Description: The vendor-specific serial number string for the physical entity. The preferred value is the serial number string actually printed on the component itself (if present).

On the first instantiation of an physical entity, the value of slEntPhysicalSerialNum associated with that entity is set to the correct vendor-assigned serial number, if this information is available to the agent. If a serial number is unknown or non-existent, the slEntPhysicalSerialNum will be set to a zero-length string instead.

Note that implementations which can correctly identify the serial numbers of all installed physical entities do not need to provide write access to the slEntPhysicalSerialNum object. Agents which cannot provide non-volatile storage for the slEntPhysicalSerialNum strings are not required to implement write access for this object.

Not every physical component will have a serial number, or even need one. Physical entities for which the associated value of the slEntPhysicalIsFRU object is equal to 'false(2)' (e.g., the repeater ports within a repeater module), do not need their own unique serial number. An agent does not have to provide write access for such entities, and may return a zero-length string.

If write access is implemented for an instance of slEntPhysicalSerialNum, and a value is written into the instance, the agent must retain the supplied value in the slEntPhysicalSerialNum instance associated with the same physical entity for as long as that entity remains instantiated. This includes instantiations across all re- initializations/reboots of the network management system, including those which result in a change of the physical entity's slEntPhysicalIndex value.