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54 Chapter 1
SNA Terms and Concepts
Basic APPN Concepts
network backbone topology—the nodes and transmission groups that
can be used for routing sessions between any pair of nodes in the
network.
In addition, the topology database on each network node contains
local information about transmission groups from that network node
to adjacent end nodes or LEN nodes.
The network node uses the topology database to calculate routes for
sessions between LUs in its domain and remote LUs, or to provide
information to other network nodes to enable them to calculate
session routes.
• Each end node has a local topology database with information about
transmission groups from that end node to adjacent nodes.
The end node provides this information to its network node server as
part of the request to locate an LU and calculate a session route to
that LU. The network node server uses the end node topology
information when calculating the session route for the end node. The
end node uses this information when establishing sessions with
predefined LUs on adjacent nodes. The end node topology database
supports communication only with adjacent nodes.
NOTE APPN network nodes and end nodes also maintain topology information
about links to a connection network (see “APPN Connection Networks”).
LEN nodes maintain local topology information. They do not forward this
information to a network node server.
As shown in Figure 1-8, “Network Topology Database in Network
Nodes,” network topology information is replicated at all network nodes,
and local topology information is stored at network nodes and end nodes.