Change the MTU setting of a VPC network

In Google Cloud, you set the MTU for each VPC network. Virtual machine (VM) instances that use that network must be configured to use the same MTU setting for their interfaces.

Linux VMs based on public OS images receive the VPC network's MTU setting from the DHCP server. DHCP Option 26 contains the MTU setting.
Windows VMs do not use DHCP to configure the MTU setting. If you change the MTU of a VPC network, you must change the MTU setting of the VM.

For more information about VMs and MTU settings, see VMs and MTU settings.

There are two ways to start using a VPC network with a different MTU setting:

You can change the MTU setting of an existing VPC network, which requires that you shut down all VMs in the network at the same time.
You can create a new VPC network with a different MTU setting, and migrate the VMs to it to it.

Change the MTU of a network

Each VPC network has an MTU. Do not change the network MTU while the VMs are running. Doing so can result in a period where VMs have different MTUs, which can lead to unreliable network connectivity.

For detailed considerations regarding changing the MTU of a network, see the maximum transmission unit overview.

The recommended way to update the network MTU is to stop all running VM instances, change the network MTU, then start all instances. VMs based on public Linux images automatically configure each of their network interfaces to use the MTU of the attached VPC network when they start. You must manually update the MTU of Windows VMs and VMs based on custom images that do not rely on DHCP for network MTU configuration.

Console

Updating network MTU is not supported in the Google Cloud console. See the Google Cloud CLI or API instructions.

gcloud

Stop all VMs.

For each zone in the network, stop all VMs in that zone.

gcloud compute instances stop INSTANCE_NAMES... \
    --zone=ZONE

Update network MTU.

gcloud compute networks update NETWORK \
    --mtu=MTU

Start all VMs.

For each zone in the network, start all VMs in that zone.
```
gcloud compute instances start INSTANCE_NAMES... \
    --zone=ZONE
```
Replace the placeholders with valid values:
- INSTANCE_NAMES... is a space-separated list of instances.
- NETWORK is the name of the VPC network whose dynamic routing mode you need to change.
- MTU specifies whether the network has an MTU of 1460 (default), 1500, or 8896. Review the maximum transmission unit overview before setting the MTU to higher than 1460.

API

Change the MTU of an existing VPC network.

PATCH https://www.googleapis.com/compute/v1/projects/PROJECT_ID/global/networks/NETWORK
{
  "mtu": MTU
}

Replace the placeholders with valid values:

PROJECT_ID is the ID of the project that contains the VPC network to modify.
NETWORK is the name of the VPC network to modify.
MTU is the maximum transmission unit of the network. MTU can be set to anything from 1300 through 8896 (default: 1460). Review the maximum transmission unit overview before setting the MTU to higher than 1460.

For more information, refer to the subnetworks.patch method.

Change the MTU setting of a Windows VM

Windows VMs based on public OS images are configured with a fixed MTU of 1460. Custom Windows VMs might have different MTU settings.

To set Windows VMs to use a different MTU, do the following on each VM:

Command prompt

Open Command Prompt (cmd.exe) as Administrator.
Run the following command to determine the index of the interface that you want to update:
```
netsh interface ipv4 show interface 
```

Update the interface:

netsh interface ipv4 set interface INTERFACE_INDEX mtu=MTU store=persistent

Update the device driver configuration:
1. Open the Device Manager with following command:
```
devmgmt.msc
```
2. Expand the Network adapters category.
3. Right-click the Google VirtIO Ethernet Adapter/Google Ethernet Adapter device.
4. Select Properties.
5. Click the Advanced tab.
6. In the Init.MTUSize field, enter the MTU setting.
7. Click OK.
The instance briefly loses connectivity, because the driver automatically restarts when the registry entries change.

PowerShell

Open PowerShell as Administrator.

Run the following command:

Set-NetIPInterface -InterfaceAlias INTERFACE_NAME -AddressFamily IPv4 -NlMtu MTU

Run the following command to update device driver settings:
```
Set-NetAdapterAdvancedProperty -Name "Ethernet" -RegistryKeyword MTU -RegistryValue MTU 
```
You will briefly lose connectivity to the VM, because the driver automatically restarts when the registry entries change.

Migrate VMs to a different MTU network

You might decide to migrate your services to new VMs in a new network rather than changing the MTU of your existing network. In such a case, you might have a server, such as a database server, that needs to be accessible to all VMs during the migration. If so, the following general approach might help you migrate cleanly:

Create the new network with the new MTU.
Create any necessary firewall rules and routes in the new network.
Create a VM with multiple network interfaces in the old network. One interface connects to the new network using the new MTU and the other connects to the old network using the old MTU.
Configure this new VM as a secondary server for the existing one.
Fail the primary server over to the secondary one.
Either Migrate VMs to the new network or create new VMs in the new network. If you create new VMs, you can create them from scratch, from an existing image, or by creating a snapshot of the existing VMs and using that to populate the new persistent disks.
Configure these VMs to use the operational server in that network.
Migrate traffic to the new VMs.
If you intend to delete the old network, create a new server in the new network, get it in sync with the existing server, and fail over to it.
Delete the old server and old network.

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