Why Change the MTU Size?
The Maximum Transmission Unit (MTU) defines the largest packet size (in bytes) that a network interface can transmit without requiring fragmentation. The default Ethernet MTU is 1500 bytes. However, if you are configuring a Storage Area Network (SAN), iSCSI targets, or heavy database replication over dedicated switches, increasing the MTU to 9000 bytes (known as Jumbo Frames) drastically reduces CPU overhead and increases throughput. Conversely, if you are routing traffic through complex VPN tunnels, you may need to lower the MTU to prevent packet loss.
Step 1: Check Current MTU Settings
To view the current MTU size for all active network interfaces on your Windows Server, open an elevated Command Prompt or PowerShell window and run the following command:
netsh interface ipv4 show subinterfaces
You will see a list of your network adapters. Look at the MTU column. Standard adapters will almost always display 1500. Note the exact name of the interface you want to modify (e.g., “Ethernet 2” or “Storage-NIC”).
Step 2: Verify Network Infrastructure Support
Critical Warning: Before enabling Jumbo Frames (MTU 9000), you must ensure that every single physical switch, router, and client network card in that specific subnet explicitly supports and has Jumbo Frames enabled. If one device in the chain does not support the larger packet size, all traffic will be dropped, instantly breaking network connectivity.
Step 3: Change the MTU via Command Line
The fastest and most reliable way to change the MTU is using the netsh command. To change the MTU of an adapter named “Storage-NIC” to 9000 bytes, run:
netsh interface ipv4 set subinterface "Storage-NIC" mtu=9000 store=persistent
The command should return an “Ok.” response. The store=persistent flag ensures the setting survives a server reboot.
Step 4: Enable Jumbo Frames in the Device Manager
While the Windows OS is now configured to send larger packets, the physical Network Interface Card (NIC) driver must also be instructed to allow them.
Open Device Manager (devmgmt.msc), expand Network adapters, right-click your specific network card, and select Properties. Navigate to the Advanced tab. Scroll through the properties list until you find “Jumbo Packet” or “Jumbo Frames”. Change the value drop-down from Disabled to 9014 Bytes (or 9000). Click OK. The network adapter will briefly disconnect and reconnect.
Step 5: Test the New MTU Size
You must verify that large packets are actually successfully traversing the network without fragmentation. You can test this by sending a ping with a specific packet size (-l) and instructing the router not to fragment it (-f):
ping 192.168.10.50 -f -l 8972
(Note: We use 8972 because the ICMP header consumes 28 bytes). If you receive successful replies, your Jumbo Frames configuration is fully operational and your storage performance will improve significantly.