Adopting Network Virtualization – Part II

Adopting Network Virtualization – Part II

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This is the second post in a series about transitioning from using VLANs for tenant segregation to Network Virtualization. This series will be an example-based walkthrough consisting of a few parts:

In the previous post, we configured a brand new environment and brought in VLANs to segregate network traffic between VM Networks. In this post, I’ll introduce the concept of Virtualized Networking and explain how to bring it into our environment. For some of the tasks I perform in this post, I’ll be skipping UI instructions that I’ve covered in a previous post.

Getting Started

I’ll assume we’ve followed steps outlined in the previous post and have a environment configured as such. If things have been operating with VLAN segregation and tenants VMs have been deployed to the tenant VM Networks, we’ll want to take the least disruptive route to introducing network virtualization.

To this end, I’ll be adding another cluster to my environment for tenants with virtualized networking environments. In this environment, I have seven hosts in three clusters. As before, all my hosts each have two NICs that are configured as trunk ports in the switch they connect to with the management network as the native VLAN. The physical cabling and cluster membership is depicted below:


As before, my HOSTCLUSTER is using VLANs to segregate its tenant traffic. My VIRTUALCLUSTER, however, will be using network virtualization to segregate tenants. My goal will be to create the network topology represented below:


Management Network (Solid Line)

Connects to all hosts and is used for VMM communication.

Tenant Network(s) (Dashed Line)

These are the networks we created in the previous post (we actually only created one) to isolate tenants using VLANs. These networks are not needed on the management cluster or the virtualization cluster. If you’ve deleted the configuration for the HOSTCLUSTER already, you won’t have this network.

Virtualization Network(s) (Dotted Line)

These networks exist as a logical entity through network virtualization. Each network is segregated but uses the same VLAN for transport. From a Layer-2 perspective, there is only a single network and the segregation is enforced higher in the network stack. In this post, the virtualized network will not have external access. We’ll cover connecting these networks to external resources in the next post. Also, we’re using a dedicated VLAN for the virtualization network in this post and we’ll do away with that in a later post. 

Adding Hosts

I’ll first need to add the new cluster. Refer to my previous post for detailed instructions on this in the UI. I’ll be creating a new host group called Virtualization and adding a cluster named VIRTUALCLUSTER to it. In PowerShell, this is:

$virtualizationGroup = New-SCVMHostGroup -Name "Virtualization" -ParentHostGroup $allHostsGroup; 
$virtualizationCluster = Add-SCVMHostCluster -Name "" -VMHostGroup $virtualizationGroup -Credential $domainAdminRAA -RemoteConnectEnabled $true;

Now, I have my cluster added:


Next, I’ll be configuring a network for network virtualization.

Adding Network Objects

The hosts in my VIRTUALCLUSTER will have connectivity to my virtualization network and to my management network. This is similar to my HOSTCLUSTER configuration. Although logically segregated, the virtualization traffic will be transported (encapsulated) over my management network.

First, I’ll create the logical network. To do this, in the Fabric pane, expand Networking, right-click Logical Networks, and select Create Logical Network.


In the wizard, name the network Virtualization and select Allow new VM networks created on this logical network to use network virtualization.


On the next page, create a network site and add an appropriate VLAN/Subnet for this network:


Finish the wizard to create the logical network. In PowerShell:

$virtualizationLogicalNetwork = New-SCLogicalNetwork -Name "Virtualization" -LogicalNetworkDefinitionIsolation $false -EnableNetworkVirtualization $true -UseGRE $true -IsPVLAN $false; 
$virtualizationSubnetVlan = New-SCSubnetVLan -Subnet "" -VLanID 802; 
$virtualizationLogicalNetworkDefinition = New-SCLogicalNetworkDefinition -Name "Virtualization" -LogicalNetwork $virtualizationLogicalNetwork -VMHostGroup @($virtualizationGroup) -SubnetVLan $virtualizationSubnetVlan;

Next, I’ll create an Uplink Port Profile for my VIRTUALCLUSTER that connects to the management network and the virtualization network. In the Fabric pane, expand Networking, right-click Port Profiles and select Create Hyper-V Port Profile.


