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VMWare vSphere iSCSI Port Binding – When to use iSCSI Port Binding, and why!

Over the years I’ve come across numerous posts, blogs, articles, and howto guides that provide information on when to use iSCSI port binding, and they’ve all been wrong!

I felt the need to write up a post and explain why and when you should use iSCSI port Binding on VMware vSphere.

This post and information applies to all versions of VMware vSphere including 5, 5.5, 6, 6.5, 6.7, and 7.0.

See below for a video version of the blog post:

VMWare vSphere iSCSI Port Binding – When to use iSCSI Port Binding, and why!

What does iSCSI port binding do

iSCSI port binding binds an iSCSI initiator interface on a ESXi host to a vmknic and configures accordingly to allow multipathing in a situation where both vmknics are residing in the same subnet.

In normal circumstances without port binding, if you have multiple vmkernels on the same subnet, the ESXi host would simply choose one and not use both for transmission of packets, traffic, and data. iSCSI port binding forces the iSCSI initiator to use that adapter for both transmission and receiving of iSCSI packets.

In most simple SAN environments, there are two different types of setups/configurations.

  1. Multiple Subnet – Numerous paths to a storage device on a SAN, each path residing on separate subnets. These paths are isolated from each other and usually involve multiple switches.
  2. Single Subnet – Numerous paths to a storage device on a SAN, each path is on the same subnet. These paths usually go through 1-2 switches, with all interfaces on the SAN and the hosts residing on the same subnet.

I.T. professionals should be aware of the the issues that occur when you have a host that is multi-homed with multiple NICs on the same subnet.

In a normal typical scenario with Windows and Linux, if you have multiple adapters residing on the same subnet you’ll have issues with broadcasts and transmission of packets, and in most cases you have absolutely no control over what communications are initiated over what NIC due to the way the routing table is handled. In most cases all outbound connections will be initiated through the first NIC installed in the system, or whichever one is inside of the primary route in the routing table.

When to use iSCSI port binding

This is where iSCSI Port Binding comes in to play. If you have an ESXi host that has vmknics sitting on the same subnet, you can bind the iSCSI initiators to the physical NICs. This allows multiple iSCSI connections on multiple NICs residing on the same subnet to transmit and handle the traffic properly.

So the general rule of thumb is:

  • One subnet, iSCSI port binding is the way to go!
  • Two or more subnets, do not use iSCSI Port Binding! It’s just not needed since all vmknics are residing on different subnets.

Additional Information

Here’s two links to VMWare documentation explaining this in more detail:

For more information on configuring a vSphere Distributed Switch for iSCSI MPIO, click here!

Stephen Wagner

14 Year IT Service and Solution Provider, Managed Services Provider, Tech Blogger, and Entrepreneur. Stephen Wagner is President of Digitally Accurate Inc. an IT Managed Services Provider in Calgary and Vancouver.

View Comments

  • Hi Fabio,

    Sorry for the delayed response! (It's Stampede week here in Calgary, busy time of the year!)

    Do you only have 1 switch between the MSA 2040 and the 3 HP Servers, or multiple switches? Also, are you using standard switches, or vSphere Distributed Switches?

    I briefly took a look at that guide and for the most part it looks good, however I might configure my vSphere switches slightly different. And as always, I always recommend using multiple subnets (and avoid using iSCSI port binding).

    Let me know and I'll see what I can come up with for you, or any advice I may have.


  • Hi Stephen,
    I have 2 Switch HP 1910 24p Gb managed.
    I can only use the standard vmware switch because I have the Essential Plus License.

    If I usedmultiple subnet, i need to use the Vlan because in the two HP Switch there is also the normal traffic of the VMs and the other clients of the network with IP 192.168.1.x

    Thanks a lot for your support, you are great...

    for the installation of the ESXi on the HP 360p Gen8 I will use a HP SDHC 32Gb. IT's a good choise about the security and the stability of the system?

  • Hi Fabio,

    I'll start off with the easiest question: The DL360p Gen8 works great with the SDHC cards for ESXi to be installed on to. I've used both the SD card and internal USB thumb drive option, and both work great!

    So if you do only use one subnet, you can use that guide you originally posted, however instead of creating multiple switches and binding them to the same NICs, I would instead create only one, configure your VLAN, and then create multiple vmkernal (vmk) interfaces on that single switch (each with their own IP on the network). Then after this you would simply go in to the iSCSI initiator settings and enable iSCSI port binding on each vmk interface.

    Keep in mind, that if you were to use both switches (with different subnets), then you would have added redundancy to your configuration in case one of the switches ever failed. This is just a consideration.

    Hope this helps,


  • Thanks for your reply, I'll do the configuration with one subnet, 192.168.1.x.
    Next week all the products arrive in my lab and then I'll write you my idea of configuration.

    Thanks a lot for your support.
    See you soon.



  • Hi, finally the MSA2040 is arrived in my lab.
    Dual controlle, 8 port iSCSI 1Gb, 7 HDD SFF 600Gb SAS.
    I do this configuration:
    I have created 2 VDSIK. The first, with SAS1-2-3 in RAID5.
    The second with SAS 4-5-6 in RAID5 and SAS7 is Global Spare.
    The first VDISK is mapped to controller A and the second vdisk is mapped to controller B.
    Each VDISK have one volume of entire capacity mapped on each port of the controller (A1, A2, A3, A4 and B1, B2, B3, B4)

    for you is a good configuration?

    Thanks a lot for your support


  • Hi Fabio,

    That should work great. When you created the Vdisks, did you choose auto for owning controller? If not, I would advise to change it.

    Other than that you should be good!


  • Hi Fabio,

    So just to confirm, when you created the volumes, when it asked for a controller ownership, you chose "Auto", correct?


  • NO. Now I have manual select Controller A and controller B.
    Tomorrow change the ownership to Auto

  • I'm looking at redesign our iscsi network to include 2 switches and put them on 2 subnets /24.
    I understand that I should not do port binding in this case. But I wanted to confirm if that this case holds true if your host has 4 nics for iscsi. I was looking at putting vmnic7 and vmnic6 on subnet 10.0.1.x and vmnic5 and vmnic4 on subnet 10.0.2.x. Would you do portbinding on the nics with in the same subnet?

    Thanks for your help.

  • Hi Devin,

    Let me see what I can find out, but I'm assuming that you would have to have port binding enabled, however it may result in some erroneous routes/paths which you may have to mark as "inactive" or "disabled" manually. This may or may not be the case, but I'm pretty sure in your case you would need to use iSCSI port binding.

    Let me see what I can find out and I'll get back to you!


  • Devin,

    Got the information faster than I thought I would! haha

    Essentially you WILL use iSCSI port binding. Make sure that pair of NICs that are on a single subnet are configured on their own vSwitch (or Distributed switch). DO NOT use the same vSwitch (or vDs) for different subnets.

    When you have the server NICs (only put NICs on the same subnet on the same vSwitch) on their own vSwitch (or vDs), then you can configure iSCSI port binding!

    Let me know if you have any questions!


  • Hi Stephen,
    I have 4 NIC's in 1 host, and 8 NIC's in SAN in test environment
    NIC's in host are each in unique subnet

    and SAN ip

    WIth Port binding enabled, I get good IO ~414MB read, if I disable it, i get 127MB read?
    I am not sure where I am going wrong here, i just removed the NIC's from the Software iSCSI addapter, do I also have to split them up into seperate vSwitches?


  • Hi Wihan,

    Just curious, how do you have everything wired? Do you have seperate physical switches? To confirm, the host is directly attached to the SAN?

    If you are using multiple subnets, you should have your vSwitches specially configured.


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