HP HP-UX SNAplus2, Administration Manual

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56 Chapter 1
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.