Name the port profile Virtualization and choose to make it an Uplink Port Profile:


On the next page, we’ll select the site to connect to. The hosts in the VIRTUALCLUSTER will connect to the management site and the Virtualization site, so we’ll select these and select Enable Hyper-V Network Virtualization. In my environment, I’m using only Windows Server 2012 R2 hosts, so I don’t need to select this option, but I’ll select it in case I add Windows Server 2012 hosts in the future.


Finish the wizard to create the Uplink Port Profile. In PowerShell:

$virtualizationUplinkPortProfile = New-SCNativeUplinkPortProfile -Name "Virtualization" -LogicalNetworkDefinition @($mgmtLogicalNetworkDefinition, $virtualizationLogicalNetworkDefinition) -EnableNetworkVirtualization $true -LBFOLoadBalancingAlgorithm "HostDefault" -LBFOTeamMode "SwitchIndependent";

Next, I’ll create a Logical Switch for my VIRTUALCLUSTER. In the Fabric pane, expand Networking, right-click Logical Switches and select Create Logical Switch.


In the wizard, on the General page, name the switch Virtualization


On the Uplink page, add the Uplink Port Profile (in PowerShell, this corresponds to creating the Uplink Port Profile Set):


Finish the wizard to create the Logical Switch. In PowerShell:

$virtualizationLogicalSwitch = New-SCLogicalSwitch -Name "Virtualization" -EnableSriov $false -SwitchUplinkMode "Team"; 
$virtualizationUplinkPortProfileSetGUID = [System.Guid]::NewGuid().toString(); 
$virtualizationUplinkPortProfileSet = New-SCUplinkPortProfileSet -Name "Virtualization_$virtualizationUplinkPortProfileSetGUID" -LogicalSwitch $virtualizationLogicalSwitch -NativeUplinkPortProfile $virtualizationUplinkPortProfile;

Next, I’ll create a Virtualization VM Network. The VM Network that I’ll be creating will be the network that transmits the virtualization traffic, but not a tenant virtualization network. Any Logical Network that uses network virtualization will need a network on which to transmit the encapsulated traffic and can only be associated with one such network. Subsequent tenant networks will all exist on the same logical network. So, first, the transport network. In the VMs and Services pane, right-click VM Networks and select Create VM Network


Name the network Virtualization and select the Virtualization Logical Network that we just created.


On the Isolation page, choose No isolation, since this is the transport network for virtualization traffic.


Finish the wizard to create the VM Network. In PowerShell,

$virtualizationVMNetwork = New-SCVMNetwork -Name "Virtualization" -LogicalNetwork $virtualizationLogicalNetwork -IsolationType "NoIsolation";

Now, we can add the Logical Switch to the hosts.

Adding the Logical Switch to Hosts

This is similar to configuring the Logical Switches on the HOSTCLUSTER in the previous post, except that we’ll be adding the Virtualization Logical Switch:


And we’ll be adding an adapter for Management connectivity:


The PowerShell is also similar:

$nodeNames = $virtualizationCluster.Nodes.Name; 
$nodeIPs = @{}; 
foreach($name in $nodeNames) { 
    $nodeIPs[$name] = @([System.Net.Dns]::GetHostAddresses($name).IPAddressToString)[0]; 
foreach($node in $virtualizationCluster.Nodes) { 
    $jobgroupGUID = [System.Guid]::NewGuid().ToString(); 
    $firstNetworkAdapter = Get-SCVMHostNetworkAdapter -VMHost $node | ?{$_.IPAddresses -contains $nodeIPs[$node.Name]}; 
    Set-SCVMHostNetworkAdapter -VMHostNetworkAdapter $firstNetworkAdapter -UplinkPortProfileSet $virtualizationUplinkPortProfileSet -JobGroup $jobgroupGUID; 
    $otherNetworkAdapters = Get-SCVMHostNetworkAdapter -VMHost $node | ?{$_.IPAddresses -notcontains $nodeIPs[$node.Name]}; 
    $otherNetworkAdapters | %{Set-SCVMHostNetworkAdapter -VMHostNetworkAdapter $_ -UplinkPortProfileSet $virtualizationUplinkPortProfileSet -JobGroup $jobgroupGUID}; 
    $networkAdapters = @($firstNetworkAdapter, $otherNetworkAdapters); 
    New-SCVirtualNetwork -VMHost $node -VMHostNetworkAdapters $networkAdapters -LogicalSwitch $virtualizationLogicalSwitch -JobGroup $jobgroupGUID -CreateManagementAdapter -ManagementAdapterName "Management" -ManagementAdapterVMNetwork $mgmtVMNetwork; 
    Set-SCVMHost -VMHost $node -JobGroup $jobgroupGUID | Out-Null; 

