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Updated 11/2/2007 - Corrected info regarding UDP DF handling.

A different kind of Jumbo

In many of our reviews, we comment on a device's support for gigabit Ethernet, as well as whether it supports jumbo frames. But what are jumbo frames and why should you care about them? In basic terms, a jumbo frame is any frame larger than a standard Ethernet frame. Their main value is that they can dramatically improve network performance by raising effective bandwidth.

Jumbo frames can be a bit of a challenge, though, as there are multiple sizes, interpretations, and technologies, making it difficult to fully understand and leverage their value, as well as fully integrate jumbo frame devices into your network. The goal of this article is to explain jumbo frames and help you determine if and how you can use jumbo frames in your network.

Gigabit Ethernet

An important fact is jumbo frames are not supported at speeds less than 1000 Mbps. There are some exceptions, with larger frame sizes possible on FDDI, POS, and Fiber Channel connections, but for typical small network purposes, you're going to need equipment with gigabit Ethernet interfaces that support jumbo frames to see network gains from larger frame sizes. We've covered the details on gigabit Ethernet pretty thoroughly in our Gigabit Ethernet Need to Know a few years back; it's an excellent discussion on the facts and details regarding gigabit network speeds.

Gigabit Ethernet interfaces are now common on PCs, NAS devices, switches and routers. Not all gigabit Ethernet interfaces support jumbo frames, though, and since there isn't a standard for jumbo frames, there are different sizes.

For example, I have a laptop and desktop PCs with gigabit Ethernet NICs (Network Interface Card). The desktop gigabit Ethernet NIC allows jumbo frame configuration, whereas my laptop NIC supports gigabit Ethernet, but does not support jumbo frame configuration. Figure 1 is a screen shot of the configuration options I have for jumbo frames on my desktop PC, showing the different sizes of jumbo frames it supports.

NIC jumbo properties

Figure 1: Properties of a NIC supporting jumbo frames

As you can see, my desktop PC NIC supports jumbo frame sizes of 16128, 4088, and 9014 bytes. Based on the 9014 byte supported size, it appears my desktop PC is one of those interpretations that doesn't consider the 4 byte CRC header as part of the countable frame size, a detail I'll cover in the "What is a Jumbo Frame?" section.

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