This topic describes Ethernet Maximum Transmission Unit (MTU), fragmentation, and MTU discovery processes for Cisco IOS XR software-based network configurations.
The Ethernet Maximum Transmission Unit (MTU) is a network parameter that
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represents the size of the largest frame excluding the 4-byte frame check sequence, varies across different physical networks along a packet's path, and
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facilitates efficient data transmission across networks.
Frame forwarding processes
Cisco IOS XR software supports two types of frame forwarding processes:
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Fragmentation for IPv4 packets fragments packets as necessary to fit within the MTU of the next-hop physical network.
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MTU discovery process determines largest packet size—This process is available for all IPv6 devices, and for originating IPv4 devices. In this process, the originating IP device determines the size of the largest IPv6 or IPv4 unfragmented packet that the system can send. The largest packet is equal to the smallest MTU of any network between the IP source and the IP destination devices. If a packet is larger than the smallest MTU of all the networks in its path, the system fragments that packet as necessary. This process ensures that the originating device does not send an IP packet that is too large.
Default Ethernet MTU values
The system automatically enables the jumbo frame support for frames that exceed the standard frame size. The default value is 1514 for standard frames and 1518 for 802.1Q tagged frames. These numbers exclude the 4-byte frame check sequence (FCS).