Info about Frames
What are Jumbo Frames?
It is a feature of Gigabit Ethernet (GbE) that allows frames up to 9 kilobytes (KB) in size.
Standard 10Base5 Ethernet, created in the early 1980s, supports frames up to 1.5 KB in length. This standard has been supported by all subsequent versions of Ethernet, including 10BaseT Ethernet, Fast Ethernet, and GbE. Unfortunately, for the speeds of 1 gigabit per second (Gbps) supported by GbE, these standard frames become inefficient, and better performance can be achieved at these speeds by using larger frames. Consequently, the designers of the 802.3z GbE standard included support for jumbo frames as an option for gigabit networking.
Jumbo frames can have payloads up to 9 KB in length and can result in much higher network utilizations for activities such as file transfers, where utilization of over 80 percent is achievable when using jumbo frames compared to typically under 20 percent when using standard Ethernet frames. Unfortunately, using jumbo frames adds issues of interoperability for enterprise networks, as these frames are not supported by slower 10 and 100 megabits per second (Mbps) versions of Ethernet. Since performing Layer-2 fragmentation and reassembly of jumbo frames is not a desirable solution, two standard methods are generally employed to incorporate GbE jumbo frames into typical enterprise networks:
• Dedicating all downstream traffic on specific ports of core GbE switches to devices supporting jumbo frames, such as GbE workgroup switches or server-switch interconnects
• Employing 802.1q virtual LANs (VLANs) to logically segregate those portions of the network that use jumbo and standard Ethernet frames
While Internet Protocol (IP) can intrinsically support frames of even larger size up to 64 KB (and IPv6 can support frames up to 4 gigabytes [GB] in length!) and using such monster frames could make gigabit networking even more efficient, there is an inherent obstacle to using such large frames. Ethernet uses a 32-bit cyclical redundancy check (CRC) at the end of each frame, and this error correction method is only efficient for frames up to about 12,000 bytes in length. The reason for choosing 9 KB as the maximum length for jumbo frames is to ensure the efficiency of such error correction while making possible the transmission of 8-KB Network File System (NFS) datagrams without the need of fragmentation.