![]() ![]() ![]() The IP MTU configured with the ip mtu interface configuration command.The hardware MTU configured with the mtu interface configuration command.Media MTU Sizes by Interface Type for M5, M7i, M10, M10i, M20, and M40 Routers Interface TypeĪdaptive Services (MTU size not configurable)Ī Cisco router configured for IP+MPLS routing also uses different Maximum Transmission Unit (MTU) values fo different purposes: If you change the size of the media MTU, you must ensure that the size is equal to or greater than the sum of the protocol MTU and the encapsulation overhead. if IP MTU is already set, so MPLS MTU = IP MTU + 20 byte.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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