VTAM® uses topology and routing services (TRS) and directory services (DS) to monitor and maintain information about the topology and resources of the network.

TRS maintains information about nodes, transmission groups (TGs), and classes of service so that appropriate routes through an APPN network can be calculated.

TRS creates a network topology database, in which it maintains information about the current topology of the intermediate routing portion of an APPN network (network nodes and the transmission groups interconnecting them). This network topology database is replicated at every network node in an APPN network. Whenever a new network node or TG is added, or whenever the characteristics of an old resource change, a topology database update (TDU) that describes the current characteristics of the resource is generated and propagated to all network nodes.

When a network node first joins an APPN network (that is, establishes CP-CP sessions with other nodes), the other nodes send the joining node a copy of the network topology database. This is known as topology exchange. (A topology exchange that occurs when a network node first joins an APPN network is also called a full topology exchange or an initial topology exchange.) If the joining node later experiences planned or unplanned down time, it can remonitor the network topology by receiving all the topology updates that were exchanged since its topology database was last updated.

Directory services maintains a database that contains information about the location of LUs in the network. Entries are added to this database by resource definition, resource registration, and dynamically through searches. This means even if a session is requested to an LU whose location is not currently stored in the database, the location of this LU can be monitored, stored in the database, and session establishment will occur. Also, if a session is requested to an LU that cannot be located, that information is stored in the database if search reduction is enabled (see Improving VTAM performance using start options).

When a session is requested with an LU, VTAM uses its search capability and the directory services database to determine what node the resource is on. The search results give the location of the resource but not the route. VTAM dynamically determines the best route for the session using the APPN Class of Service requested for the session and the characteristics of the intermediate nodes and links stored in the topology database.

The defaults for the definition of the node and link characteristics and the VTAM ability to dynamically update the contents of the databases allow sessions to be established without requiring any definitions for resources outside the domain of that VTAM.

There are techniques you can use to improve the efficiency of resource location and session setup. These techniques include coding a central directory server and registering resources to a central directory server or a network node server. Resource registration occurs by default for some resources, but you can modify which resources are registered.

You can also checkpoint the topology and directory databases to ensure that information is retained during system outages. Checkpointing the directory database increases the chances that a resource can be located with a directed search instead of a broadcast search. Directed and broadcast searches are described in Types of searches. Checkpointing the topology database reduces the number of topology database updates that must flow at startup.

You may also need to adapt the Class of Service definitions, link and node characteristics to your network, such as coding some links as secure and requesting a secure Class of Service for certain sessions.

The following sections give information about searches, checkpointing, and APPN Class of Service.