two server sharing the same MAC address

[zhichao]

We plan to install two servers (active and standby). The two servers will be using the same MAC due to some reason. When they are conencted to the same vlan, will this cause problem? I worry the switch may claim the mac-address flapping, and disable the ports....

[milan.kulik]

Hi,

you are correct the switch will claim the mac-address flapping.

It will not disable the ports.

You will just be receiveing syslog error messages "mac-address flapping ..." and your active server will loose the network connection any time the standby server sends a frame (with the same MAC source address as the active server) and vice versa.

I can imagine this solution works in the case the standby server is absolutely "silent" (not sending any frames, some UNIX machines behave this way, definitely not Windows machines) while the active server is working OK and starts sending frames only in the case of the active server failure.

Regards,

Milan

[zhichao]

Thanks. I think the solution is to configure the mac address table so the same mac is associated with two switch ports. Supported on all platform of switches?

[milan.kulik]

I'm afraid it's impossible with a unicast MAC address in one VLAN.

Have you considered using multicasts?

Regards,

Milan

[tbaranski]

Someone reported a few months ago that he was able to statically assign the same unicast MAC address to multiple ports on a 3550 to get it to flood traffic destined to that MAC address out all the ports. A 3750, however, wouldn't let him do this -- it complained that the address already existed on another port.

So your mileage may vary depending on what switch model you have.

[#6]

Hi, there

Could you please tell me how to use the same MAC for two server?

Thanks.

[smif101]

Yeah with most clusters or load balancing servers today they provide a way to use multicast MAC addresses or the ability to use the actual MAC address of NIC. This will avoid the pitfall of the MAC flapping. Just check with the documentation or your system.

Jason Smith

www.smif101.com

[sachin]

I have done clustering for exchange servers which share a virtual IP and virtual MAC address too, but I have not seen flapping on switches as switches will always learn actual physical MAC address and as for as Virtual MAC address is concern , the entery will be there in one switch only where Active server is connected.

One more thing how both servers will have same (physical) MAC address ? are you changing the Physical MAC address on servers ? Then surely you will get flapping. But I have not seen such type of achieving redundancy , it always work with virtual MAC only.