This is the question Connectivity issues when setting guest MAC to the same as the host MAC on ESXi 5

As some of you may see - this question has a little pragmatic problems:

  1. This was not the original question I asked. The original question had a lot of miisng points.
  2. After some people edited it (and answered at (and upvoted the answer)) - I edited the question again to show them that they don't understand something very basic which I am trying to do - instead they just pick on me for "arp poisoning" and doing bad things I shouldn't.
  3. After more research for last response (from MDMarra) - the VMware station itself gave me a message to repeat the steps I stated at the question body.

I think that this question was changed too much. It's a bit of a mess and it might mislead other readers.

  • 6
    I've been through the entire revision history for this question and I don't see any obvious problems with the edits from other users. What exactly was the problem? Commented Nov 17, 2012 at 20:59
  • The problem was that the guest OS received data only after pinging the HOST OS. Commented Nov 19, 2012 at 9:17

2 Answers 2


There's a major difference between configuring a pass through NIC (which makes it transparent to the host) and just configuring a pNIC and vNIC with the same MAC. This is apparent to anyone that's worked with vSphere. You also say VMWARE instead of VMWare vSphere or ESXi. VMWare is a company not a product. This is the sign of a user that's inexperienced with a product.

The problem is that this is a site for professionals and you asked a question that showed that you have almost zero knowledge of how ESXi works. When users tried to correct you, you got defensive, which lead to more friction. At least three people (myself included) that participated in your question all run production vSphere clusters and have significant experience with it.

The fact that you keep insisting that your steps were mostly right and that the edits somehow mangle your question only serves to underline that you don't really understand how vSphere works.

Of course, questions about learning how a new technology works are on topic, but when you start telling experts in the field that they are wrong, that's not a way to get a positive response. If you had said "this is the problem I'm trying to solve, how should I do it?" instead of just saying that duplicating the MAC in the guest wasn't working, you'd have gotten much more constructive answers. This is why it's always a good idea to phrase a question in the form of: "I'm trying to solve ______. So far, I've tried ______ but it isn't working. Is this the best way to go about it, or am I missing something?" If you had explained from the beginning what you were actually after, there wouldn't have been a problem.

  • I think you missed the point of what I am actually trying to do, as a result you think I am getting a problem I don't get. I don't have ARP poisoning, I don't have MAC conflicts. Everything is working fine in my basic "very bad - you don't know anything" setup I did. Commented Nov 19, 2012 at 9:26
  • 3
    If everything was working fine, why did you ask the question in he first place? Obviously everything wasn't working fine.
    – MDMarra
    Commented Nov 19, 2012 at 11:37
  • Everything was not working fine UNTIL I pinged from HOST to outside, since then everything is working smoothly. Commented Nov 19, 2012 at 12:50
  • 4
    Sorry, but I think you're beyond help. You lack such a fundamental understanding of how ESXi networking works that I don't think anyone can show you why you were wrong. Rather than getting into an argument with a brick wall, my final piece of advice is to go buy a copy of Mastering VMware vSphere 5 by Scott Lowe.
    – MDMarra
    Commented Nov 19, 2012 at 12:59

I don't see any problems with the edits (with the exception of yours about the question being "hijacked" -- please confine meta discussion like that to this site).
If you can articulate a specific reason the edits are invalid I'll take another look, but the question seems to have been substantially improved by the edits.

Re: people "picking on you for doing bad things you shouldn't" - the answer is "Don't Do Bad Things!"

There is a strong culture of Doing It Right on Server Fault. If you are clearly Doing It Wrong (and having multiple devices on the same ethernet network sharing a MAC address is certainly Wrong, per the ethernet specification) you should expect to be told that you shouldn't be doing that. As professionals we're not going to tell you how to make a mess of you're environment: We're going to tell you not to make a mess, and aim you in the direction of proper, stable, supportable solutions.

If you have a REALLY GOOD REASON for doing something that you know is wrong you can defend yourself from the lectures by explaining that you're aware what you're doing is wrong, and explaining clearly why you have to do it anyway. (Your question does not currently acknowledge the inherent badness of duplicate MAC addresses, nor does it articulate anything I would call a GOOD reason to violate core networking specifications. If I understand what you're trying to do this isn't the right way to go about it per my comment on the question.)

  • 4
    I think part of the edit problem is that I rolled back his most recent edit, which contained an update about what he was actually trying to accomplish. I noticed this a couple of minutes after I did the rollback and edited it back in to the question. So, for about 5 minutes, there were some missing details. In my defense, those details should have been there from the beginning :)
    – MDMarra
    Commented Nov 18, 2012 at 11:46
  • Right, I should have put all the details in the beginning. However - all those details are still missing in current post. There is no MAC conflict if VM shares MAC address with host machine on ESXi 5. Wthout sharing MAC address, features on pNIC do not become apperant on guest OS (vNIC). Commented Nov 19, 2012 at 9:45
  • @HenryAloni When you use bridged networking, each virtual machine is entirely equivalent to a separate physical machine on your network. Of course you are creating a MAC address conflict if you assign the same address to the host and the guest. If you're certain that you're right, even when you're wrong, we cannot help you... but you may have a bright future in politics.
    – Skyhawk
    Commented Nov 19, 2012 at 17:48
  • How do I detect those MAC conflicts? I tried to see what's happening, and everything looks fine through the wire. There seems like there's no mac conflict in my setup. Commented Nov 21, 2012 at 9:12

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