What is automatic network routing?

Automatic network routing is a low-level routing mechanism that minimizes cycles and storage requirements for routing packets through intermediate nodes. Unlike APPN, intermediate ANR nodes are not aware of SNA sessions or RTP connections passing through the node. All an ANR node must do is read the header in a network layer packet and forward the information to the next node on the path. The ANR information is learned by RTP during establishment of the RTP connection by sending a "Route Setup" message which flows through all nodes on the prospective HPR path.

Tip: On nodes configured as border nodes (BN=YES is specified) with HPR fully enabled (HPR=RTP is specified), code RTPONLY=YES on ADJCP definition statements in adjacent CP major nodes if you want the border node to maintain awareness of all sessions established to, from or through the adjacent node being defined. The RTPONLY operand is valid only when the activating node is configured as a border node, and RTPONLY=YES is meaningful only when the adjacent node being defined is a nonnative border node or nonnative network node. By coding RTPONLY=YES, you are instructing VTAM® to disallow use of the ANR function for any new RTPs that are established or path switched to, from, or through the adjacent nonnative node being defined. (Allowing the use of ANR could result in RTPs being established through this border node, thereby preventing the border node from maintaining awareness of any sessions that use these RTPs.) Instead of allowing RTPs to be established through this border node, coding RTPONLY=YES will result in VTAM forcing these RTPs to terminate on this border node or forcing the use of intermediate session routing (ISR) instead of HPR (or a combination of the two).

Restrictions:

Use of RTPONLY=YES at any APPN subnetwork boundary on a session setup path prevents the use of global VRNs (GVRNs) for intersubnetwork connectivity, because using GVRNs could result in sessions being established across this subnetwork boundary without this border node maintaining awareness of these sessions.
Use of RTPONLY=YES can result in an increase in network traffic in the form of additional Route_Setup flows used for RTP establishment. These additional Route_Setup flows will occur only during the establishment of sessions that cross subnetwork boundaries defined with RTPONLY=YES.
Use of RTPONLY=YES can result in an increase in storage and CPU utilization, because of VTAM maintaining awareness of these sessions and performing ISR instead of HPR/ANR for these sessions.