Normal standards-compliant IEEE-defined ethernet frames have a maximum MTU of 1500 bytes (plus 18 additional bytes of header/trailer for srcaddr, dstaddr, length/type, and checksum).
Do they exist ..?
Juniper talks about 1514 rather than 1518 (excluding just the 4 byte FCS of ethernet frames when specifying MTUs).
Cisco InterLink Switch Frame Format takes the max encapsulated ethernet frame size out to 1548 bytes
Jumbo Frames
jumbo frame is basically anything bigger than 1522 bytes, with a common size of 9000 bytes, which is exactly six times the size of a standard Ethernet frame.With Ethernet headers, a 9k byte jumbo frame would be 9014-9022 byte.how large should an Ethernet frame be?
Ethernet's 32-bit cyclic redundancy check is effective for detecting bit errors at frame sizes under 12,000 bytes, thereby drawing a logical upper limit. Within that, the optimum large frame size can be determined by an application's block size. For example, Network File System (NFS) transfers data in 8,192-byte blocks. So adding room for headers, an attractive maximum Ethernet frame size for NFS applications is 9,000 bytes.
Why Jumbo Frames?
Every data unit on a network has to be assembled by the sender, and its headers have to be read by the network components between the sender and the receiver. The receiver then reads the frame and TCP/IP headers before processing the data. This activity, plus the headers added to frames and packets to get them from sender to receiver, consumes CPU cycles and bandwidth.
A single 9k jumbo frame replaces six 1.5k standard frames, producing a net reduction of five frames, with fewer CPU cycles consumed end to end. Further, only one TCP/IP header and Ethernet header is required instead of six, resulting in 290 (5*(40+18)) fewer bytes transmitted over the network.
- There are other jumbo frame sizes used, but larger sizes don't always lead to better performance.
- Jumbo frames require gigabit Ethernet.
- Gigabit Ethernet Layer 2 switches forward or drop jumbo frames; they don't fragment.
- Fragmentation is a Layer 3 (routing) function.
