![]() MPLS MTU = physical interface MTU – L2 encapsulation overhead – 12.Default IP MTU = Default media MTU – L2 encapsulation overhead.Default media MTU = Default IP MTU + L2 encapsulation overhead.Currently, the software provides for three labels of four bytes each, for a total of 12 bytes. From this value, the software subtracts the encapsulation-specific overhead and space for the maximum number of labels that might be pushed in the Packet Forwarding Engine. If you do not configure an MPLS MTU, the JUNOS software derives the MPLS MTU from the physical interface MTU. The physical MTU for Ethernet interfaces does not include the 4-byte frame check sequence (FCS) field of the Ethernet frame. ![]() ![]() For example, the media MTU for a Gigabit Ethernet interface is specified as 1500 bytes, but the largest possible frame size is actually 1504 bytes you need to consider the extra bits in calculations of MTUs for interoperability. NOTE: The actual frames transmitted also contain cyclic redundancy check (CRC) bits, which are not part of the media MTU.
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