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  1. #1
    Join Date
    May 2002
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    Raleigh, NC
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    Post 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?

  2. #2
    Join Date
    Aug 2005
    Location
    Los Angeles, CA
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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. #3
    Join Date
    Jan 2004
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    North Yorkshire, UK
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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. #4
    Join Date
    Apr 2004
    Location
    SF Bay Area
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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. #5
    Join Date
    Apr 2005
    Location
    Jacksonville, FL
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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. #6
    Join Date
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    690
    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. #7
    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. #8
    Join Date
    Apr 2004
    Location
    SF Bay Area
    Posts
    876
    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).
    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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