The value of vRtrIfMtu is used to reflect the MTU size of this
interface in the header of an ICMP MTU exceeded message when
a packet is too large to be transmitted on this interface.
The default size is calculated by subtracting the DLC header
size from the physical MTU size of the interface.
When a SNMP SET request is received with a value of zero (0), the
agent actually sets vRtrIfMtu to the calculated default size.