When defining the CP-CP sessions in an APPN network, it is important to remember that APPN networks do not require meshed CP-CP sessions the way subarea networks require meshed SSCP-SSCP sessions. In APPN networks, direct CP-CP sessions between all pairs of network nodes are impractical because of the number of network nodes, and it is not possible in many cases because CP-CP sessions are allowed only with physically adjacent nodes. Instead, only a path of active CP-CP sessions between the origin and destination nodes is needed to deliver a session establishment request, determine an appropriate session route, and establish the session.

In fact, too many CP-CP sessions in an APPN network can cause network nodes to spend too much time processing duplicate requests. For example, when network topology database updates and broadcast search requests are performed, every network node must send or receive every request over every pair of CP-CP sessions that it has with other network nodes. (This ensures that every network node receives the request.) For this reason, establishing CP-CP sessions with every adjacent node might also prove to be undesirable.

At the same time, not enough CP-CP sessions in an APPN network can cause portions of the network to become disjoint, so that CP-CP sessions fail and network topology database updates and broadcast search requests can no longer be delivered to every network node. Therefore, you need to consider (which takes into account network availability requirements) the number of CP-CP sessions and the placement of those sessions. The ideal configuration is one that minimizes the number of CP-CP sessions while still ensuring that, at any time, every network node is logically connected to every other network node by a path of active CP-CP sessions.

The same considerations are necessary when defining VR-based TGs and CP-CP sessions through a subarea network. The following recommendations can simplify the task of determining how many VR-based TGs and CP-CP sessions should be defined, and where in the network they should be located.