Set the Protocol MTU

For each interface, you can configure an interface-specific MTU by including the mtu statement at the [edit interfaces interface-name] hierarchy level. If you need to modify this MTU for a particular protocol family, include the mtu statement at the [edit interfaces interface-name unit logical-unit-number family family] hierarchy level:

[edit interfaces interface-name unit logical-unit-number family family]

mtu bytes; 

The default protocol MTU is 4470 bytes for ATM PVC, Cisco HDLC, Frame Relay, and PPP encapsulations. For Ethernet encapsulation on IPv4, the default protocol MTU is 1500 bytes. For Ethernet encapsulation on ISO, the default protocol MTU is 1497 bytes.

When you initially configure an interface, the protocol MTU is calculated automatically. However, if you subsequently change the media MTU, the protocol MTU on existing address families does not automatically adjust. If you increase the size of the protocol MTU, you must ensure that the size of the media MTU is equal to or greater than the sum of the protocol MTU and the encapsulation overhead. If you reduce the media MTU size, but there are already one or more address families configured and active on the interface, you must also reduce the protocol MTU size. (You configure the media MTU by including the mtu statement at the [edit interfaces interface-name] hierarchy level, as discussed in Configure the Media MTU.)

For Ethernet encapsulation when the family is mpls, the default protocol MTU is 1500 bytes, including 4 to 12 bytes of overhead. The maximum number of DCLIs is determined by the MTU on the interface. If you have keepalives enabled, the maximum number of DLCIs is 1000, with the MTU set to 5012.

The actual frames transmitted also contain cyclic redundancy check (CRC) bits, which are not part of the MTU. For example, the default protocol 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.