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SRobertJames
Asked: 2011-01-16 20:41:37 +0800 CST

Can I replace a router and DHCP server without disrupting traffic?

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I have a device which acts as a router and DHCP server. I'm replacing it, and would like to minimize down time.

  1. If I unplug it, and plug in a different device with the same IP, will all the PCs with DHCP leases keep on working? (I have DHCP Conflict detect on, so it shouldn't reassign a DHCP address already used).

  2. What if I want to change the IP (new subnet) - is there anyway to tell all the clients (Windows PCs) to release their DHCP leases and request new ones in a minute?

  3. If before unplugging the old device, I have it release all DHCP leases, will the Windows PCs automatically ask for new DHCP addresses?

windows networking dhcp cisco high-availability

2 Answers

  1. DCHP lease are client based. It's the client that request lease, ask later if it can renew it (to extend it and so keep the ip).

    While the lease is valid, clients keep their config. But they will contact again the dhcp server before the end to try to renew it.

    So you could have a dhcp lease big enough to handle the change. Take care, removing the router will make client unable to reach what was behing the router (internet for example), while the new one is not ready.

    If the new router provide ip using another subnet, client will use this new subnet when their current lease expire. To handle the change, your router must have ip in both new and old (2 ip on same ethernet card), and provide lease only on the new one. So old client still works, and new one too.

    You could do a batch using psexec for example to connect remotely on each computer and do an ipconfig /renew to force the change before lease expire.

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  2. The DHCP should continue to work until the leases expire, since once a machine has an IP address it will try to hold onto it unless told otherwise.

    So to answer your questions:

    1) Yes.

    2) Not if you want to change the subnet, no.

    3) Yes, probably. I say probably, but the DNS might be cached so you may need to run ipconfig /flushdns and ipconfig /registerdns after the fact.

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