Configuring Inter VLAN routing and DHCP-Scopes help ...

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Hey guys, hoping you can help get my head around this scenario .... im struggling.

Basically i want to setup DHCP for my different VLANs, i found various examples around the web but still fail to understand how the correct DHCP scope is selected for a DHCP request...

If you refer to the diagram on this link Network City: Configuring Inter VLAN routing and DHCP-Scope-VLAN Bindings

Im not sure how a DHCP broadcast from say from a client in VLAN 2 gets an offer from the configured DHCP pool from the router (200.1.1.2) i totally get the ip helper config and i can see the DHCP scopes setup, but how does the router know how to assign the correct IP from the correct subnet for that VLAN.

I found some explanations online that hint towards the the DHCP server (router) sees that the request is coming from the Vlan2 with the IP address 10.1.2.x/24 so it knows to give the client on that vlan an address from the 10.1.2.x/24 scope.

But i dont see how this information is passed onto the router, ..... as the client machine in vlan 2 has no ip/gateway information?? it just sends the broadcast to the layer2 switching network (tagging the frame with the VLAN ID) it gets passed to a trunk on the Catalyst 3550 (our layer 3 switch which will do the routing between the domains) so i presume (as i dont have a layer 3 switch kicking about) the packet is tagged somehow with the correct VLAN interface IP info and then this is forwarded to the router which then just magically selects a IP from the correct DHCP pool?? *sarcasm* lol

becuase we can name them pools anything.....

Please shed some light on this as it is driving me mad! thanks

 

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right, done some more diggin about and i can see the clue is in the VLAN tag, the information on the cisco learning network cleared up here:

https://learningnetwork.cisco.com/message/151790#151790

 

Basically it knows which router interface (subnet) the request came through first so the DHCP server assigns it an address from that scope. That is a very basic explanation though!