Jumbo frames

Jumbo frames are a technique for maximizing the throughput of Ethernet networks by increasing the frame size from the default 1,518 bytes up to 9,000 bytes. To gain the maximum benefit, all devices on the network have to support the frame size; non-jumbo routers will break the frames down to 1,518-byte frames.

Network World columnist and jumbo-frames advocate Kevin Tolly explains the rationale for jumbo frames:

While the speed of Ethernet has increased 1,000-fold over the past decade - from 10M bit/sec to 10G bit/sec - the maximum frame size hasn't grown at all. With the grand exception of 4-bytes added to accommodate virtual LANs (VLAN) and priority, which in any case can't be used for payload, the size has remained at 1,518 bytes.

Relative to the transport capacity, then, the maximum frame size has shrunk to almost microscopic proportions. Indeed, at 1G bit/sec it is like having a maximum frame size of 15 bytes. Divide by 10 again when cranking up to 10G bit/sec and we'd have 1.5 bytes per frame. And these are "best-case" examples using the maximum-allowable Ethernet frame size.

This low ceiling on frame size causes stations to slice up data into pitifully small chunks. Imagine loading an 18-wheeler carrying sugar - one cube at a time.

The processing load on end stations is enormous. And the load also increases on switches as they peer into and process each frame.

The alternative, Jumbo Frames, increases the frame size maximum by a factor of about six up to the 9K realm. Data streams better, and processing load is reduced dramatically. Tests have proven that.

Just consider a few numbers. At gigabit speeds, it takes only about 14,000 frame/sec to fill a pipe with 9K Jumbos compared with more than 81,000 frame/sec for 1,518-byte frames. These numbers jump to 140,00 and about 812,000 frame/sec at 10G bit/sec.

One drawback of jumbo frames is that there is no standardization for them, so users who want to implement them must often buy all of their gear from the same vendor.

Additional resources

Time for Jumbo Frames - again!
The column from which the above was excerpted. Network World, 01/07/02.