[PLUG-TALK] PXE and Class B

Larry Brigman larry.brigman at gmail.com
Thu Mar 24 08:39:39 PDT 2011

On Thu, Mar 24, 2011 at 8:21 AM, Daniel Herrington
<dherrington at robertmarktech.com> wrote:
> All,
> Anybody out there work with Boot servers? I'm seeing an issue with a
> vendor supplied boot server using a class B subnet. I have a DHCP
> server, a bootp server at, a target server at,
> and a gateway at There is no routing hardware between the the
> bootp, DHCP, and target servers.
I run a pxe/dhcp server on a class B network.  The info you provided above is
a class A network.

> Form my understanding, the target and bootp servers should talk to each
> other without needing to go through the gateway. It should be MAC
> address to MAC address since they are within the same subnet.

It uses a broadcast.  The dhcpd process should be listening on the
same subnet but
if you have things configured correctly; you could have the pxe/bootp server
on any subnet you want.

Here is an example from my configuration.
subnet netmask {
  option domain-name-servers;
  option domain-name "mydomain.com";
  option routers;
  option broadcast-address;
  default-lease-time 7200;
  max-lease-time 7200;
# add host entries in order

group {
  filename "linux-install/pxelinux.0";
    <....host entries go here...>
> The problem is the target gets a DHCP address, but never gets the
> pxeboot file. The reason could be that the pxe request is never recieved
> by the bootp server because PXE doesn't go across subnets. I'm unclear
> if it will traverse class b networks though, or just stay at the Class C
> level. The other cause could be the vendor's implementation of the bootp
> server.
That depends on the pxe implementation.  By spec it should pick up the
route from the dhcp-answer

> My question is, does standard PXE traverse class B subnets?

It can depending on your configuration. next-server and routers
entries plus pxe implementation will
determine if this will work for you.

The other course of action is to boot to a gpxe client with will allow
you to finish the boot from a http server.

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