The Datagram configuration parameters override the automatic configuration of the datagram (DG) layer.
By default, the database server loads the single, most efficient DG layer shared library for the platform. Communication between coservers on the same node passes through the same network protocol layers (and possibly even onto the network media) as messages passed between coservers on different nodes.
On some platforms, Extended Parallel Server supports the DG layer with the shared-memory extension. This DG layer uses shared memory to pass messages between coservers on the same node, thus bypassing network protocol layers. When the database server uses shared memory for intranode communication, performance improves because shared-memory communication eliminates the overhead of network communication. The database server creates and uses shared-memory segments automatically when multiple coservers are configured on a node. (Messages passed between coservers on different nodes continue to use the network protocol layers.)
For more information on these parameters, including the correct string format, see the machine notes.