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We have two systems connected together, and the end system (system 2) relies on differentiating packets via MAC address from system 1 (it has two routes).

For testing purposes, we want to put a Cisco switch in the middle. But doing so obviously changes the MAC address presented to system 2.

System 2 cannot be changed to RX a different MAC address.

Is there a way to "pass through" the MAC address on the switch from system 1 to 2, or is it possible to spoof the address on the port facing system 2?

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    Sounds like the words "switch" and "router" are getting mixed up. Maybe in your lab you're actually using a router with some features turned off to make it seem like a switch, but they are different devices with different characteristics.
    – JPhi1618
    Commented Sep 7, 2018 at 18:26
  • What model of switch do you have? If it's a so-called "layer 3" switch, you might have a switch with routing facilities enabled, with the same effect as @JPhi suggests.
    – jonathanjo
    Commented Sep 8, 2018 at 10:33
  • No, definitely a switch, and I cleared up what the request was. Like I mentioned below, it was being tee'd off to something else.
    – user10021657
    Commented Sep 10, 2018 at 14:51
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 25, 2018 at 9:22

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Switches do not change MAC addresses, so your application, if it is on the same subnet, will see the sender's MAC address.

If you the application is on a different subnet, then I'm afraid you can't see the sender's MAC. It is stripped off by the router.

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  • hmmm, that's what I thought. So either system 1's MAC is wrong, or it's being tee'd off to something else first in the switch, before being sent to system 2
    – user10021657
    Commented Sep 7, 2018 at 14:16
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    As I said, switches do not change MAC addresses.
    – Ron Trunk
    Commented Sep 7, 2018 at 15:06
  • 1
    Not only do switches not change MAC addresses, they work based on MAC addresses, so they are crucially reliant on correct MAC addresses. A switch that changes MAC addresses would be so completely, utterly, fundamentally broken that it would never get into the hands of a customer.
    – Jörg W Mittag
    Commented Sep 7, 2018 at 17:57
  • @JörgWMittag I think that's a bit extreme. Routers don't typically change IP addresses, but they can when doing NAT. Similarly, switches can do layer 2 NAT of MAC addresses. Among other things, that capability is sometimes used to bridge wireless LANs at layer 2 when, for one reason or another, WDS is not an option. It's definitely not typical though.
    – David Schwartz
    Commented Sep 7, 2018 at 19:06
  • @JörgWMittag Plenty of experience with MOXA equipment begs to differ.
    – chrylis -cautiouslyoptimistic-
    Commented Sep 7, 2018 at 21:40

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