After adding the Logical Switch to all the hosts in the cluster, we’re almost ready to proceed with onboarding Tenants. First, we’ll need some IP addresses for the transport network.

Adding Provider IP Pools

Static IP Pools can be assigned to a logical network so that deployed VMs can have an IP without a DHCP server. It’s a convenient way to manage IP addresses on a network. To create one, in the Fabric pane, expand Networking, select Logical Networks, right-click the Virtualization Logical Network, and select Create IP Pool.


You can name the pool Default and make sure the Virtualization Logical Network is selected.


Select the previously created network site:


Supply the IP range, or leave the defaults:


On the next pages, you can supply additional IP information (Gateway, DNS, WINS), but this is not necessary for this environment, as the VLAN that I’m using does not have any of these components. For now, just complete this wizard to create the IP Pool. In PowerShell:

$virtualizationStaticIPPool = New-SCStaticIPAddressPool -Name "Default" -LogicalNetworkDefinition $virtualizationLogicalNetworkDefinition -Subnet "" -IPAddressRangeStart "" -IPAddressRangeEnd ""

We have a static IP pool:


And now we can start adding Tenant Networks.

Adding Tenant Networks

In this scenario, we’ll bring in a tenant called Fabrikam2 (no doubt a subsidiary of Fabrikam). With network virtualization, this is as simple as adding a VM Network. So, in the VMs and Services pane, right-click VM Networks, and select Create VM Network.


Name the network Fabrikam2 and select Virtualization for the Logical Network.


On the next page, you’ll choose to Isolate using Hyper-V network virtualization and I’ll be choosing IPv4 for this network.


A quick hiatus: For IP protocols, you’ll notice that you can elect to either choose IPv4 or IPv6 in the tenant environment, but you can only choose IPv4 for the logical network (the transport network created earlier). This is because we only provided an IPv4 address for our network site. If you added an IPv6 address to your network site…


Then you can choose IPv6 transport for the logical network:


Note that dual-stacking is not supported for network virtualization. Moving on, we get to a VM Subnet page. On this page, we will define VM Subnets, which are akin to network sites. Creating VM Subnet allows you to then create a static IP pool for this subnet so your tenant can get a VM. I’ll explain in a moment how the IPs are used by a VM using network virtualization. For now, choose an arbitrary private IP range, as this is an isolated network.


On the connectivity page, you will likely see a notification that the VM Network will have no external connectivity:


This is because there is no gateway device added to VMM. In a later post, I’ll discuss external connectivity. For now, simply finish the wizard. In PowerShell:

$fabrikam2VMNetwork = New-SCVMNetwork -Name "Fabrikam2" -LogicalNetwork $virtualizationLogicalNetwork -IsolationType "WindowsNetworkVirtualization" -CAIPAddressPoolType "IPV4" -PAIPAddressPoolType "IPV4"; 
$fabrikam2Subnet = New-SCSubnetVLan -Subnet ""; 
$fabrikam2VMSubnet = New-SCVMSubnet -Name "Default" -VMNetwork $fabrikam2VMNetwork -SubnetVLan $fabrikam2Subnet;

We’re almost ready, but the VMs will need IP addresses.

