Router as DHCP server and using ip helper at the same time?

Hello all,

Has anybody here been successful with or know how to set up the following?

Using the Cisco router to hand out DHCP addresses to all but one device on a
subnet.
That one device needs its boot request to be forwarded to a server on a
different subnet.

I can get either the router to hand out addresses OR forward with ip helper
but not at the same time.

Thanks in advance.

 

My educated guess on this matter is that this will not be possible. My
first thought was some kind of policy routing to pick out the one
device on the subnet that needs special treatment and maybe do
something sneaky like send it into a loopback interface which has
perhaps an "ip helper-address" configured and hope that the helper
address command overrides the DHCP server running on the same system.

However, note that the one device that needs this special treatment
does not even have an IP address yet! So, policy routing is out. The
only way to identify that device would be its layer 2 address and I
can't see how with that constraint we could do this mix of DHCP server
and DHCP relay functionality on the same box.

Cisco da Gama
http://ciscostudy.blogspot.com

 

What are the requesting devices attached to - a switch ?
If so are the swiutch and the router capable of trunking ?

Please explain the requirement to have the boot request for one device
to go to a different server

 

Yes, they are attached to a switch that is capable of trunking to the
router.
One device needs to get its address from a DHCP server that will then load
an OS to it. All other devices get there address from the router no problem.

 

so you could put the switch port for that device on a separate vlan and
enable trunking between switch ant the router.

Then on one vlan the router can be the local DHCP server, while the
speical device is helpered to the desired server

 

The problem originally stated that a solitary device on a subnet needed
this special treatment with a different DHCP server while other devices
on that same subnet should use the IOS DHCP server. Moving the device
to a separate vlan would mean moving it out of that subnet and putting
it in a subnet of its own. It does seem reasonable to do something
like that for this special case and so if you can afford to have two
subnets instead of the one initially specified, the above suggestion
should work and is very nice.

I was earlier assuming that for some reason you need to keep all those
devices on the same subnet.

Cisco da Gama
http://ciscostudy.blogspot.com

 

Another thing I would investigate is if the server can be configured to
unicast instead of broadcast its bootp request. AN IP helper would
not be required in the case.

 

So put this one device in a new vlan by itself, same IP address but trunk
the old and new vlan on the same router port?

 

Using a new vlan will mean a new IP subnet for the device using bootp.

BTW what is the device in question (make,model, OS version)

 

Yes, this can be done.

Yes. I am doing this with a Cisco 2621 running IOS (tm) C2600 Software (C2600-IK9O3S3-M), Version 12.3(12), RELEASE SOFTWARE (fc3)

I am running vlans with subinterfaces on my FastEthernet 0/0. I also have FastEthernet 0/1, but don't use it with vlans or DHCP, as it is a standart ethernet (access) port that connects to my ISP.

My applicable config is:

Global config:
ip dhcp excluded-address 192.168.254.1 192.168.254.99
ip dhcp pool RepairVirusLAN
network 192.168.254.0 255.255.255.0
default-router 192.168.254.1
domain-name RepairVirusLAN.internal
dns-server 69.39.19.247

ip dhcp-server 192.168.131.50


Interface config on the interface that I want the ip helper to send dhcp broadcasts to my windows dhcp server:
ip helper-address 192.168.131.50

No extra config needed on the interface that I have the Cisco router doing DHCP for. As long as that interface has an ip address assigned that is in the range defined my my dhcp pool (shown above), then it just works. My router ip on that interface is 192.168.254.1, which is why I have excluded it from the dhcp pool.

I'm not sure whether all of these commands are necessary. I have seen some posts that indicate that the ip dhcp-server command may not be, but I'm not sure. I haven't tried to disable it. But in any case, what I currently have works exactly as you have requested.

Good luck!!

-
Doug