Is there a way to “Pass through” a MAC address on a Cisco Switch?

Is there a way to “Pass through” a MAC address on a Cisco Switch?



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?






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
Sep 7 '18 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
Sep 8 '18 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
Sep 10 '18 at 14:51






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– Ron Maupin
Dec 25 '18 at 9:22




1 Answer
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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.






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
Sep 7 '18 at 14:16






As I said, switches do not change MAC addresses.

– Ron Trunk
Sep 7 '18 at 15:06






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
Sep 7 '18 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
Sep 7 '18 at 19:06







@JörgWMittag Plenty of experience with MOXA equipment begs to differ.

– chrylis
Sep 7 '18 at 21:40



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