SGE2000 / SRW208G VLAN questions

Question

So we've got a few SGE2000P's and several more SRW208G's, and it was decided to use a GSM7312 for the core L3 functionality (price was one of the top issues to be taken into consideration, we couldn't find anything else suitable for our needs of features in that range). We just got the GSM7312, before it the entire network was flat
We are going to have multiple VLANs that need to communicate with a few shared resources on one particular VLAN. The one particular VLAN mentioned will have, amongst other things, the DC (doing DNS/DHCP) and our RAS box which is also doing all web filtering, the RAS box is connected directly to our T-carrier line.
as of right now, everything is in 1 VLAN pointing to the RAS box as its gateway. right now all SGE/SRW switches are also pointing to the RAS box as their gateway.
So now that we have L3 functionality in place and can assign IPs to VLANs, am I correct in saying that all SGEs and SRWs should point to the GSM as their default gateway, while the GSM only is pointing to the RAS box as its gateway?
What about pushing DHCP from the DC in 1 VLAN to all the others? I see that both the SGE and the GSM support DHCP relaying- is that what I need to be using? Something doesn't seem right about that to me.. for example. Lets say I have the DC in VLAN 200 (192.168.2.0/24), and clients in both VLAN 300 (192.168.3.0/24) and 400 (192.168.4.0/24). If the switch relays a DHCP broadcast request to the DC, how is the DC going to know from which scope/pool to give the address from? The server would be blind to the fact that the request was relayed at all, much less would it know it was relayed from a node in VLAN 400.
There are a few resolutions in my mind to this problem, but I'm not sure I am barking up any of the right trees.
One thought would be to set specific reservations for DHCP leases by MAC address in the servers DHCP pools (assuming that the destination MAC isn't changed when the switch relays it... I didn't even consider that at all until I just typed this).
Another thought would be to multihome the server and place it into every single VLAN, just to serve DHCP addresses
Another thought would be to buy an 802.1q capable NIC for the server and trunk all VLANs to it.
These last two kind of seem to defeat some of the purpose of VLAN'ing the network because I've once again placed at least 1 server into every layer 2 broadcast domain.
Someone please tell me what simple thing I am overlooking
Solved!
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Answer 1

Both SGE and SRW switches are layer 2 switches. You use them to create and extend VLANs. For any interconnection you have to use layer 3 functions/routing in the GSM.
The GSM will have an IP address in each VLAN, e.g. VLAN 1 192.168.1.254, VLAN 200 192.168.2.254, VLAN 300 192.168.3.254, etc. The GSM will do the routing between all VLANs. If you want to limit the traffic between VLANs you have to set up filters on the GSM to filter out anything not allowed.
If you want to use your DC for DHCP you configure the DHCP relay function on the GSM and point it to the IP address of the DC. You have to set up several DHCP IP address pools on the DC, one for each VLAN, e.g. one pool with 192.168.2.* addresses for VLAN 200. The DHCP relay in the GSM will add its relay address into the DHCP request, for instance the DC on VLAN 1 would receive a DHCP request via 192.168.2.254. (i.e. the DHCP request will be modified by the relay) Using the relay address the DC is able to identify the address pool to use for assignment. Creating fixed leases for specific MAC addresses won't change a thing in this respect. The lease is only valid if the request came through the according relay IP address...
If you put your internet gateway/RAS box into VLAN 1 and the GSM is the main L3 router, then the default gateway for each VLAN 100,200, etc. would be the GSM address in that VLAN, i.e. default gateway in VLAN 200 would be 192.168.2.254 if that's that IP address of the GSM in VLAN 200. The default gateway on the GSM itself would be the RAS box inside VLAN 1, i.e. any traffic which is not routed into any of the connected VLANs would be sent to the RAS box.
Multi-homing can be a good idea if bandwidth is an issue. Without multi-homing any server traffic will be routed through the GSM. (The internet traffic as well but I guess you won't have a gigabit internet connection thus it won't create a bottleneck here.) With multi-homing the server and the internet gateway is accessible directly in each VLAN. Using a 802.1q NIC would be a good idea to implement this. If you have multiple NICs support 802.1q and teaming you could even bundle multiple NICs to the server to increase bandwidth to the server.