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clear-dont-fragment-bit (Interfaces GRE Tunnels)


Hierarchy Level


Clear the Don’t Fragment (DF) bit on all IP version 4 (IPv4) packets entering the generic routing encapsulation (GRE) tunnel on Adaptive Services (AS) or Multiservices interfaces. If the encapsulated packet size exceeds the tunnel maximum transmission unit (MTU), the packet is fragmented before encapsulation. The statement is supported only on MX Series routers and all M Series routers except the M320 router.

When you configure the clear-dont-fragment-bit statement on an interface with the MPLS protocol family enabled, you must specify an MTU value. This MTU value must not be greater than maximum supported value, which is 9192.

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.

Statement introduced in Junos OS Release 10.2 for MPC1 and MPC1Q.


On SRX platforms the clearing of the DF bit on a GRE tunnel is supported only when the device is in packet or selective packet mode; This feature is not supported in flow mode. As a result, when in flow mode, a packet that exceeds the MTU of the GRE interface with the DF bit set is dropped, despite having the clear-dont-fragment-bit configured on the GRE interface.