In OSPF environments in which there might be a problem with some remote hardware (for example, router, switch, or network cable) that is beyond detection by z/OS® hardware or software, OMPROUTE can get into an infinite (or futile) neighbor state loop over one of its interfaces with a neighbor. This loop might contribute to increased workload. In LAN configurations in which there are parallel OSPF interfaces that can reach the same neighbor for adjacency formation, unless you are using OMPROUTE futile neighbor state loop detection or unless you manually fix the problem, the backup interfaces are not used until after an outage occurs for the OSPF interface that was initially involved in an adjacency formation attempt with a designated router. For information about parallel OSPF interfaces, see step 5.
Futile neighbor state loops can occur during adjacency formation with a neighboring designated router before full adjacency has been reached. According to OSPF protocol, all routers on a multiaccess network attempt to establish adjacency with the designated router (DR) and backup designated router (BDR) after two-way communication has been established. When a problem occurs during the adjacency formation process, the adjacency formation attempt is restarted. Repeated adjacency attempts can continue until full adjacency has been established or an interruption occurs. Interruption occurs from an interface outage to the neighbor or from a neighbor outage. Whether or not parallel interfaces are used, OMPROUTE repeatedly attempts to establish full adjacency with the neighbor, and if the problem is due to a cabling error or failure or some other non-transient condition, it is highly unlikely that the problem will be resolved and the loop will become futile.
To stop a futile neighbor state loop, you can either manually deactivate the interface or suspend the OSPF interface within OMPROUTE so that OMPROUTE will stop attempting to form the adjacency over the interface. If the OSPF interface is suspended, any active sessions using static routes over the interface will not be disrupted. If the interface is deactivated, all active sessions over the interface will be disrupted. If a parallel OSPF interface is available, OMPROUTE will then attempt to form adjacency with the neighbor over the parallel interface.
OMPROUTE futile neighbor state loop detection removes the need for manual detection of futile neighbor state loops, manual intervention to resolve the loops, and the disruption of existing sessions due to deactivating the interface. Code the DR_MAX_ADJ_ATTEMPT parameter on the OSPF or IPV6_OSPF statement, or both, in your OMPROUTE configuration file to enable this function. OMPROUTE will then report and control futile neighbor state loops during the adjacency formation process. For a list of interfaces that support the futile neighbor state loop detection function, see z/OS Communications Server: IP Configuration Reference
If you use the DR_MAX_ADJ_ATTEMPT parameter, futile neighbor state loops are automatically detected and reported using message EZZ8157I. If a parallel OSPF interface is not available, adjacency formation attempts continue to be tried over the same interface. If parallel OSPF interfaces are available, an interface change is reported using message EZZ8158I and adjacency formation attempts are tried again over a parallel OSPF interface. The problematic interface is suspended within OMPROUTE, but the interface is not deactivated and active sessions over the interface are not disrupted.
After a problematic OSPF interface is suspended in OMPROUTE and adjacencies are formed on a parallel OSPF interface, you might want to switch back to the original interface after you have fixed the problem that caused the futile neighbor state loop. To accomplish this, activate the repaired interface and then suspend the parallel interface. After OSPF adjacencies are established over the repaired interface, the parallel interface can be reactivated so it is again available as a backup for the repaired interface. Use the ACTIVATE function of the OMPROUTE MODIFY command to activate a suspended OSPF interface. Use the SUSPEND function of the OMPROUTE MODIFY command to manually suspend an OSPF interface. For more information about these functions of the MODIFY command for OMPROUTE, see z/OS Communications Server: IP System Administrator's Commands.