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mtu

 

Syntax

Hierarchy Level

Release Information

Statement introduced before Junos OS Release 7.4.

Statement introduced in Junos OS Release 9.0 for EX Series switches.

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

Statement introduced in Junos OS Release 12.1X48 for PTX Series Packet Transport Routers.

Statement introduced in Junos OS Release 12.2 for ACX Series Universal Metro Routers.

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.

Statement introduced in Junos OS 17.3R1 Release for MX Series Routers.

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.

To route jumbo data packets on an integrated routing and bridging (IRB) interface or routed VLAN interface (RVI) on EX Series switches, 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).

Caution

For EX Series switches, setting or deleting the jumbo MTU size on an IRB interface or RVI while the switch is transmitting packets might cause packets to be dropped.

Note

The MTU for an IRB interface is calculated by removing the Ethernet header overhead [6(DMAC)+6(SMAC)+2(EtherType)]. Because, the MTU is the lower value of the MTU configured on the IRB interface and the MTU configured on the IRB’s associated bridge domain IFDs or IFLs, the IRB MTU is calculated as follows:

  • In case of Layer 2 IFL configured with the flexible-vlan-tagging statement, the IRB MTU is calculated by including 8 bytes overhead (SVLAN+CVLAN).

  • In case of Layer 2 IFL configured with the vlan-tagging statement, the IRB MTU is calculated by including a single VLAN 4 bytes overhead.

  • 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) or for loopback, multilink, and multicast tunnel devices.

  • 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.

  • Starting in Release 14.2, 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.

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

Options

bytes—MTU size.

Range: 256 through 9192 bytes, 256 through 9216 (EX Series switch interfaces), 256 through 9500 bytes (Junos OS 12.1X48R2 for PTX Series routers), 256 through 9500 bytes (Junos OS 16.1R1 for MX Series routers)

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: 1500 bytes (INET, INET6, and ISO families), 1448 bytes (MPLS), 1514 bytes (EX Series switch interfaces)

Required Privilege Level

interface—To view this statement in the configuration.

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