To enable a network node to provide routing functions to and from itself and the end nodes it serves, every network node maintains a network topology database that has complete and current topology information about the network. This topology information consists of the characteristics of all network nodes in the network and of all transmission groups (TGs) between network nodes. The end nodes in the network and the TGs connected to them are not considered network topology information.

This section describes the two kinds of topology databases in an APPN network and explains how each is used. Local topology information and network topology information is maintained at each network node. The local topology information is unique to the node; the network topology information is replicated at all network nodes.

Local topology database

A local topology database contains information about a node and about the TGs connecting the node to other type 2.1 nodes. Both end nodes and network nodes have local topology databases.

In an end node, the local topology database is used to calculate a route to the network node server and to supply the endpoint TG vectors (vectors that describe the TGs connected to the end node) to the network node server. In a network node, the local topology database includes information about the attached end nodes. The local topology database is not saved when VTAM® is stopped and restarted.

Network topology database

The network topology database is referred to as the topology database. The network topology database is kept only in network nodes and contains complete and current information about all network nodes in the network, and about all transmission groups connecting them. It does not contain any information about LUs, end nodes, or LEN nodes.

The network topology database dynamically records changes in network topology and can be checkpointed to a file and reloaded when VTAM is restarted to reduce the amount of network traffic when a node is restarted.

To keep the topology database current, network nodes in an APPN network send each other topology database updates (TDUs) over CP-CP sessions whenever a resource (node or link) is activated, becomes inoperative, deactivated, or its characteristics change. Only the current changes are included in the TDU, not the complete topology. The TDUs contain information about the network nodes and the TGs between the network nodes. Every network node receives the TDUs, so all have the same view of the network.

A resource sequence number (RSN) is associated with each resource, and is incremented each time that resource changes status. The RSN is used to determine when all network nodes have received the most current information about the node or TG, at which time the TDU activity for that resource ceases. As long as all network nodes follow the same rules with regard to TDU processing, the RSN guarantees that a TDU war does not occur. A TDU war is an endless exchange of TDUs in contention over the same topology resource, resulting in continuous performance degradation of the APPN network. See the Display TDU information section in z/OS Communications Server: SNA Diagnosis Vol 1, Techniques and Procedures for information about diagnosing TDU wars.

In an APPN network, several mechanisms are used to ensure that unnecessary or excessive TDUs are not propagated throughout the network. For example, multiple TDUs can be grouped and sent out together if there are updates to multiple local TGs.

The topology database is used by the network node to select routes for sessions that originate at the LUs in it and at the ENs that it serves.