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Scenario:

We are migrating DHCP off our Cisco gear and onto a Red Hat 6.7 server. I am concerned about clients who will not release and renew for several days. Since the new server will not have any bindings once it goes into production, how does it know NOT to hand out an IP that may already be in use? I have read that linux may ping the IP before handing it out, but I have not seen any official documentation on that and could not see that type of activity in my dhcpd logs when I was testing.

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Could you not shorten the lease time on your CISCO device before finally closing it down ?

That would ensure that the clients were more likely to re-request using the new DHCP device.

  • We could, although we are in a bind and may not have that luxury of waiting for the clients to pick up a shorter lease time before we make the change. – user53029 Feb 18 '16 at 18:14
  • So make the DHCP Lease change now. – Tim Seed Feb 18 '16 at 18:16
  • Not sure I follow what you are suggesting. And I appreciate the help, but do you have any insights on the specific question that was asked? – user53029 Feb 18 '16 at 18:18
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    Alter your current CISCO DHCP lease time now to being 1 Hour.... As soon as you switch the hardware (assuming it is in 2-3 days) then the chances of DHCP clashes will be minimised - more so if you can swap the DHCP server during quiet times. – Tim Seed Feb 18 '16 at 18:22
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    The DHCP server should check that the address is not in use before offering it to a client, but that function can be disabled. I'd have to imagine though that it is enabled in every commercially available OS/DHCP server implementation. Upon receiving an offer a client also should check that the address is not in use. So you have two independent mechanisms that should ensure that the new DHCP server doesn't offer an address that is already in use. Shortening the DHCP lease duration on the existing DHCP server will have no bearing on DHCP clients that don't have an existing lease. – joeqwerty Feb 18 '16 at 20:49

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