Multiple devices with the same IP

Unanswered Question
Feb 8th, 2011

Hi,

I am a test engineer and do not have a ton of networking experience so I'm hoping someone here in the community can help me with some ideas.  I am in a manufacturing test scenario and need to connect multiple  devices with the same IP address (identical hardware all with the same  fixed IP) to one Windows 7 desktop for the purposes of testing.

My switch supports VLAN and I have set up a number of VLANs, 1 for  each device, with the host machine residing in all of the VLANs.  I need  to connect to these devices via SSH, preferably all simultaneously.

I'm thinking this will work except I have no idea how to differentiate ssh traffic to/from any 1 of the VLANs to the host.

I considered setting up virtual NICs on the windows 7 machine and seeing  if it was possible to set up some sort of bridge between a virtual NIC  and a VLAN but I wasn't able to find any convenient software to do this.

I believe that using virtual machines on the host is not a possibility  because I am writing 1 piece of software to communicate to all of the  devices.  Any ideas you might have for me would be appreciated, even ideas that  are along a totally different path than I have taken to accomplish this.

Thanks!

I have this problem too.
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phiharri Wed, 02/09/2011 - 09:35

Hey Christopher,

Interesting question, I see the main problem being that the "testing station" Win7 desktop won't be able to distinguish each device being tested based on IP address alone.

You could run a VM for each test device and statically pin the VMs to specific VLANs, that'd mean a lot of potential overhead running several OS instances for what I presume is a simple application, and running multiple instances of your test software which I understand is outside your requirements.

Putting all test devices in the same VLAN and using static ARP bindings on the test station to direct 10.0.0.1 -> device_a, 10.0.0.2 -> device_b .. would at least get frames to the devices, but most likely these would be rejected by the IP stack on those devices as the address wouldn't be the fixed one it expects (worth testing though as that would be a simple-ish solution, at most some scripting to pull the MAC entries on each switchport and populate the Win7 ARP cache).

Assuming neither of these are options, the only way I can see this working would be again with static ARP bindings and NATting the destination address of packets towards the devices, which would require different VLANs/L3 interfaces to each device and introduce a further problem of overlapping address space, which you could possibly solve using VRFs and selective route-leaking on top of the NAT. That'd probably require a more advanced router - what model of switch are you using?

With the complexity and ugliness of solving this issue with IPv4, it does beg the question - are these devices IPv6 capable (and if not why not?! :)), as you wouldn't have this problem using unique link-local addresses.

I'm interested to hear input from others on this.

Cheers,
/Phil

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