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New Member

Helper address with secondary IP address.


We have an DHCP server in subnet of which have a ranges configured for One of the router ( is configured as relay agent to convert DHCP discover broadcast to unicast. Clients can easily obtain the IP address from network configured at DHCP server (using ip helper-address < DHCPServer IP address> ). Recently we have configured a secondary IP adress on the interface with primary address as We wish that certain client should get the IP address from the DHCP server but from the range. We have tried to configure two helper address for that still all the clients are getting IP address from primary subnet.

Please suggest.


Deepak Sharma

Hall of Fame Super Silver

Re: Helper address with secondary IP address.


If it is certain PCs that you want to have addresses in the second scope you may be able to configure reservations so that they always get a certain address out of the second scope. Otherwise I am not sure that you can do what you describe. The normal operation of DHCP server is that it will assign addresses from the primary scope until its addresses are exhausted before it will assign addresses from the second scope.



Re: Helper address with secondary IP address.

The reason your clients still get IP from primary subnet is that original request is sent within the same broadcast domain (LAN segment, VLAN) as primary subnet. DHCP request in a nutshel looks like "Hi everyone (IP:, MAC: FFFF.FFFF.FFFF) out there. I am new here (IP:, MAC: ). Someone cares to tell me which IP should I use?" So there is no way for router to know that one of such requests is for one subnet and another one for another. Router simply listens for broadcasts on bootps port and forwards them.

Imaging that your DHCP server would actually be directly on the same subnet as clients. How would it distinguish between different subnets? If you can determine criteria for this, then the same answer applies to DHCP server being on another subnet.

New Member

Re: Helper address with secondary IP address.

You need to make dhcp scopes for both your /23 and /24 networks rather than the entire /8. In ISC the new scope would look similar to:

subnet netmask {


default-lease-time 604800;

max-lease-time 2419200;

option subnet-mask;

option broadcast-address;

option routers;


Re: Helper address with secondary IP address.


Cisco supports a feature called 'dhcp smart relay' feature that would enable users acquire IP from a secondary address (subnet)configured on a router interface. However, this would work only if the DHCP server isn't configured with the primary IP address scope.

If you have another ethernet interface on the router you could configure the two interfaces to forward dhcp requests to two different servers and smart relay function should help. Check out the following link for more info on this feature.

Hope that helps!



New Member

Re: Helper address with secondary IP address.

Why don't you use dot1q vlan subinterfaces rather than secondaries? If you did that the giaddr in the discover message would contain your 10.200 address and it would work fine.