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Jay Gough avatar Server

Windows Server 2016 DHCP Issue (Reserved IPs, Static & DHCP)

cn flag
Jay Gough

I've got a problem thats driving me nuts now.

I've got my DHCP server set up to provide addresses over a number of different VLANs.

On the main Processing VLAN the lease time is set to 15 mins, after which all devices keep dropping off for a few seconds and reconnecting. I was under the impression that if a device was still active on the network it would skip the drop out and continue using its assigned address until it was removed from the network. This is not happening and is becoming really annoying.

I have even reserved the DHCP addresses against devices to try and fix it but they still drop off for a few seconds whenever the lease expires.

The only way i've found is to reserve them AND set the devices to static addresses physically on them to stop it happening. I thought a DHCP reservation would suffice.

Can anyone offer any advice?

Finally, is there a best practice way to trunk all of the DHCP scopes over one NIC? I couldnt get it to play properly so i've had to create a separate virtual NIC on the DHCP server VM for each VLAN even though it all goes out over the same physical NIC on the VMware host.

Im losing the will to live. But i know ive messed up somewhere.

Thanks.

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joeqwerty avatar
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joeqwerty
`1.` Why such a short lease time? That seems counterproductive. `2.` The only time a DHCP client should release it's ip address is **A.** it can't communicate with the original DHCP server, **B.** it can't communicate with any other DHCP server, and **C.** it's lease expires. `3.` If a DHCP client can communicate with the original DHCP server then the DHCP client lease should never expire, it should be renewed. I'd suggest troubleshooting why they aren't able to renew their leases. That's the core of the problem.
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Jay Gough avatar
cn flag
Jay Gough
lease time is so short because we process devices for resale and have thousands of unique devices every day. our processing network is a /22. all vlans can see the dhcp server fine or they wouldnt get addresses to begin with. the issue arises when then lease time expires. they just drop off for a few seconds and then carry on which im sure it shouldnt do if its still active. they renew their leases. its just the drop out thats winding me up.
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joeqwerty avatar
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joeqwerty
You're missing my point. A DHCP client that can reach the DHCP server that assigned it's ip address should never expire. You're saying "expire" but that isn't what's happening. They're not expiring. So when the lease is renewed they drop off for a few seconds? Run a packet capture on the DHCP server and if possible on one of the devices. Then when the issue occurs on that device analyze the capture and see what's going on.
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Zac67 avatar Server
ru flag
Zac67

the lease time is set to 15 mins, after which all devices keep dropping off for a few seconds and reconnecting.

That's not normal behavior. A standard DHCP client tries to renew a lease after half the lease time. If that fails it'll keep retrying until the lease runs out and then fall back to some default (link-local/APIPA mostly).

You might want to check why the DHCP clients behave like that. Recheck their settings and eventually run a packet trace to find out what's going on.

Also, you might want to consider a longer lease period. For most purposes, something between a few hours and a few days might be more useful.

lease time is so short because we process devices for resale and have thousands of unique devices every day. our processing network is a /22.

You should have a dedicated subnet with short lease period for that.

The only way i've found is to reserve them AND set the devices to static addresses physically on them to stop it happening. I thought a DHCP reservation would suffice.

That disables DHCP altogether and defies its purpose.

is there a best practice way to trunk all of the DHCP scopes over one NIC?

You can either connect the DHCP server to each VLAN directly or use a DHCP relay on an intermediate switch or router.

i've had to create a separate virtual NIC on the DHCP server VM for each VLAN

That's the connect the DHCP server to each VLAN directly venue - should be working.

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