How to increase time to get DHCP lease?

  • Hi guys,

    Hope you can help me. My current network is slow getting DHCP leases. I suspect from STP, but right now I can’t reconfigure our switches.

    Maybe it’s possible to increase the time to get a DHCP lease during the pxe boot?


  • Moderator

    @asch75 said in How to increase time to get DHCP lease?:

    Next thing: redesign our network topology and reconfigure the cisco switch/es.

    Glad you have it sorted out. All you “need” to do is enable portfast on all of the interfaces where you need to pxe boot from. As you found the dumb switch is the quickest check if its spanning tree or not. Plus with it inline you can image before you get your infrastructure updated. So it has an extra benefit.

  • @george1421 Thanks for your support! Tested the dumb switch and… BINGO! You’re right. Mistery solved.
    I’m actively studying for CCNA right now, so I understand the STP behaviour.
    Next thing: redesign our network topology and reconfigure the cisco switch/es.


  • Moderator

    @asch75 I have access to a fog server now.

    So in dhcp option 67 you will want to configure into that field

    Bios: 10secdelay/undionly.kpxe
    uefi: 10secdelay/ipxe.efi

    Realize that 10 seconds is not enough for standard STP to start forwarding packet. That forwarding timeout is 27 seconds. But I’m hoping that PC boot time plus the 10 second delay will be just enough.

  • @george1421 Thanks for your ultrafast response.
    I will try to place a dumb swith and see the results.
    Also I will try the image in /tftpboot/10secdelay/


  • Moderator

    @asch75 To test for a spanning tree issue place a dumb (cheap unmanaged switch) between building switch and the pxe booting computer. When you pxe boot that should mask a STP issue. This way you know for sure or not if its a problem.

    There is a ipxe boot image that delays dhcp for 10 seconds that you can try. Its in the tftpboot directory there is a folder called 10second or something like that (I’m not in front of a fog server at the moment). Try that boot file.

    If you can absolutely not fix a spanning tree issue we can look into creating a custom boot loaders. We can provide instructions for you.