Configuring the Media MTU
The media maximum transmission unit (MTU) is the largest data unit that can be forwarded without fragmentation. The default media MTU size used on a physical interface depends on the encapsulation being used on that interface. For a listing of MTU sizes for each encapsulation type, see Media MTU Sizes by Interface Type.
To configure the media-MTU size:
- In configuration mode, go to the [edit interfaces interface-name] hierarchy level.[edit ]user@host# [edit interfaces interface-name]
- Include the mtu statement.[edit interfaces interface-name]mtu bytes;
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. You configure the protocol MTU by including the mtu 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]
Changing the media MTU or protocol MTU causes an interface to be deleted and added again.
Because tunnel services interfaces are considered logical interfaces, you cannot configure the MTU setting for the physical interface. This means you cannot include the mtu statement at the [edit interfaces interface-name] hierarchy level for the following interface types: generic routing encapsulation (gr-), IP-IP (ip-), loopback (lo-), link services (ls-), multilink services (ml-), and multicast (pe-, pd-). You can, however, configure the protocol MTU on all tunnel interfaces except virtual tunnel (vt) interfaces. Starting in Junos OS Release 17.1R3, you cannot configure the maximum transmission unit (MTU) size for vt interfaces because the mtu bytes option is deprecated for vt interfaces. Junos OS sets the MTU size for vt interfaces by default to unlimited.
If you configure an MTU value by including the mtu statement at the [edit interfaces interface-name unit logical-unit-number family mpls] hierarchy level, the configured value is used.