Jumbo, MTU and MFL

MTU  is short for maximum transmit unit, which represents the length of an L3 packet or IP data packet.

MRU is short for Maximum Received Unit, which represents the maximum frame which can be received on the link.

MFL  is short for maximum frame length, which represents the length of an L2 packet or Ethernet MAC frame.

To better explain, let’s look at the structure of an Ethernet frame:

From the diagram, we can see that:

– The IP data packet ranges from 46 bytes to 1500 bytes. So the MTU is 1500 bytes.
– The Ethernet MAC frame ranges from 64 bytes to 1518 bytes. So the MFL is 1518 bytes.
– The MFL increases by four bytes for each added VLAN tag.

Jumbo Frames are also called “Giant Frames”. The name appeared during the jump from 10M to 1000M (GE) when the original 1518 payload for a typical Ethernet frame was deemed too small to achieve the required transmission efficiency.
IT & Telecom equipment manufacturers began to use Jumbo Frames from 1518 bytes to 9000 bytes.

Jumbo Frames raise transmission efficiency by reducing overhead. However, there are trade-offs:
 1. Jumbo Frames are not included in any international standard, so compatibility is lacking.
 2. As the frames are larger, the impact from even a single lost packet could significantly affect service quality!
 3. Jumbo Frames reduce overheads but may have an adverse effect on low-quality links.

Different vendors have different MTU sizes because depending on the vendors and devices, the IFG, CRC and Ethernet encapsulation (DMAC, SMAC, and TAG ID) can or cannot be included on the interface MTU. Below are some vendor’s standards:

• Cisco Ethernet MTU: 1518 bytes / 1522 bytes / 524 bytes
• Huawei Ethernet MTU: 1500 bytes
• Juniper Ethernet MTU: 1514 bytes
• Alcatel Ethernet MTU: 1514 bytes (Access Ports), 1518 bytes (Trunk Ports)
• Ericson Ethernet MTU: 1500 bytes