EE connection network reachability awareness in a mixed-release environment

EE connection network reachability awareness can coexist in a mixed-release environment. However, if the PLU of a session resides on an NN with a release earlier than V1R6 (or, without EE connection reachability awareness enabled), or on an EN whose NNS is a release earlier than V1R6 (or a node without EE connection network reachability enabled), then that session will continue to fail as it did before the introduction of this function.

Consider the following configuration:

Figure 1. EE connection network reachability awareness in a mixed-release environment

Diagram of EE connection network reachability awareness in a mixed-release environment.

Assume the PLU resides on NETA.AA, a pre-V1R6 network node or a node without EE connection network reachability awareness enabled. The SLU resides on NETA.A2, a network node that supports EE connection network reachability awareness and has the function enable for NETA.VRN.

The intermediate node, NETA.A1, is also a network node that has EE connection network reachability awareness enabled for NETA.VRN. Both NETA.A1 and NETA.A2 have the unreachable time value for the EE virtual node, NETA.VRN, set to 180 seconds. There is a problem with the connection that underlies the connection network path through NETA.VRN and it has already been detected in NETA.A1 and NETA.A2.

A DISPLAY NET,TOPO,LIST=UNRCHTIM entered at NETA.A1 would show the following output:

      IST20571   UNREACHABLE PARTNER INFORMATION 
      IST924I  - - - - - - - - - - - - - - - - - - - - - - - - - 
      IST2150I   VIRTUAL NODE NETA.VRN 2 UNREACHABLE PARTNERS
      IST2052I   ORIGIN NODE  PARTNER NODE    UNRCHTIM   EXPIRES 
      IST2055I   NETA.A1      NETA.A2         180S     07:54:10 
      IST2055I   NETA.A2      NETA.A1         180S     07:54:12 
      IST314I   END

A request is made to VTAM® Topology and Routing Services (TRS) in the PLU network node (or if the PLU is on an end node, the NNS of the EN) to select the route that is to be taken to reach the SLU node. So if the PLU is on NETA.AA, which is a pre-V1R6 node, TRS in NETA.AA has no knowledge of the unusable path through NETA.VRN and, assuming that is the best (lowest weight) path between NETA.A1 and NETA.A2, will select that route.

When NETA.A1 becomes aware of the route to be taken, a DIAL request is made across the bad connection using a dynamic PU (these PUs usually default to a name of CNVxxxx). After a period of time, the DIAL request will timeout and the PU will INOP with the following messages:

      IST259I   INOP RECEIVED FROM CNVxxxx CODE = 01 
      IST1411I  INOP GENERATED FROM linename 
      IST430I   REASON FOR INOP IS XID OR LDLC COMMAND TIMEOUT 
             .
             .
      IST1903I  FAILURE OVER vrnname TO CP partner_cpname 
      IST2050I  THIS PATH WILL NOT BE SELECTED FOR UNRCHTIM = seconds            
                           SECONDS 
      IST314I   END

The session will fail in NETA.AA with sense code 08010000. In NETA.A1, where the IST1903I and IST2050I messages were received, the EXPIRES time for the path with NETA.A1 as the origin and NETA.A2 as the partner will be reset to 180 seconds after this failure is detected. The EXPIRES time for the path with NETA.A2 as the origin and NETA.A1 as the partner will not be reset because NETA.A2 did not detect a problem in the reverse direction.

A DISPLAY NET,TOPO,LIST=UNRCHTIM entered at NETA.A1 would now show the following output:

       IST2057I   UNREACHABLE PARTNER INFORMATION: 
       IST924I  - - - - - - - - - - - - - - - - - - - - - - - - - - - 
       IST2150I   VIRTUAL NODE NETA.VRN - 2 UNREACHABLE PARTNERS 
       IST2052I   ORIGIN NODE      PARTNER NODE    UNRCHTIM  EXPIRES 
       IST2055I   NETA.A1          NETA.A2         180S    07:58:45 
       IST2055I   NETA.A2          NETA.A1         180S    07:54:12 
       IST314I   END

Sessions routed through the VRN by the pre-V1R6 node (or a node without EE connection network reachability awareness enabled) will continue to fail as long as the connection underlying the EE VRN is not working. However, if the PLU resides on any of the nodes with EE connection network reachability awareness enabled (for example, where that node is selecting the route), sessions will be routed around the bad connection until the unreachable time expires. This is true even when the PLU is on NETA.BA, which is not connected to either of the nodes with EE connection network reachability awareness enabled that detect the failure. This node is informed of the unreachable partner information with Topology Database Updates (TDUs). The pre-V1R6 node, NETA.AA, does not recognize the unreachable partner information about the TDUs, but will pass that information about to NETA.BA, the node with EE connection network reachability awareness enabled.