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DHCP fails to properly assign IP's for a 2nd subnet on network
Hello - we have a SuperScope on our 2008 R2 DGCP AD server, which runs DHCP for 2 subnets - 192.168.1.x and 192.168.5.x. The respective VLANs for each subnet are VLAN2 & VLAN 6, respectively. What we are seeing is the following scenario:
1) Computers that do not have a lease previously on the network (they have been off for several days) will be properly assigned a new lease on the 2nd subnet of 5.x on VLAN6, and work fine.
2) Computers that have an active lease, however, on the 1st subnet of 1.x on VLAN2, will not receive a new IP on the 2nd subnet when they are in our neighboring building that has a WIFI network going thru a switch that is configured for VLAN6. Cisco captured and inspected packets from test clients coming thru a VLAN6 port on the network, and see the DHCP Request is made by the client, and the DHCP server answers, but does not properly hand out a 5.x IP address - and the client ends up retaining the 1.x IP it had originally. Even if we issue a "ipconfig /release" it does not help.
3) If I hard-code a static address on the 5.x subnet, however, the client will connect properly.
Is there something that is not configured correctly on the DHCP server to cause this? I will attach a screenshot of our Superscope. Let me know what else I can send that might help disagnose this.
Thanks for your help!
Damian
DHCP-SuperScope.pdf
1) Computers that do not have a lease previously on the network (they have been off for several days) will be properly assigned a new lease on the 2nd subnet of 5.x on VLAN6, and work fine.
2) Computers that have an active lease, however, on the 1st subnet of 1.x on VLAN2, will not receive a new IP on the 2nd subnet when they are in our neighboring building that has a WIFI network going thru a switch that is configured for VLAN6. Cisco captured and inspected packets from test clients coming thru a VLAN6 port on the network, and see the DHCP Request is made by the client, and the DHCP server answers, but does not properly hand out a 5.x IP address - and the client ends up retaining the 1.x IP it had originally. Even if we issue a "ipconfig /release" it does not help.
3) If I hard-code a static address on the 5.x subnet, however, the client will connect properly.
Is there something that is not configured correctly on the DHCP server to cause this? I will attach a screenshot of our Superscope. Let me know what else I can send that might help disagnose this.
Thanks for your help!
Damian
DHCP-SuperScope.pdf
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2 Solutions
Bryant - thanks for your reply. We have only 1 site here - pretty simple network structure. We USED to have a IP helper setup on the 2nd VLAN, but Cisco engineer said it actually was not necessary, with how we're configured. He does have a DHCP relay configured on the network, which had not been there before. but he indicated the IP helper was actually unnecessary, and removed it.
IP helper is not needed if DHCP server and DHCP clients are part of the same VLAN (or that L3 device has SVI that belongs to that VLAN), otherwise you need IP helper address configured.
possibly, but it sounds like the cisco engineer removed a command and it was working. To me, a quick diagnostic step, just to check
Thanks for your replies, everyone. Managed to get it working. two things combined fixed it - 1) the IP helper WAS needed on the 2nd VLAN side, so we put it back in. 2) The SuperScope was screwing things up because there were 2 different VLANs being used. we removed the subnets from the SuperScope, and it worked.
Thanks for your help!
Thanks for your help!
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