the quiet geek

Windows DHCP Superscope – NOT a convenient way to organize VLANs at a site

Add this to the list of things I learned the hard way.

I am working on a project to implement VLANs at a site, and set up a DHCP scope for each VLAN under a superscope that included all VLANs on-site. BIG MISTAKE. If you do this, you will have problems if you change a computer’s VLAN.

What happens with this is that any address within the superscope is considered valid for the client. When the client sends it DHCP Discover message, it will include DHCP Option 50 – ‘Requested IP Address.’ If the client had and IP that was valid for a different VLAN, it will request that same IP, and the DHCP server will IGNORE the giaddr sent by the router, which would normally determine from which scope the IP is assigned.

So you end up with a computer that keeps getting an address for the wrong VLAN. The solution is to remove the scopes from the superscope, and then everything will function normally.

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