dublicate MAC on seperate subnets

Answered Question
Jan 14th, 2010

I have some units which have configurable MAC addresses on my network.

I am afraid that there will be a conflict if someone changes MAC addresses on two units to the same MAC address.

I am certain that ALL MAC addresses within the same subnet are always different.

The problem that may occur is two MAC addresses that are configured to be the same on two different subnets.

Will this cause a problem?

Because I am working in a regulated environment it is vital that I can get some written fact on this subject to be able to document it.

BR Torben

I have this problem too.
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Correct Answer by Ganesh Hariharan about 7 years 1 week ago

Hi Torben,

If you have the same MAC address assigned within the same network segment, subnet, or broadcast domain you will undoubtedly run into problems in layer 2 (Data Link Layer). What will happen is that traffic will be split between the MAC addresses, leading to errors and possibly a total network shutdown depending upon the equipment that you use. The most likely scenario is that the machines with duplicate MAC addresses will be the only ones affected, and won't have network connectivity.

Same MAC in different vlan will work without any issue check out the below link hope that clear your query !!

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml

Regards

Ganesh.H

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Giuseppe Larosa Thu, 01/14/2010 - 03:18

Hello Torben,

the same MAC address can be used on different Vlans.

so it is important that each IP subnet is associated to a different Vlan.

if multiple IP subnets are associated to a single Vlan broadcast domain (use of secondary IP addresses) this would be a problem.

Actually, cisco multilayer switches reuse the same MAC address on multiple Vlans as described in another recent thread.

So until different vlans are involved you should be fine

Hope to help

Giuseppe

Correct Answer
Ganesh Hariharan Thu, 01/14/2010 - 03:40

Hi Torben,

If you have the same MAC address assigned within the same network segment, subnet, or broadcast domain you will undoubtedly run into problems in layer 2 (Data Link Layer). What will happen is that traffic will be split between the MAC addresses, leading to errors and possibly a total network shutdown depending upon the equipment that you use. The most likely scenario is that the machines with duplicate MAC addresses will be the only ones affected, and won't have network connectivity.

Same MAC in different vlan will work without any issue check out the below link hope that clear your query !!

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml

Regards

Ganesh.H

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