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We currently have 3 VLANs setup in our network. On our DHCP server (Windows 2008) we have 3 scopes created for the 3 separate subnets. Our clients can't seem to pull addresses on the subnets, except for the one the server is attached to. DHCP forwarding is enabled on our procurve switches (ip-helper)

If we make the scopes part of the same superscope in windows DHCP, then addresses are handed out fine, but if a client roams from one VLAN to another, they will always pull the same address from their existing lease, even if it's from another subnet. If the scopes are not part of the same superscope, the DHCP server responds with a NAK to the client's DHCP request if they're not on the same subnet as the server.

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    Turns out the switchport the server was plugged into was tagged for the other 2 vlans, so it was seeing the broadcasted DHCP requests and not the relayed requests from the switch. After making sure the only vlan assigned to the server's port was the main vlan, everything works as expected. Commented Aug 21, 2013 at 20:37
  • Good Deal...please post your comment as the Answer and accept your own answer. That is perfectly acceptable here, and could help others with the same/similar situation in the future. Commented Aug 21, 2013 at 20:57

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Turns out the switchport the server was plugged into was tagged for the other 2 vlans, so it was seeing the broadcasted DHCP requests directly from the clients, and not the relayed requests from the switch. After making sure the only vlan assigned to the server's port was the main vlan, everything works as expected.

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  • One thing I would like to know is why the server would even respond to those client broadcasts from the other VLANs. They're in a separate broadcast domain, and the server's network interface isn't tagged for the other VLANs, so the only traffic it should've been able to see would be from it's native VLAN. Commented Aug 22, 2013 at 14:27

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