With EE connection network reachability awareness, you can use a start option, UNRCHTIM, to indicate that paths to partner nodes over Enterprise Extender VRNs should not be used for route selection for a period of time after initial dial failures or connection INOPs; this provides time for the underlying connection problem to be corrected. An alternate route to the partner node is selected, if an alternate route is available, when new sessions are established and HPR path switches occur. If an alternate route is not available, the session or HPR path switch fails with sense code 80140001. UNRCHTIM can also be coded on a PORT or GROUP definition statement that defines a connection network (with the VNNAME and VNTYPE operands) in an EE XCA major node. This enables a specific connection network to have a different unreachable time value than the value specified on the UNRCHTIM start option. See the UNRCHTIM start option in z/OS Communications Server: SNA Resource Definition Reference for more information.

The ramifications of the unreachable time value you choose are important. The results can vary depending on the duration of failures that occur. A value that is appropriate for a short-term failure might not be appropriate for a long-term failure. For example, if the value is set very low and a long-term outage occurs, you introduce the performance impact of added network flows to communicate the unreachable partner information to all nodes each time a path is determined to still be unreachable. In this case, many session or HPR path switch failures can still occur, even though an alternate route is available. Alternatively, if the value is set high and a short-term outage occurs, any paths to unreachable partners detected at that time will remain unavailable for use until the unreachable time expires or until the unreachable partner information is cleared with the MODIFY TOPO,FUNCTION=CLRUNRCH command.

You can change the UNRCHTIM start option value while VTAM® is running with the MODIFY VTAMOPTS command. You can also change the unreachable time for a specific Enterprise Extender connection network using the VARY ACT,UPDATE=ALL command with the Enterprise Extender XCA major node after changing UNRCHTIM on the PORT or GROUP definition statement. However, the unreachable time value is not changed for paths to unreachable partners that were determined to be unreachable before the value was modified. When the existing unreachable time value for a path expires, the new unreachable time value will be used if the partner node through that path is again determined to be unreachable.

An end node that has detected an unreachable partner path notifies its network node server (NNS) of the unreachability so that the NNS can use that information when calculating routes for the end node. Beginning with z/OS® V1R8 Communications Server, the NNS will override the unreachable time value sent by the EN using its unreachable time value for the associated connection network, if configured, or the value specified in its UNRCHTIM VTAM start option. With this enhancement, changing the unreachable time value for an Enterprise Extender connection network will no longer require you to update every attached end node. Only the network nodes and those attached end nodes whose NNS is running z/OS V1R6 or V1R7 Communications Server will need to be updated with the new value. You must still configure an unreachable time value on every end node to activate the EE connection network reachability awareness function.