Are these layer 2 or layer 3? If layer 2, then they are (and should be) paying zero attention to the IP address. Layer 2 cares only about MAC addresses. Layer 3 is more subtle. Technically a real switch should attempt only to insert the address in the forwarding table and then the latest entry wins (eg it should eject the previously registered host as ARP entries expire in the communicating guests with cached info about IP to MAC mappings).
So, I'd say that if you are using layer 2 switches, it is neither a bug nor a feature. It's working correctly, and it's your problem to avoid this situation. In the layer 3 case, it's arguably doing the right thing, but there is a case for it dropping the first registration when a new host registers the same address. From: The IBM z/VM Operating System [mailto:IBMVM@LISTSERV.UARK.EDU] On Behalf Of Mark Wheeler Sent: Friday, April 29, 2011 3:49 PM To: IBMVM@LISTSERV.UARK.EDU Subject: Duplicate IPs on VSWITCHes - Feature or Defect Greetings all, We've been pulling our hair out for several days trying to figure out a networking issue involving VSWITCHes. A server (LNXA1) attached to VSWITCHA on VMSYSA can connect to a server (LNXB1) attached to VSWITCHB on VMSYSB but a server (LNXC1) attached to VSWITCHC on VMSYSC cannot. We moved LNXC1 to VSWITCHA on VMSYSA and it worked. All on the same subnet, BTW. Unbeknownst to us, a server (LNXC2) had an interface on VSWITCHC that used the same IP as LNXA1. It couldn't be registered to the outside network because it was already being used, yet it was still registered to VSWITCHC. Hence, anyone else on VSWITCHC would try to connect to LNXC2 when it in fact was trying to connect to LNXA1. Q VSWITCH VSWITCHC DETAILS shows the duplicate IP, identifiable by the "Local" designation under the list of unicast IP address(es). The VSWITCH is able to detect the fact that this is a duplicate IP. Is this a feature or a defect? Should VSWITCHC drop the IP address when it identifies the duplicate situation? What would a real switch do? Best regards, Mark Wheeler UnitedHealth Group -- "Excellence. Always. If Not Excellence, What? If Not Excellence Now, When?" Tom Peters, author of "The Little BIG Things"
