Topology data update (TDU) flows

An example shows how the network topology data is updated.

Figure 1 represents an APPN with three network nodes (NN2, NN3, and NN4). NN1 is about to join the APPN Network. Prior to connecting NN1 to the APPN network, every network node has an identical representation of the network in its topology database. NN1 starts with a topology database containing only itself.

Figure 1. Network node joins the APPN network

When NN1 joins the network, topology data updates are sent over the CP-CP sessions. These topology database updates are sent whenever a node or transmission group state or characteristic changes or when CP-CP sessions are started or ended.

The topology database updates are propagated to all other adjacent network nodes; see Figure 2.

Figure 2. Topology database update (TDU) flow

TDU (1) is originated by NN1 and is sent from NN1 to NN2. NN2 propagates TDU (1) to all its adjacent network nodes (NN4 and NN3). The adjacent nodes that received the TDU (1) propagated TDU to their adjacent network nodes.

NN2 sends TDU(2) that describes the entire network topology, including the transmission group from NN2 to NN1.

As you can see, NN3 receives two identical TDUs (1), one from NN4 and one from NN2. How does NN3 determine whether to update its database when it receives an identical TDU? NN3 uses the resource sequence number.

Resource sequence number (RSN)

A resource sequence number is associated with each transmission group and node record in the topology database. Thus, when receiving resource updates in a topology database update, a network node can determine if the information in the update is new or old by comparing the resource sequence number in the update request with the resource sequence number of the stored record in the topology database (if the record has already been created).

If the resource sequence numbers are equal or the resource sequence number in the update is less than the resource sequence number in the stored record, the information is old and the network node does not need to update the topology database with the updated record and also does not need to forward this resource update in a topology database update to its network node partners. Only new information needs to be forwarded.

There is also the flow reduction sequence number.

Flow reduction sequence number (FRSN)

Network nodes assign flow reduction sequence number (FRSN) values to topology resource records when the records are modified or newly created. The assigning of a FRSN number to topology database updates and topology resource records allows a network node to track the most recent time the local network node sent a topology database update to each of its network node partners, as well as the most recent time each resource record in its topology database was last modified and broadcast in a topology database update.

The FRSN identifies how much of the topology database must be exchanged when the network node rejoins the network.

Once all topology database updates have been sent and received, all network nodes have the same view of the network. Figure 3 depicts the topology database that exists in all network nodes once topology database has reached "steady state."

Figure 3. APPN network topology - steady state