Setting the Protocol MTU with Frame Relay Encapsulation
For each interface, you can configure an interface-specific MTU by including the mtu statement at the [edit interfaces interface interface-name] hierarchy level. If you need to modify this MTU for a particular protocol family, include the mtu statement:
You can include this statement at the following hierarchy levels:
[edit interfaces interface-name unit logical-unit-number family family]
[edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family family]
For Frame Relay encapsulation, the default protocol MTU is 4470 bytes.
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. (You configure the media MTU by including the mtu statement at the [edit interfaces interface-name] hierarchy level, as discussed in Configuring the Media MTU on Frame Relay Interfaces.)
When the family is mpls, the default protocol MTU is 1488 bytes. MPLS packets are 1500 bytes and have 4 to 12 bytes of overhead.