mtu (interfaces)

Syntax

Hierarchy Level

Description

Specify the maximum transmission unit (MTU) size for the media or protocol. The default MTU size depends on the device type. Changing the media MTU or protocol MTU causes an interface to be deleted and added again.

If a packet whose size is larger than the configured MTU size is received on the receiving interface, the packet is eventually dropped. The value considered for MRU (maximum receive unit) size is also the same as the MTU size configured on that interface.

Not all devices allow you to set an MTU value, and some devices have restrictions on the range of allowable MTU values. You cannot configure an MTU for:

• Management Ethernet interfaces (fxp0, em0, or me0)

• Service interfaces (ms, vms, or ams)

• Loopback, multilink, and multicast tunnel devices

MTU for IRB interfaces is calculated by removing the Ethernet header overhead (6(DMAC)+6(SMAC)+2(EtherType)), and the MTU is a minimum of the two values:

• Configured MTU

• Associated bridge domain's physical or logical interface MTU

  • For Layer 2 logical interfaces configured with flexible-vlan-tagging, IRB MTU is calculated by including 8 bytes overhead (SVLAN+CVLAN).

  • For Layer 2 logical interfaces configured with vlan-tagging, IRB MTU is calculated by including single VLAN 4 bytes overhead.

  Note:

  Changing the Layer 2 logical interface option from vlan-tagging to flexible-vlan-tagging or vice versa adjusts the logical interface MTU by 4 bytes with the existing MTU size. As a result, the Layer 2 logical interface is deleted and re-added, and the IRB MTU is re-computed appropriately.

On EX Series switches, to route jumbo data packets on an integrated routing and bridging (IRB) interface or routed VLAN interface (RVI), you must configure the jumbo MTU size on the member physical interfaces of the VLAN that you have associated with the IRB interface or RVI, as well as on the IRB interface or RVI itself (the interface named irb or vlan, respectively).

On ACX Series routers, you can configure the protocol MTU by including the mtu statement at the [edit interfaces interface-name unit logical-unit-number family inet] or [edit interfaces interface-name unit logical-unit-number family inet6] hierarchy level.

• If you configure the protocol MTU at any of these hierarchy levels, the configured value is applied to all families that are configured on the logical interface.

• If you are configuring the protocol MTU for both inet and inet6 families on the same logical interface, you must configure the same value for both the families. It is not recommended to configure different MTU size values for inet and inet6 families that are configured on the same logical interface.

For more information about configuring MTU for specific interfaces and router or switch combinations, see Configure the Media MTU.

Options

bytes—MTU size.

• Range: 256 through 9192 bytes

  • ACX710-48L, ACX710-32C, ACX7509, and ACX7024: 256 through 9216 bytes

  • EX Series switch interfaces: 256 through 9216

  • MX Series routers: 256 through 9500 bytes

  • PTX Series routers: 256 through 9500 bytes

  Starting in Junos OS Evolved Release 21.2R1, MTU 16KB is only for transiting traffic of WAN interfaces on the PTX10001-36MR, PTX10008 and PTX10004 routers. MTU is 9500B for protocols and 16KB for transit traffic.

  Note:

  Starting in Junos OS Release 16.1R1, the MTU size for a media or protocol is increased from 9192 to 9500 for Ethernet interfaces on the following MX Series MPCs:

  • MPC1

  • MPC2

  • MPC2E

  • MPC3E

  • MPC4E

  • MPC5E

  • MPC6E

• Default:

  • INET, INET6, and ISO families: 1500 bytes

  • EX Series switch interfaces: 1514 bytes

  • MPLS: 1448 bytes

Required Privilege Level

interface—To view this statement in the configuration.

interface-control—To add this statement to the configuration.

Release Information

Statement introduced before Junos OS Release 7.4.

Support for Layer 2 VPNs introduced in Junos OS Release 10.4.

Support at the[set interfaces interface-name unit logical-unit-number family ccc] hierarchy level introduced in Junos OS Release 12.3R3 for MX Series routers.