Adding Customer IP Pools

Now is probably a good time to discuss the difference between a Provider Address (PA) and a Customer Address (CA). The PA is the IP address the VM will use to communicate when passing traffic over the transport network (in our case, the Virtualization VM Network) and the CA is the IP address the VM thinks it has. So, VM1 transmits a packet for VM2.


So the packets from VM1 on destined for VM2 on are encapsulated to be sent as packets from to In this case, the range is the CA space and the range is the PA space. Since the CA space resides in a virtualized network, you can have multiple CA spaces with overlapping subnets on a single PA space. Keep in mind that these numbers are just examples.

We created an IP Pool for the PA space earlier, now we need to create one for the CA space. To do this, in the VMs and Services pane, click VM Networks, right-click Fabrikam2, and select Create IP Pool.


Just like a IP Pool in the PA space corresponds to a subnet of a Network Site, an IP Pool in the CA space corresponds to a VM Subnet. In the wizard, supply the name Default and continue creating the IP pool as before.


You can accept the default range or supply a different range:


Finish the wizard to create the IP Pool. In PowerShell:

$fabrikam2StaticIPPool = New-SCStaticIPAddressPool -Name "Default" -VMSubnet $fabrikam2VMSubnet -Subnet "" -IPAddressRangeStart "" -IPAddressRangeEnd ""

Now, we’re ready to deploy Fabrikam2 tenant VMs.

Deploying VMs

I created a VM Template with a Windows Server 2012 R2 and configured the network card to be attached to Fabrikam2 and use static IPs:


I also added some OS Configuration to supply a machine name, Product Key, and local administrator password:


Now, I’ll deploy this VM Template. Although I chose my destination as all hosts, only my hosts that support network virtualization are suitable due to network configurations:


I’ll deploy two VMs. In PowerShell, deploying a VM from a template is similar to the GUI. VMM Generates a temporary hardware profile and a temporary VM template before generating a configuration object, running placement on that object, and then using that configuration object to create a VM. So, the PowerShell equivalent would be:

$vmTemplate = Get-SCVMTemplate -Name "Tenant VM";

$jobGroupID = [System.Guid]::NewGuid().ToString();
New-SCVirtualScsiAdapter -JobGroup $jobGroupID -AdapterID 7 -ShareVirtualScsiAdapter $false -ScsiControllerType DefaultTypeNoType;
New-SCVirtualDVDDrive -JobGroup $jobGroupID -Bus 1 -LUN 0;
New-SCVirtualNetworkAdapter -JobGroup $jobGroupID -MACAddress "00:00:00:00:00:00" -MACAddressType Static -VLanEnabled $false -Synthetic -EnableVMNetworkOptimization $false -EnableMACAddressSpoofing $false -EnableGuestIPNetworkVirtualizationUpdates $false -IPv4AddressType Static -IPv6AddressType Dynamic -VMSubnet $fabrikam2VMSubnet -VMNetwork $fabrikam2VMNetwork;
Set-SCVirtualCOMPort -NoAttach -GuestPort 1 -JobGroup $jobGroupID;
Set-SCVirtualCOMPort -NoAttach -GuestPort 2 -JobGroup $jobGroupID;
Set-SCVirtualFloppyDrive -NoMedia -JobGroup $jobGroupID;
$hwProfile = New-SCHardwareProfile -CPUType $vmTemplate.CPUType -Name "TemporaryHWProfile" -CPUCount 1 -MemoryMB 512 -DynamicMemoryEnabled $false -MemoryWeight 5000 -VirtualVideoAdapterEnabled $false -CPUExpectedUtilizationPercent 20 -DiskIops 0 -CPUMaximumPercent 100 -CPUReserve 0 -NumaIsolationRequired $false -NetworkUtilizationMbps 0 -CPURelativeWeight 100 -HighlyAvailable $true -HAVMPriority 2000 -DRProtectionRequired $false -NumLock $false -BootOrder "CD", "IdeHardDrive", "PxeBoot", "Floppy" -CPULimitFunctionality $false -CPULimitForMigration $false -CapabilityProfile $vmTemplate.CapabilityProfile -Generation 1 -JobGroup $jobGroupID;

