SNA Terms and Concepts

Basic APPN Concepts

topology information because it does not have any links to end nodes). For example, Node NNB includes information for Link f to Node EN2 and Link g to Node EN3, but it does not include information for Link i, which connects Nodes EN2 and EN3.

End nodes include information only for links to adjacent nodes. For example, Node EN2 includes information about Link f to Node NNB and Link i to Node EN3.

Topology Database Updates. APPN network nodes use CP-CP sessions to exchange network topology information when a resource (such as a node or a link between two network nodes) is activated or deactivated, or when the characteristics of an existing resource change. When such a change occurs, a network node generates a topology database update (TDU) that contains node identification node and link characteristics, and update sequence numbers identifying the resource to be updated and the changes for the resource. Each TDU is sent to all active network nodes to ensure that the network topology database is kept current throughout the network.

Route Selection in an APPN Network. APPN directory services locates a specific session partner; topology and routing services calculates the optimal session route after the session partner has been located in the network. Each network node provides route selection services for sessions originated by its own LUs and by LUs at the end nodes or LEN nodes that it serves. A network node uses its own local topology information, plus information from the shared network topology database, to dynamically calculate routes between nodes.

Once the session partner has been located, the network node performs the following steps to select a route:

1.Obtains required characteristics for the session route.

The LU requesting the session specifies a mode name that identifies session characteristics. The associated mode identifies a class of service that specifies requirements for the links used to route session traffic.

2.Obtains all transmission groups and network nodes for possible routes:

•If the session request comes from an end node, the end node provides information about links it has to its network node server and to a connection network, if one exists.

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Chapter 1