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It is known that VMs, through their vNIC connect to vSwitch which is in turn connected to the physical machine or ESXI host's network adapter (vmnic). The vmnic is then connected to an actual L2/L3 switch.

[VM's vnic] <---> [vswitch] <---> [vmnic] <---> [physical switch]

Now what i don't understand is how does the vmnic works ? I can't really come up with a single question to explain my doubt but below are the questions that I have been asking myself ->

  • How is it able to assign IP off to the internal VMs ?
  • When a external packet from the physical switch is being send to the vmnic, how does it pass it internally to the internal VMS ?
  • How does it even know that the designated IP belong to a VM inside and will pass it "inwards" to the vswitch ?
  • Is the vmnic using the IP or MAC address to decide whether to pass the packet inwards to the switch ?
  • When a arp request for a VM's IP is send out from external, will the vmnic reply with its mac address ?
  • When a arp request is send out from an internal VM, does the vmnic keep track of the mac address of the VM so that next time when it saw a reply to that MAC, it know to "forward" inside to the vSwitch
  • Is it working something like a "Bridge mode" bridging the internal vswitch to the external physical switch ? Is there a mac address table belonging to the vmnic ?
  • Whenever VM's IP is being assigned, will the IP be bind to the vmnic ?
  • How do we list all the IPs or MACs (if there are) that are associated with a vmnic ?

Can some gurus elaborate further on the the actual flow of a IP packet/frame in and out from a VM to the physical switch ?

Thank you.

Regards, Noob

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  • It seems you assume there’s something highly sophisticated going on. Instead, the “vmnic” is a physical port of the virtual switch. It behaves like any other switch port, including learning and whatnot. So yes, it’s like “Bridge Mode”.
    – Daniel B
    Jun 10, 2015 at 16:21
  • Hi daniel, thanks for your reply. If it is a like a physical port in the virtual switch and the interface itself has a IP, then i suppose it is like a L3 switch with a routed port. But then again, vswitch are L2 if i am not wrong. Are the vm IPs "bind" to the physical NIC on the ESXI OS layer ? or is there a list of mac addresses tie to the physical nic ? I am just curious on how does the physical nic actually pick up a frame and knows that it must forward inwards to the vswitch..
    – Noob
    Jun 11, 2015 at 17:54
  • Promiscuous mode. Like I said: It behaves like any other switch port.
    – Daniel B
    Jun 11, 2015 at 18:09
  • Hi Daniel, do you mean that the physical nic will be in promiscuous mode ? in that case, for frames/packets send to it, will it be recording the mac address or ip address ? will there be multiples mac address or ip addresses tie to it and are we able to list them down ? I mean from the physical OS (esxi), are we able to e.g. do a ifconfig and see a list of ips attached to the network interface ?) or the network interface will accept all frames send to it and it will be up to the internal "vnic"to drop the frame if it is not meant for the vm.
    – Noob
    Jun 12, 2015 at 7:34
  • hi @Noob - did you find answers to your questions? I'm trying to understand how networking in VMware is different from overlay networking in Kubernetes. Your questions list is just perfect!
    – dsatish
    Sep 17, 2017 at 17:37

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