Damian Gardner
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DHCP has stopped working on our wireless network
We have a network with two VLANs associated with their own subnets. VLAN 1 is tied to SUBNET 1, and VLAN 2 is tied to SUBNET 2. We use subnet 2 in a second section of our building. The backbone is Cisco catalyst 3850, and the problem right now seems to be almost entirely happening with only WIFI users in the "subnet 2" area of the building, which are Cisco Meraki APs. While users have been getting IP addresses dynamically without issues for some months now, they suddenly are no longer able to get an address thru DHCP. If I statically assign the IP, it works. There are hard-wired desktops on the 2nd subnet that are working fine - those are DHCP, with 4-hour leases, incidentally. In the "SUBNET 1" area of the building, users are working fine on WIFI or LAN, without issues. Nothing that I know of has been changed on the netowkr - no work or anything like that has been done over the last week, when this started happening. Could this simply be a glitch with the Meraki APs or the AD server??
Thanks for your help
Thanks for your help
Craig Beck
Is the DHCP server on VLAN1? If so, check the IP helper address on the SVI for VLAN2 on the 3850. It should point to the AD server (if that's where your DHCP service is).
Saying that, if wired devices work it could be the APs not passing DHCP. Have you tried rebooting one to see if that helps?
ASKER
Thanks for your help Craig. yes the DHCP server is on VLAN 1, and I believe Cisco helped me with setting the IP Helper command to forward the DHCP requests from VLAN 2. Yeah I did reboot all of the APs on VLAN 2, but I'm going to down the switch AND the APs again tonight, plus the AD DHCP server too. maybe that will help.
ASKER
I rebooted the switch on the VLAN 2 subnet as well as the AD DHCP server, to no avail.
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ASKER
ok, will do. thanks for your help
ASKER
So a new twist on the case now - I've determined that it seems to be a problem with computers that are FIRST connected on one subnet or the other, and they are unable to switch over to the OTHER subnet / VLAN. I determined that DHCP requests ARE being processed without issue across the backbone to the IP helper server without issues. It's just something to do with whether the computer has an existing lease on 1 or the other maybe...not sure. but when I take a computer that has not been in use for awhile (ergo it has no existing lease on the first subnet/vlan, and I turn it on in the 2nd building with the 2nd VLAN, it works without issue, and is assigned an IP address. What could cause that?
In this scenario, your observation is correct. OLD computer need reboot and flush IP.
ASKER
Thanks for your help Sandeep. Unfortunately, that doesn't seem to work. I've noticed that it also only works going one way - from the 2nd subnet back to the 1st (main) subnet - only then does deleting all traces of the machine on DHCP and DNS does it allow the machine to take a new address automatically. But if I try that going the other way (subnet 1 over to subnet 2), it fails. This is something that used to work without manual intervention for months - and now suddenly has stopped. Users would walk their laptops from 1 building to the other - wait 30 seconds - and it automatically switched over. what happened I wonder? It seems to be something with the DHCP server somehow. Do I maybe need to think about adding a separate DHCP server onto the 2nd subnet?? Perhaps its not best practices to have the same DHCP server handling both subnets or something? but I need to figure out how to get it working automatically again...
ASKER
When I check the DHCP log, I can see the server is handing out subnet 1 IP's to clients in subnet 2 now. IT should be handing out subnet 2 addresses
There must be a link between vlan1 and 2.
something like inter vlan routing??
ASKER
Update: Microsoft is asking me to remove recent updates installed, and turn off Symantec Antivirus, and test again.
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I can't believe it....it was superscoped. here's the thing on this - we ALREADY removed the scopes from a superscope 4 months ago. we never thought it could move back in to a superscope on its own. I'm baffled. but that's what did it.