface does not cause Spanning Tree issues. Even 
with fastboot enabled on switches and routers, there is a normal delay 
for STP.  That delay may be just long enough to prevent pxe from getting 
a response.  It is really an issue of a second or two.

Since the host is playing the role of the 'bridge', it is trying to play 
nice on your network, which is correct.  However, since the host 
interface is already connected, a spanning tree issue is extremely 
unlikely and it is normal and desirable to turn off the STP delay.

You can confirm this behavior using tshark on the bridge interface.

Your second question is whether the network sees the VM MAC or the host 
MAC on that interface.

A network bridge device can have its own MAC and IP as well as pass 
other packets 'raw' through its bridged interfaces.  Those bridged 
packets will not be wrapped and the VM client MAC(s) will appear in the 
MAC table on the switch for that interface.  They will work exactly as 
if you had plugged the VM into the same switch as the host.

So your dhcp server for pxe needs to watch for the VM client interface 
data, not the host.

Good Luck!