How is the MTU of an AR interface changed

The network layer limits the size of data frames. When a network layer device receives an IP packet, it determines the outbound interface and obtains the MTU configured on the interface. The IP layer then compares the MTU with the packet length. If the packet length is longer than the MTU, the device fragments the IP packet. Each fragment has the smaller or equal size as the MTU.
If forcible unfragmentation is configured, some packets may be discarded when transmitted at the IP layer. Therefore, large packets must be forcibly fragmented to prevent packet loss on the network. The mtu command is used to set the size of a fragment.
The MTU value can be configured for Layer 3 interfaces only. The commands are as follows:
[Huawei] interface gigabitethernet1/0/0
[Huawei-GigabitEthernet1/0/0] mtu 1200
[Huawei-GigabitEthernet1/0/0] restart

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