The size of an MTU is governed by the physical properties of the communications media. If encapsulation of Ethernet in MPLS pushes the size of the MPLS frame past the MTU of the MPLS edge switches, the switches will drop it. In that case the size of the Ethernet frames must be taken into consideration. For instance, a device on an Ethernet LAN might want to send a payload to a device on an Ethernet LAN in another city and have to cross an MPLS connection on the way. MTU size also comes in to play when, for a frame to get from its source to its destination, it may have to cross a network that use a different protocol than that used by the source and destination networks. The intent is for the source device resend the payload in smaller packets, but it might not be configured to support this. So, if 2000-byte Ethernet packets arrive at a router, it will split their payloads in two and repackage them into two packets that are each smaller than 1500 bytes and so meet the MTU.Īn alternative is that the router drops the packet but sends the source device an internet control-message protocol (ICMP) packet-too-big message. The maximum transmission unit (MTU) is the largest number of bytes an individual datagram can have without either being fragmented into smaller datagrams or being dropped along the path between its source and its destination.įor Ethernet frames-and many other types of packets-that number is 1500 bytes, and it generally meets the requirements of traffic that can cross the public internet intact.
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