$jobGroupID2 = [System.Guid]::NewGuid().ToString();
$template = New-SCVMTemplate -Name "TemporaryVMTemplate" -Template $vmTemplate -HardwareProfile $hwProfile -JobGroup $jobGroupID2;
$vmConfig = New-SCVMConfiguration -VMTemplate $template;
Set-SCVMConfiguration -VMConfiguration $vmConfig -VMHostGroup $virtualizationGroup;
Update-SCVMConfiguration -VMConfiguration $vmConfig;
New-SCVirtualMachine -Name $vmConfig.ComputerName -VMConfiguration $vmConfig -BlockDynamicOptimization $false -JobGroup "$jobGroupID2";

In the above script, all of the new commands that are part of the first job group will be picked up by the New-SCHardwareProfile cmdlet because it uses the same JobGroup ID. I deployed two VMs in my environment, one to each host, called TenantVM01 and TenantVM02.

Now, we can see which IPs are reserved from the pools:


If I connect to TenantVM01 on the console, I can ping TenantVM02 (after configuring firewalls appropriately):


Next, I’m going to create another VM Network called Fabrikam3 with the same range for the VM Subnet and the same range for its IP pool:


And I’ll deploy two VMs to this VM network, one to each host, as well.

Looking at IP leases again, we see the CA and PA IP addresses:


You can see more IP addresses for the new VMs and additional IPs for the transport network. Now, notice the descriptions for the HostNetworkAdapter IPs. At first glance, it looks like there is a one-to-one mapping of CA space to PA space IP addresses, but that’s not the case. Actually, each host will need a PA IP address for each virtualized network.

So, we have four VMs in two virtualized networks on two hosts. When TenantVM01 sends traffic to TenantVM02, it sends it from to The host CON-HyperV6 will encapsulate that packet and sent it from to CON-HyperV7 then receives the packet and sees that it’s destined for a VM on the network with the ID 3535905 and an IP address of, and will send it to TenantVM02. If we added more VMs to either network on either host, no more PA IP addresses would be allocated. To verify this, you can deploy another host and run the command again:


This means the maximum number of PA IPs you’ll need is [Number of Hosts]*[Number of VM Networks using Network Virtualization]. There are some technical details on this below.

Next Time

At this point, we have the ability to add and remove tenant VM networks on the fly with very little overhead. Just create a VM Network, create an IP pool in that network (or deploy a DHCP server to the network), and start deploying VMs. Right now, however, the VMs can only communicate with each other. In the next post, I’ll be adding a gateway to our VM networks so they can communicate externally.

Technical Details

For those interested in technical details of how NVGRE works, read on. If uninterested, it’s safe to move on to the next post without reading the rest of this one.

Many have the tendency to relate network virtualization in Windows and VMM to VLANs. While conceptually similar, that’s technically incorrect. The basis for NVGRE is GRE, an encapsulation protocol, which differs from simply tagging frames. I’ll briefly summarize the difference.


With VLAN tagging, a header is inserted into the Ethernet frame and the FCS is recalculated:


All this happens at Layer-2; the Layer-3+ information remains inside the payload of an Ethernet frame unchanged. The resultant frame is now tagged, and cannot be interpreted by a network device that is not expecting for this information. Similarly, a network device that is expecting this tag generally cannot interpret untagged frames.

Comparatively, GRE happens at Layer-3, but leaves the original Layer-2+ frame intact through encapsulation. Encapsulation is not unique to GRE; it happens at every layer in the network stack:


Note: Not to scale – the sizes of the headers at each layer are not all the same.

An application that communicates with another application on the network simply relies on the layer below it to perform the appropriate transportation.

For a VM using network virtualization, the VM believes that its NIC is sending bits on the wire, but the bits are just going to the vSwitch on the host, adding another layer to the stack. Once there, the vSwitch can then encapsulate the VM’s frame into a Layer-3 packet to be sent to another host:


Note: Not to scale – the sizes of the headers at each layer are not all the same.

