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How To Enable Jumbo Frames

Enabling jumbo frames can significantly increase your network’s throughput while consuming fewer CPU cycles (we’ll explain why in a moment). But before you configure your PCs to use jumbo frames, you should know that their value lies primarily in speeding up large file transfers within your network (versus to and from the Internet).

You should also be aware that enabling jumbo frames might cause problems with latency sensitive network applications, such as VoIP and online games. Lastly, jumbo frames are available only on gigabit networks, and every device in the path of the file transfer—all your switches (starting with the one in the router), your PC, server, and/or NAS—must all be equipped with gigabit Ethernet interfaces. What’s more, each of those devices must be capable of passing the same size jumbo frames. Okay, here’s another twist: There is no such thing as a standard-size jumbo frame.

Confused? Here’s a brief Ethernet primer: Data travels over an Ethernet network in frames, with each frame containing a 1,460-byte payload (the data being transferred) plus a 40-byte TCP/IP header. Add these together and you have Ethernet’s 1,500-byte maximum transmission unit (MTU). Now add the 12-byte Ethernet frame header, two bytes that identify the Ethernet type, and four bytes of error-detection data and you get a standard-sized Ethernet frame of 1,518 bytes.

A jumbo frame is simply an Ethernet frame that’s larger than 1,518 bytes. Since the header sizes remain the same, each frame can carry a larger payload. You’ll see the greatest benefit from enabling jumbo frames when you transfer large files across your network. Since fewer frames are needed to carry the same amount of data, transfer speeds go up and CPU utilization goes down. (CPU utilization probably isn’t an issue on your PC, but your server or NAS almost certainly has a fraction of that horsepower.)

The tricky part is determining the optimum size frame that everything supports, and that will take a little trial-and-error benchmarking to figure out. Gigabit Ethernet supports a maximum MTU of 9,000 bytes; but if you push your network too hard, performance might deteriorate as dropped and fragmented frames are constantly resent.

Your PC’s network interface probably came from the factory with jumbo frames disabled.

As we mentioned, the value of using jumbo frames manifests itself during large file transfers. So before you make any changes to your gear’s MTU settings, determine your network’s baseline performance by using a stopwatch to measure how long it takes to copy a large file—a DVD ISO image, for instance—from a client to a server or a NAS box across your network and back again.

On a PC running Windows 7, click the Start menu and then right-click on Computer and choose the Manage option. Click on Device Manager in the left-hand column, click on Network adapters in the center column, and then right-click on the network adapter you’re using to connect to your network and choose Properties. Click on the Advanced tab and look for Jumbo Frame in the Property window. Now, choose the MTU size you wish your NIC to operate with.

You’ll need to do the same thing with your server or NAS device (most NAS boxes have a Web interface for this purpose, so you should refer to its documentation for specifics). Although each device in the path of the transfer needs to be configured to use the same size jumbo frame, you probably won’t be able to manually configure the switch in your router or even a stand-alone switch if it’s an unmanaged model (the most common type in the consumer market). Fortunately, you won’t need to: You’re golden as long as the router’s and any stand-alone switch you use support jumbo frames.

Once you’ve tweaked the MTU values on each device’s network interface, measure how long it takes to copy that same file across the network. If you get a bump in speed with a 4K MTU, try 9K. If performance degrades at that size, dial it back until you find the optimum value.

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