Can the MTU of the GRE tunnel interface take effect


If you set an MTU value on a GRE tunnel interface, forwarding of data packets through the GRE tunnel will be affected. If the packet length exceeds the MTU value on the tunnel interface, the device fragments the packet.

Other related questions:
What is the function of the tunnel interface (GRE interface)
The tunnel interface (GRE interface) encapsulates and decapsulates data packets using GRE. The tunnel interface that sends encapsulated packets is called the tunnel source interface, and the one that receives these packets on the peer end is called the tunnel destination interface. Generally, the local WAN interface is used as the tunnel source interface, and the peer WAN interface is used as the tunnel destination interface.

The reason of S series switches TUNNEL port MTU does not take effect
S series switches (except S1700) MTU of the TUNNEL port is valid only for the uplink control plane and the packets sent from the control plane. The MTU does not take effect for the data packets directly forwarded at the hardware level. The hardware level MTU need to configure the MTU value of the physical interface.

Why does not the MTU configuration take effect on tunnel interfaces of a switch
The maximum transmission unit (MTU) configuration is valid for packets sent to and from the control plane, but invalid for packets directly forwarded by hardware. To limit the size of packets forwarded by hardware, configure the MTU on physical interfaces.

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