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Odd behaviour with an add MAC Address

Hi Guys,

I'm currently installing some new switches (2960-S) for a customer.  They have this building managemenet syste,m that requires connectivity throughout the LAN.  I've segmented them into a VLAN (VLAN4) and given them an ip arange of

They have these controllers that all talk back to a WInXP machine, everythign is on the same VLAN.  If I plug the controller directly into the PC with a crossover, it works fine however if I plug them both into a switch (with ports in VLAN4) I get no conenctivity at all to the controller.  I definitely have connectivity to the XP PC so this isn't the problem.  All controllers are having the same issue.

I SPAN'd the port that the controller is connected to with wireshark and I can see ARPs going out to it from the XP PC but I see no return traffic at all, not even a broadcast on plugging it it.  If I set the controller to DHCP, I don't even see a broadcast for an IP.  The switch doesn't learn the source MAC off that port at any stage.

I plugged my laptop into the controller and I could ping it no problem.  I sniffed my link and I could see that the source MAC coming from the controller was 55-55-55-55-55-55.  I can't see any drops on the interface at all and there are no errors.  We've plugged everything into a NetGear switch and it all works fine.  We've changed cables and ports but this has be baffled now.

The only strange thing is the MAC address, would the switch be dropping this source MAC for some reason??

I'm all out of ideas, hoping someone can give me something to go on,

THanks in advance,


cadet alain


If I'm not mistaken  in 55-55-55-55-55-55 the first octet from left to right is 0x55 which is 01010101 in binary and the I/G bit( the least significant bit is set to 1 which means a group address so it can't be a valid source address as far as the Cisco gear is concerned.



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Hi Alain - thanks for the reply.

I did notice from the captures that it was a group address but I'm not sure what exactly this is or why it's just Cisco that has problem with it.



*** EDIT ***

OK After some reading the group address means that it's a broadcast/multicast address.  But I would have thought all switches would have an issue with

Glenn Matthys

Can you show us the output of "show running-config"?

Thanks Glenn, however I got them to change the MAC to the ones they "should" have been using and not 55-55-55-55-55-55 it's all working now.