APPN network routing through a composite network node (CNN)

In a configuration containing a composite network node (CNN), it is possible that the optimal route computation does not occur. Route computation is performed by the network node serving the origin logical unit (OLU). For example, in Figure 1, a route from LU to APPL is computed that uses TG1-TG4. TG1-TG3 would be a better route.

Figure 1. Routing example through a CNN node

VTAM® in the composite network node (CNN) node affects APPN routing by:

Selecting an optimal route through the CNN
When computing a route and choosing among TGs of equal weight, VTAM searches for matching subarea numbers for the entry and exit TGs to a CNN (TG1 and TG3 in this example, because they share NCP1A). If TGs with matching subarea numbers are found, VTAM selects that route. In situations where no match is found (for example, if TG3 did not exist), the original route is used (either TG1–TG4 or TG1–TG2).
Changing the route during BIND processing
The CNN node affects routing during BIND processing. When a BIND reaches the CNN node, the next hop in the RSCV contained in the BIND may be changed by VTAM in the CNN node when the entry and exit TGs use two different subareas. VTAM changes the route during BIND processing when the following conditions are met:
- The subarea number of the new exit TG is the same as the subarea number of the entry TG.
- TG characteristics are the same as the original TG.

Figure 2 shows an example of VTAM changing the selection of a route. In Figure 2 mesh connectivity exists between VTAMA, VTAMB, NCPA, NCPB, and NCPC. In selecting a path between EN1 and NN2, VTAMA determines that using TG2-TG3 is the optimal route. Because NN2 does not use subarea numbers in route calculation, it might choose TG2-TG4 when selecting a route from NN2 to EN1.

Figure 2. CNN route calculation example

Figure 3 shows another example of route calculation through a CNN node. VTAMA, the dependent LU server for ENA, calculates a route of TG5-TG6.

Figure 3. Composite network node route calculation example

Diagram that shows another example of route calculation through a CNN node.

Figure 4 is an example of the CNN node changing the route when receiving a BIND request. A BIND request sent from an LU located on NN1 to an LU on NN2 contains TG1-TG2 in the RSCV. VTAMA changes the RSVC to use TG1-TG3. By rerouting the BIND to a TG with the same subarea as the entry TG, VTAM prevents unnecessary hops within the CNN.

Figure 4. Composite network node route BIND reroute example

Diagram that shows the CNN node changing the route when receiving a BIND request.

Non-optimal routes might result because the topology of the CNN is not known by the APPN topology and route selection process, or because the route was calculated by a non-VTAM node that does not support the use of subarea numbers in route calculation. For example, referring to Figure 4, if NN2 calculates TG2-TG1 as the route to go from NN2 to NN1, when the BIND arrives at VTAMA it cannot be changed to use TG3. It is too late in the process for the CNN node to change the route. However, if CNNRTMSG=NOSUPP is in effect, VTAMA issues the IST1774I message group to indicate that an optimal route through the CNN does exist but was not chosen during session activation.

Note that all the optimizations concern equally weighted TGs. For this reason, making the characteristics of TGs between two identical nodes allows VTAM more flexibility in optimizing paths through CNNs.