Set the Protocol MTU

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 change.

For a list of default protocol MTU values, see Table 3 through Table 7.

If you need to modify the MTU for a particular protocol family, include the mtu statement:

mtu bytes; 

You can configure this statement at the following hierarchy levels:

• [edit interfaces interface-name unit logical-unit-number family family]
• [edit logical-routers logical-router-name interfaces interface-name unit logical-unit-number family family]

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. For a lift of encapsulation overhead values, see Table 8. 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.)

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 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.