In a VTAM APPN network, what is a TDU?

To keep VTAM' APPN topology database current, network nodes in an APPN network send each other topology database updates (TDUs) over CP-CP sessions whenever a resource (node or link) is activated, becomes inoperative, deactivated, or its characteristics change. Only the current changes are included in the TDU, not the complete topology. The TDUs contain information about the network nodes and the TGs between the network nodes. Every network node receives the TDUs, so all have the same view of the network.

In the D NET,TOPO,ID display:

• The sent value is the number of TDUs about the node that were sent since the last time that the TDU counts were reset.

• The received value is the number of TDUs about the node that were received since the last time that the TDU counts were reset.

• The accepted value is the number of TDUs about the node that were accepted since the last time that the TDU counts were reset. The TDUs contain new information about the node and the topology database was updated.

• The rejected value is the number of TDUs about the node that were rejected since the last time that the TDU counts were reset. The TDUs were rejected because the TDUs contain outdated information about the node. TDUs that are built from the local topology database information for the node were sent as corrections. Any TDU received having a Resource Sequence Number (RSN) lower than the current RSN in the topology database is considered as outdated information.

• The ignored value is the number of TDUs about the node that were ignored since the last time that the TDU counts were reset. The TDUs were discarded because the TDUs contained no new information.