Once the bits are on the wire, there’s be an Ethernet frame header containing an IP packet header. Following that header will be a GRE header, and then another Ethernet frame header. Inside the payload of that frame will be the original IP packet:


The diagram above may be too difficult to read without following its link to see a larger version. I’ve shaded the boxes that are worth discussing:

IPv4 Protocol:

This byte of an IPv4 header indicates the protocol of the packet, which will dictate how a L3 device treats it. And a additional header that might follow. Common protocols you might be familiar with are ICMP (1), TCP (6), and UDP (17). There are also types such as routing protocols like IGRP (9), EIGRP (88), or OSPF (89), authentication protocols like ESP (50) or AH (51), and tunneling protocols like L2TP (115) or GRE (47). Since NVGRE uses GRE, all virtualized packets on the network will have 0x2F (47 in hexadecimal) in the protocol field and will also have a GRE header.

GRE Header:

The GRE header is pretty small and we’re only really interested in one field. The protocol type is always 0x6558, so we’ll just be interested in the 3-byte VSID. When a VM Network using network virtualization is created, VMM chooses a VSID in a range from 4096 up to about 16.7 million (three bytes worth). This VSID is how the receiving host knows where to forward the CA packet.

Host Configuration Objects

Earlier, I showed that the CA addresses and PA addresses are leased from VMM, but let’s go a bit deeper with a goal of understanding how the configuration on a host really works. First, I deleted all my tenant VMs so I can start from scratch. I’ll deploy a VM called TenantVM01 into Fabrikam2 on CON-HyperV7.

Connecting to CON-HyperV7’s console, I can use the Get-NetVirtualization* tools. First, we can investigate the LookupRecord. This will show the mapping of CAs to PAs. When I run it, I see:


What we’re looking at here is two records. One record ( is for the VM I just deployed (TenantVM01). Its PA is, which is in my Virtualization Logical Network. The other address I see my gateway address for the Fabrikam2 network, Its Provider Address is non-routable. If I ping that gateway address from within the VM, it will respond. In the next post, we’ll deploy a Network Virtualization gateway and I’ll discuss this further. For now, we will ignore the GW record. Notice the VirtualSubnetID is 3535905, just like it showed in the Get-SCIPAddress command in VMM.

The next command we can look at is the Get-NetVirtualizationProviderAddress. This command will show you all of your PAs on this host, which can be useful. It will also show the VLAN ID that the PA uses:


This information doesn’t show up on CON-HyperV6 (the other machine in the VIRTUALCLUSTER), yet, since it doesn’t have any VMs in that virtual network deployed to it. Next, we’ll deploy a VM called TenantVM02, also into Frabrikam2, onto CON-HyperV6. After deploying, I can use Get-SCIPAddress in VMM to see the assignments:


Now, on either CON-HyperV6 or CON-HyperV7, I should see the same thing:


Now, if CON-HyperV6 gets a packet from TenantVM02 destined for, it knows to send that packet to This configuration is controlled by VMM. If TenantVM02 need to migrate to another host, VMM could notify any host with a VM in Fabrikam2 to update its lookup records.

That’s all for the technical dive for this post. In the next post, I’ll be discussing network virtualization gateways and I’ll have another technical dive at the end to expand on these concepts.

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  • Nice post,

    Can you give us a bit more clearance how MTU 1458 is set on vNICs because of the fragmentation and GRE payload overhead.

    Also it will be interesting to explain the lookup table of the Gateway Hyper-V host as its different in your next post.

    Thank You

  • Hi YSDimov,

    Thanks for the feedback, I'll be sure to go into more detail on the next posts. I have been considering adding a full "deep-dive into NVGRE" as an additional post to this series, and it sounds like there's some interest in that.

    To your question around MTU settings for fragmentation avoidance: are you asking if the MTU size on the vNIC is reduced by VMM or are you asking how to adjust the MTU size on the vNIC?

  • Hi Kurt,

    I know that MTU is reduced by VMM during the OS boot process when it validates vNIC is using VSID but don't know how it is forced within the OS. Does it works only with 2012/8 guests? IS it applied only for dynamic or static IP

    I guess that maybe VMM DHCP extension is used but im not sure. What I know also is that for us process is not working so im wondering how it works to be able to validate our configuration or to open a case :)

    To adjust it I can use Run it once post script but the problem is that it will change MTU size for any adapters even they are not using NVGRE like public adapter.

