HP-UX SNAplus2 R7 Administration Guide

SNA Terms and Concepts
Basic APPN Concepts
The shared network topology database is duplicated at Nodes NNA, NNB, NNC, and NND. In addition, each of
those nodes includes local topology information (except Node NNC, which does not have any local topology infor-
mation 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.
• If the session partner is not on an adjacent node, the network node server for the LU requesting the session
uses the network topology database to identify network nodes and intermediate transmission groups in the
route to the session partner.
• If the session partner is on an end node, the end node (or its network node server) provides information
about the link between the network node server and that end node (or the link between the end node and a
connection network).
3. Excludes all network nodes and transmission groups that do not meet the specified characteristics for the session
route.
4. Computes the optimal route for the session.
Depending on the specified class of service, the route calculation algorithm computes a weight value for each node
and logical link and then totals the weights for each route. To select the optimal path, the network node computes
the current least-weight route from the node containing the originating LU to the node containing the destination
LU.
Intermediate Routing
Intermediate routing enables an APPN network node to receive and route data destined for another node. The origin
and destination of the data can be an end node, another network node, or a LEN.
Intermediate routing supports sessions between LUs that are not on adjacent nodes. After a route has been selected
for a session, APPN network nodes in the route use intermediate routing to forward session data to the next node
in the route.
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