Background Information
This section describes the basic terminology that this document uses. This section also explains the background theory for the configurations in this document.
Term Definitions
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MTU: MTU is short for Maximum Transmission Unit, the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packets before transmission.
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Jumbo: Jumbo frames are frames that are bigger than the standard Ethernet frame size, which is 1518 bytes (including Layer 2 (L2) header and FCS). The definition of frame size is vendor-dependent, as these are not part of the IEEE standard.
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Baby giants: The baby giants feature allows a switch to pass or forward packets that are slightly larger than the IEEE Ethernet MTU. Otherwise, the switch declares big frames as oversize and discards them.
Background Theory
In order to transport traffic across switched-networks, ensure that transmitted traffic MTU does not exceed the MTU that the switch platforms support. Here are the reasons why the MTU size of certain frames is truncated:
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Vendor-specific requirements: Applications and some Network Interface Cards (NICs) can specify an MTU size outside of the standard 1500 bytes. Much of this drive has been due to studies undertaken, which prove that an increase in the size of an Ethernet frame can increase average throughput.
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Trunking: In order to carry VLAN-ID information between switches or other network devices, trunking has been employed to augment the standard Ethernet frame. Today, the two most common forms of trunking are Cisco's proprietary InterSwitch Link (ISL) encapsulation and IEEE 802.1q. Refer to these documents for more information on trunking:
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MultiProtocol Label Switching (MPLS): When you enable MPLS on an interface, MPLS can also augment the frame size of a packet, depending on the number of labels in the Label stack for an MPLS-tagged packet. The total size of a label is four bytes. The total size of a label stack is n x 4 bytes. If a label stack is formed, the frames can exceed the MTU.
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