We are running virtual servers on our windows server, I noticed that one of the server wont connect and when connected through virtual machine interface we found that server is up and running and we can access network/internet within the server but no outsider can connect to server.
We removed virtual network interface and added new one (That will generate new MAC address for virtual network interface) and then server was accessible.
Same problem occured both in VMWare as well as HyperV, not both at same time but with gap of 3-4 days.
I want to know that is it possible that two network interface on same LAN with same MAC address but different IP can create problem?
Hell yes, an unreservedly bad idea - they NEED to be unique.
The Ethernet protocol doesn't know anything about IP adresses. You always address hosts by their MAC address (IP addresses get mapped to MAC addresses) so it's not allowed to have duplicates.
Whilst it is unusual to have a single MAC address tied to multiple interfaces, or multiple servers, there is nothing preventing one from doing so; and there are in fact cases where this is done.
For example, there are load balancing configurations, where multiple servers are given the same MAC address, one, or both of the servers broadcast gratuitious ARP, announcing that the IP in question can be found at that physical address.
The switch to which the devices are connected adds an entry to it's port to mac table for both ports and will happily then forward packets to both ports when routed to the mac in question.
The load balance servers will then choose which packets they ACK (in the case of tcp), usually based on the source IP mod of the packet, allowing for example, one server to handle 50% of the traffic (assuming mod 2 is used)
With regard to ARP lookup, there is no issue here. When an ARP request is sent by a client for the IP in question, both servers respond with exactly the same answer, you can find IP x at Mac Y. This is subsequently stored in the clients ARP table as a single entry.
Just to add a little more detail to the answers. Yes, it's VERY bad to have two devices share a MAC address. The access switch(es) servicing these devices will be unable to maintain a decent mac address table. Each time they receive a packet from the device not currently in the table they'll update the device's port of entry.
Also, regarding the answer about the ARP query. The client will record the IP address of the LAST device to respond. The first response (I have MAC address XYZ and am at IP address 1) will be recorded, but then overwritten when a new "update" is received (I have MAC address XYZ and am at IP address 2). Depending on the circumstances, the client could end up continually updating its ARP cache and really messing up its TCP sessions.
Yes, the ARP protocol is used to map IP addresses to ethernet addresses so that the bits can find their way to the proper ethernet port. If you have two systems with the same ethernet address, then the bits will go to whichever system happens to answer the ARP query first.
The have to be unique to avoid this confusion.