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Current situation: we have a C3850 switch serving DHCP lease to desktop clients without communicating with any upstream DHCP server

Objective: We would like the C3850 switch continue to serve DHCP lease on VLAN60 while Microsoft DHCP server could see all the DHCP lease from C3850 on VLAN60

Remark: I know "ip helper-address" feature exists on the Cisco switch, but it will forward DHCP client request to the Microsoft DHCP server instead of having a local DHCP server in the C3850 switch, that's not what we want to do. C3850 could communicate with Microsoft DHCP server on layer 3 routing without problem.

Question: how exactly could I implement DHCP relay on C3850 with Microsoft DHCP server without impacting existing DHCP lease on C3850 right now?

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    The DHCP servers built into network devices are really minimal, and you should not use them if you have a real DHCP server or even any (minimally) complex situations. – Ron Maupin May 28 at 2:44
  • in that case, I just need to use ip helper-address and point to the "real" DHCP server, everything should be fine then? – Thomas G. Lau May 28 at 3:44
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    Yes, I'm really not fond of MS product, but their DHCP server works quite well and you will have much more option. Otherwise apart performing some elaborate scripting to query the leases on the Cisco I can't think of any way to do what you ask. – JFL May 28 at 6:39
  • hit another issue after using ip helper address, serverfault.com/questions/1019016/… – Thomas G. Lau May 28 at 8:43
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    Really nice that you put this second question on the appropriate S.E. site.. Answered ;) – JFL May 28 at 8:55
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Short answer: you can't.

First, the switch cannot be configured to do both server and relay at the same time, on the same interface.

Second, you cannot have two DHCP servers. The switch, being the first device to see the request, will almost always be the first one to answer, even if the MS DHCP server is on VLAN60. In this case, as the host is already associated with the switch's server, it will continue renewing its address with that DHCP server. (but we're running into #1 again.)

The only way I can see to move forward is to copy the reservation to the MS server, and let the client (re)discover the DHCP server. Yes, there will be a few seconds where the host(s) will lose their lease, but with static reservations, they'll get the same address. (Even the dynamic hosts could end up with the same address. But this depends on the host, DHCP server, and if the requested address is available.)

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