    Thanks in advance

  • Hi YsDimov,

    VMM doesn't do anything to reduce the size of the MTU; that should be built into PMTU discovery in the networking stack.

    When the vNIC comes up, if the MTU is too large to traverse the network, the device that can't send the packet should drop it and reply via ICMP that the packet is too large (Fragmentation Needed).

    When the vNIC receives this ICMP message, it should reduce the size of its MTU and set the DF bit. The PMTU discovery follows a standard (RFC 1191) and I think it's been the same since Windows 2000.

    If ICMP packets are filtered, this can affect the process. This would be the first place I would check.

  • Hi Kurt,

    I think this is not the case with NVGRE vNICs. And the problem is within the GW compartment.

    For example I can ping but not from VM within NVGRE network. What I know is that this was a problem in the Preview and still persist for us in R2. The MTU should be reduced for NVGRE adapters with some kind of other mechanism.

  • I don't believe responds to pings.

    Let me try and give a sufficiently clear answer.

    VMM deploys the VM and VMM doesn't manage any MTU settings. The VM should detect MTU settings end-to-end and adjust, providing ICMP traffic is allowed. Generally speaking, allowing ICMP traffic of type 3, code 4 to pass on your network unfiltered should fix the issue.

    Now, as to where the MTU 1458 setting comes from... The maximum MTU for a VM's NIC should be set by the VM's OS when initializing the NIC. I can't speak to the details of the Microsoft Hyper-V Network Adapter driver, but when it initializes NDIS, the Hyper-V Virtual Switch should expose the MTU settings.

    If the Hyper-V Virtual Switch is using network virtualization, the maximum MTU it exposes should consider the 42 bytes of overhead for NVGRE (giving 1458). Doing a 'netsh int ipv4 sh int' should display the MTU size in the VM. As this should be set during the NDIS initialization phase, static versus dynamic IP doesn't matter. This functionality is only in Windows Server 2012 R2 hosts and the driver setting is only in guests that are Windows Server 2008 and above. Other combinations will need their MTUs set manually.

    Manually setting MTUs is a somewhat boring process of pinging with different size packets until one doesn't pass and then adjusting the MTU accordingly. On Windows, you can do 'ping -f -l <n>' to send an n+28 byte ping. If your MTU is properly set, you should get fragmentation notices above a certain threshold and echo replies below. If you get timeouts, the MTU should be adjusted accordingly to close the gap. All of this will obviously rely on ICMP traffic to traverse the network.

  • Hi Kurt, I have a question about this picture, as your description, the nvgre packet has 2 fcs fields. I did not see any information about that in the draft of nvgre. In v-switch layer, I believe the inner fcs should be stripped. The encapsulated nvgre packet should only has one fcs according to the draft. The fcs should be added before a physical layer process. So the inner fcs is meaningful in the overlay L2 network only. It is not necessary to bring it into the outer L3 encapsulation's payload. The gateway or hyper-v should check the packet from VM by inner fcs and guarantee the correctness of packet by generating an outer-fcs when doing encapsulation. In de-encapsulation case, it should check the outer fcs and generate an inner one and send the packet to VM. The 2-fcs encapsulation is not a GRE(NvGRE) format encapsulation. In VxLAN, it is clearly stated in the encapsulation format. So please check if it is different in NvGRE case?

  • Hi Kurt. Thanks for a really useful and informative series. I'm really looking forward to the rest of it. :) Just a small error in the text though. Where you write "To verify this, you can deploy another host and run the command again" I think you mean "To verify this, you can deploy another VM and run the command again".

  • no words to thank you kurt , I read several posts on NW virtualization before , but none explained in detail like this , thanks a lot kurt krishna

  • Hi.....Can you explain me why you created or is required create a transport network (No Insolation) for Hyper-v Network Virtualization, then will be use for all tenants?