Forwarding Data - Is this possible

Unanswered Question
Jul 20th, 2009

Hi All,

Have a configuration issue:

I have a device on a LAN that Gathers data and then forwards that data to another IP address on the same subnet. It is custom software that is hard-coded with the destination address, the source address and destination address cannot be modified or changed. The problem that I have now is that the data needs to be sent to another device on another network - but connected via routers (which can be configured with NAT).

What I was thinking of was like an IP Helper-Address type of solution, except that the router would act as the destination Address but then forward the data to the destination address, as it would a DHCP request. Is there a configuration command that could accomplish this?

Any assist would be appreciated

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Collin Clark Mon, 07/20/2009 - 10:55

As you stated you would want to use NAT or have the developers fix their program ;-)

The router would simply translate (original IP) to (new IP).

Hope that helps.

r.blanchard Mon, 07/20/2009 - 11:06

That I agree, the issue that I am having is that the device that transfers the data, sends it to an IP address on the same subnet, so IP would send that data to It would not hit the router, so if I could have the router pretend that it was and then forward it to the that would solve the problem.

Collin Clark Mon, 07/20/2009 - 11:14

The only way I can think of to do that would be to add a secondary IP on your router and NAT to the new destination. I've never done that so I'm not 100% sure you can. Other than that, the developers will have to fix the app.

Istvan_Rabai Mon, 07/20/2009 - 11:29

Hi Robert,

This is an interesting requirement from you.

On simple solution could be to split the subnet to sub-subnets.

For example: - would be part of this subnet - would be part of this subnet.

You could configure these 2 subnets on different interfaces of the router.

In this case the router would proxy-arp for the address and forward the packet to its other interface.

If you wanted to forward this packet to somewhere else, you could configure policy routing.



r.blanchard Mon, 07/20/2009 - 11:40


As with the IP address I am unable to change the mask as well, I was wondering if Cisco had a hidded command that could have helped - as in the secondary address suggestion, and then a Forward-Protocol or ACL type of command to pass the data to the NAT address at the distant end.

Tks. for the suggestion tho.


Istvan_Rabai Mon, 07/20/2009 - 11:57

Hi Rob,

There is the need to change the mask on the router only.

But if you have so many hosts on the network that you can't break it up into subnets, then I understand your point.



r.blanchard Mon, 07/20/2009 - 14:56

Thank you all for your suggestions, going to check out using a GRE tunnel beween the networks, with destination device with NAT.


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