The tunnel gre smart-fragment enable command enables the GRE packet pre-fragmentation function.
The undo tunnel gre smart-fragment enable command disables the GRE packet pre-fragmentation function.
By default, the GRE packet pre-fragmentation function is disabled.
To ensure that the physical interface of the tunnel interface does not fragment packets after GRE encapsulation, the MTU of the tunnel interface must use the smaller value of the following: 1. the configured tunnel interface MTU, 2. the value obtained by deducting the GRE header length from the physical interface MTU. Therefore, the actual tunnel interface MTU is always smaller than or equal to the second value. Even though the GRE packet is larger than the physical interface MTU after GRE encapsulation, the device will fragment the GRE packet on the tunnel interface in advance. This is the GRE packet pre-fragmentation function. Therefore, when the GRE packet pre-fragmentation function is enabled, the tunnel interface MTU may not be the same as the configured MTU. If you need to keep the tunnel interface MTU the same as the configured MTU, run the undo tunnel gre smart-fragment enable command to disable the GRE packet pre-fragmentation function.