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Chris Sears
Asked: 2012-03-02 17:22:08 +0800 CST

Overlapping DHCP scopes for different VLANs?

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Should the following scenario work?

I have a DHCP server on VLAN 101, with the IP 192.168.0.5. I have two other VLANs 102 and 103 where I want to hand out IPs the same range 10.20.30.40-240, but from different scopes. So a client on VLAN 102 could get 10.20.30.40 and a different client on vlan 103 could also get 10.20.30.40.

Their routing doesn't overlap, so there is no layer 3 issue.

I will configure the DHCP IP helper setting on the firewalls for each VLAN to forward the DHCP traffic to my DHCP server on VLAN 101.

I know the MAC addresses of all the clients in VLAN 102 and 103, so I can use a filter or static mapping to assign clients to the VLAN 102 scope or the VLAN 103 scope.

Thanks!

vlan dhcp pxe-boot

2 Answers

  1. If these two VLANs are separate and there will be some form of NAT-ing happening between them, then yes, this is certainly feasible. Though in my own corporate network I would try to eliminate possible confusion (human or technological) and just give them all their own IP ranges.

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    1. I'm not sure it's possible to create two DHCP scopes using the same ip address range on one DHCP server. In fact I'm pretty sure it isn't possible.

    2. How is it that the VLAN routing doesn't overlap? How is routing accomplished if clients from VLAN 102 and 103 need to communicate outside of their own VLAN? How are you going to route traffic to two different VLAN's that use the same subnet addresses?

    3. I don't see how this would work because the router needs to have an ip address on each VLAN (which will be in the same subnet) and will relay the DHCP broadcast packets from the DHCP clients as unicast packets to the DHCP server, but from an ip adress in the same subnet. How is the DHCP server going to differentiate between the two? The DHCP server allocates ip addresses via the relay based on the subnet of the incoming relay packet, not the ip address of the incoming packet so how will it know which scope to assign the ip address from, if this were even possible?

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