The MTU of each logical interface MUST be configurable within the range of legal MTUs for the interface.
Many Link Layer protocols define a maximum frame size that may be sent. In such cases, a router MUST NOT allow an MTU to be set which would allow sending of frames larger than those allowed by the Link Layer protocol. However, a router SHOULD be willing to receive a packet as large as the maximum frame size even if that is larger than the MTU.
If a network is using an MTU smaller than the maximum frame size for the Link Layer, a router may receive packets larger than the MTU from misconfigured and incompletely initialized hosts. The Robustness Principle indicates that the router should successfully receive these packets if possible.