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Network boot across many VLANs


Hello,

I'm not sure exactly how to phrase the question. But, I'm researching how to PXE boot a server without having a DHCP/PXE server in each vlan.

Scenario: Datacenter with dozens of servers. 1 VLAN per server. Cisco switches and routers. Each server has a serial console available for remote management (OS and BIOS are configured for serial console). If an admin wants to re-install OS, they should be able to reboot the server and tell the BIOS to initiate a PXE boot request. A central install server is available to provide the DHCP and PXE boot images.

Has anyone tried this? I have been reading about the 'ip helper-address' for Cisco to relay DHCP requests. Interested in hearing about real-world setups. Or is there a better way to accomplish remote OS installs?



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  #2  
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I haven't tried it, but relaying DHCP requests should probably work. Another idea--if DHCP relay doesn't work for some reason--is to set up a trunk to the boot server and let it appear on whatever VLAN it wants. However your provisioning system configures the helper address on each segment, you can configure the boot server with an appropriate interface. You probably don't want DHCP always active on each segment or people will use it on accident.

  #3  
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We've done this in the past by simply creating one massive vLan (I believe that's how it works, I didn't set it up personally), and adding all ports to both their local vLan and the mass vLan meaning that DHCP addresses can be picked up if a static address isn't already configured.

It's not ideal but it does work pretty well.

Dan

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  #4  
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Join Date: Apr 2004
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Quote:
Originally Posted by sloop
Hello,
Reading about the 'ip helper-address' for Cisco to relay DHCP requests. Interested in hearing about real-world setups. Or is there a better way to accomplish remote OS installs?
Sure, I do it everyday.

Real simple:

int vlan 100
desc Example VLAN, of course
ip helper-address <ip of DHCP server>

Then the DHCP server which is capable of sending the PXE boot parameters (ISC DHCP, Windows DHCP, etc.) has scopes configured for each subnet and the requisite DHCP options in the DHCP server config file, which, in ISC's case is usually only next-server and filename. There are additional parameters that can be passed to a second-stage PXE bootloader like Grub, but in general I don't think you need to get into that kind of detail.

  #5  
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Join Date: Apr 2005
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Although it's possible to change the VLAN just to do a PXE boot, I wouldn't recommend it due to the added complexity. As has already been suggested, use 'ip helper-address' in the SVI/L3 subinterface configuration and save yourself a headache or two.

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  #6  
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Quote:
Originally Posted by serverminds
Sure, I do it everyday.

Real simple:

int vlan 100
desc Example VLAN, of course
ip helper-address <ip of DHCP server>

Then the DHCP server which is capable of sending the PXE boot parameters (ISC DHCP, Windows DHCP, etc.) has scopes configured for each subnet and the requisite DHCP options in the DHCP server config file, which, in ISC's case is usually only next-server and filename. There are additional parameters that can be passed to a second-stage PXE bootloader like Grub, but in general I don't think you need to get into that kind of detail.
The only way I can see this working is that the DHCP server must know the MAC address of every server that may want to network boot, and know the ip config of that server's vlan to pass it along appropriately. Am I right?

Rather than maintaining all of that configuration in the DHCP server it may actually be easier as tical suggested to have a network admin change the vlan that the server is in while network booting. Or am I missing something?

  #7  
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Join Date: Dec 2006
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I agree with serverminds. An ip helper-address will send DHCP requests thru the router to your DHCP server, and that should be all you need!

  #8  
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Join Date: Apr 2004
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Quote:
Originally Posted by sloop
The only way I can see this working is that the DHCP server must know the MAC address of every server that may want to network boot, and know the ip config of that server's vlan to pass it along appropriately. Am I right?
No, you're not

Well, actually, you're sort of right in the sense that you could use DHCP to pass PXE netboot parameters to a server based on MAC address, but in practice it's highly improbable that you would. That's a really ugly implementation in any kind of large-ish environment (>50 servers).
Quote:
Rather than maintaining all of that configuration in the DHCP server it may actually be easier as tical suggested to have a network admin change the vlan that the server is in while network booting. Or am I missing something?
In practice you'll use the second-stage boot loader (pxelinux, pxegrub) to send across the proper configuration based on the MAC address -- if you even want to do it that way. You can have your standard server profiles defined in the boot loader's configuration and automatically or manually load the OS based on that config. Just pick the one you want when the boot loader boots off the network.

Every VLAN will point to the DHCP server that will pass the PXE netboot parameters back to every VLAN. It's really very simple and as long as you set the PXE boot options on to the global scope options you don't have to define them for every VLAN (potentially thousands in a big enough environment).

Trust me on this--I've tried to commercialize a PXE service. Hopefully I can get the open source re-coding done soon so others can take advantage of whatever insights I gained when setting it up.

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