strange task from customer

Unanswered Question

Hello everybody,

we got a strange task from one of our customers. As you will see it is a really individual solution which is asked by the customer:

He produces a box which includes a 2-Port-Switch with just ONE MAC-Adress and only one IP-Adress can be configured on this box. Now packets should be sent to two different destinations (pls see topolgy.jpg) from both ports. If I connect this box with two LAN-cables in a Cisco 3400, the switch gives me MAC-Adress error because it recognizes that one MAC-Adress can be found on two different physical ports. I have tried to find a solution on Layer 2 and on Layer 3 but without success.

Regarding Layer 3 solution we will get the problem that both destinations can be reached but the reply packets only go one way back to the box. The Packets take the way in which the box is situated.

My only solution is using two switches. The customer wants to connect several of these boxes to the switch. Please have a look on the attachment and the problem should become more clear.

Would be nice if anyone has an idea how to solve this problem with just one L3-Switch.

Best regards,


I have this problem too.
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Edison Ortiz Thu, 01/10/2008 - 07:38

If both interfaces are broadcasting the same mac-address, I'm afraid they can't be connected to the same switch /or/ even in the same physical network.

What kind of device is this?



Hi Edison,

it a self made box for industrial traffic. The goal is it to test both interfaces on Tx/Rx functionality. Therefore this box sends echos to those two destinations and expect a request on both sendings ports.

I am affraid, too, that there won't be a solution. I spent a lot of time on this task and tried to bind those interfaces in different VLANs but also wihtout success.

Would there be a possibility to force a L3-Switch to send a packet back to the box via the network which 'does not belong' to the box?

Concretely: If you have a look again on the topology and PC 2 ( sends an echo reply to the box (, the packet takes the way through Port 1 of the L3-Switch because the IP-Adress belongs to the Network of

I need to force the packet to leave out of Port 2 via the Network to the the second Port of that mystical box.


Edison Ortiz Thu, 01/10/2008 - 08:32

In order for the packet to be sent, the device needs to be listed in the switch CAM table.

If the CAM table is detecting the same MAC address on multiple ports, erroneous decision can occur.

I'm not sure what you are proposing is going to work.

As I tried to say I expected the packets leaving the box on both ports to two different destinations and the reply packets should be sent back to the sending ports.

I prepared a new graphic. Maybe I can show you that way what I am proposing to work.

Regarding the CAM table I have never come across it, I guess.


mheusing Fri, 01/11/2008 - 00:55


An odd idea for an odd problem:

You could use a normal L2 port and a SPAN port monitoring the normal port and connect the two ports of the box to them. This way I would assume it would duplicate packets to the box (maybe need a static MAC table entry for the normal port as well). Not sure it will help from an application point of view, but at least the box will see traffic on both ports. I have no clue what happens, when every packet comes in twice to the box, but it might be worth a try.

Regards, Martin


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