A GRE tunnel is established between two routers. When a user on one end sends traffic with a large packet size to a user on the remote end, the packet never arrives. However, the problem does not exist for smaller packets.
What is the problem?

A.
The tunnel is down on the remote end.

B.
The don’t-fragment bit is set on the traffic.

C.
GRE tunnels do not support IP traffic.

D.
No dynamic routing protocol exists to handle the traffic.

Explanation:
Enabling Fragmentation on GRE Tunnels

To enable fragmentation of IPv4 packets in generic routing encapsulation (GRE) tunnels, include the clear-dont-fragment-bit statement and a maximum transmission unit (MTU) setting for the tunnel as part of an existing GRE configuration at the [edit interfaces] hierarchy level:

[edit interfaces]gr-fpc/pic/port {unit logical-unit-number {clear-dont-fragment-bit;…family inet {mtu 1000;…}}}

This statement clears the Dont Fragment (DF) bit in the packet header, regardless of the packet size. If the packet size exceeds the tunnel MTU value, the packet is fragmented before encapsulation. The maximum MTU size configurable on the AS or Multiservices PIC is 9192bytes.

Note: The clear-dont-fragment-bit statement is supported only on MX Series routers and all M Series routers except the M320 router.

Fragmentation is enabled only on IPv4 packets being encapsulated in IPv4-based GRE tunnels.

Note: This configuration is supported only on GRE tunnels on AS or Multiservices interfaces. If you commit gre-fragmentation as the encapsulation type on a standard Tunnel PIC interface, the following console log message appears when the PIC comes online:
gr-fpc/pic/port: does not support this encapsulation
The Packet Forwarding Engine updates the IP identification field in the outer IP header of GRE-encapsulated packets, so that reassembly of the packets is possible after fragmentation. The previous CLI constraint check that required you to configure either the clear-dont-fragment-bit statement or a tunnel key with the allow-fragmentation statement is no longer enforced.