CP-CP sessions

To perform directory services and topology and route selection services, adjacent APPN nodes throughout the APPN network use a set of two CP-CP sessions to exchange network information. CP-CP sessions are established between type 2.1 nodes. A network node or composite network node can establish CP-CP sessions with any network node or composite network node to which it has an APPN link that supports CP-CP sessions, with the exception that CP-CP session between network nodes having different Net IDs are permitted only if at least one of the network nodes supports the border node function. It also establishes CP-CP sessions with each of the end nodes that it serves. End nodes can be attached to several network nodes but can establish a CP-CP session pair with only one network node server at a time.

When two nodes have one or more CP-CP capable links between them, VTAM® requires a CP-CP session to be active between the nodes, as long as none of the following conditions disallowing CP-CP sessions are true:

The CP-CP capable links exist between two adjacent network nodes with different net IDs, and neither network node provides border node support.
The CP-CP capable links exist between an end node and a network node, and the end node already has CP-CP sessions with a different network node.
The CP-CP capable links exist between two adjacent end nodes.

If the CP-CP session is deactivated, you need to either reactivate the CP-CP session or deactivate all the CP-CP capable links and reactivate them with CPCP=NO.

In an APPN network you do not need meshed connectivity of CP-CP sessions with every node having a CP-CP session with every other node in the network. With APPN it is necessary only for each APPN node to have a CP-CP session into the network. A CP-CP session is not required between the end points of an LU-LU session. Defining CP-CP capable links to every other node in your network might cause excessive topology update flows. It is recommended that you define a primary and backup link with CONNTYPE=APPN and CPCP=YES for each node in your network. It is also recommended that you define additional APPN-capable links with CPCP=NO to provide efficient routing for your LU-LU sessions.

CP-CP sessions between two network nodes are used to perform searches for resources, exchange topology information, and can be used to register resources with a central directory server.

After an APPN connection has been established, identification information is exchanged between the nodes, and CP-CP sessions can be started between the control points in the directly attached nodes.

After the CP-CP sessions are established, the two nodes exchange CP capabilities, which indicate the level of network services provided by the control point. When both nodes are network nodes, they exchange topology database update (TDU) messages. The TDU messages contain identifying information, node and link characteristics, and resource sequence numbers that identify the most recent updates for each of the resources described in the TDUs.

When an LU-LU session is requested, information is transported across the CP-CP sessions to request that an LU-LU session be established between the two session partners. The control points that own the two session partners do not have to have a direct CP-CP session between them in order to establish the LU-LU session.