Troubleshooting
Problem
Omproute OSPF has the logic to add outbound multipath routes to a destination over each interface in a common subnet. Many other OSPF routers do not have this logic and will only add outbound multipath routes to the same destination over interfaces in different subnets. As a result, inbound routing to z/OS might not appear to use multiple interfaces in the same subnet.
Cause
Omproute OSPF has the logic to add outbound multipath routes to a destination over each interface in a common subnet. Many other OSPF routers do not have this logic and will only add outbound multipath routes to the same destination over interfaces in different subnets. As a result, inbound routing to z/OS might not appear to use multiple interfaces in the same subnet.
The following diagrams illustrate the differences in inbound multipath:
Inbound multipath to z/OS via two interfaces on SAME subnet
In the above scenario, the OSPF router can determine that both interfaces i1 and i2, which can reach the VIPA, are in the same subnet. From the OSPF router's perspective there is only one path to the z/OS system: out the router's own interface i3 and though the subnet N1. This behavior causes all inbound traffic for the VIPA to use only one interface out of i1 or i2. The inbound traffic is not load balanced across the interfaces. If the OSPF router is another instance of omproute, which has logic to support this scenario, then both i1 and i2 are used on the inbound path.
Resolving The Problem
Inbound multipath through different subnets
In this scenario, there are two equal cost multipath routes to z/OS through different subnets. From the OSPF router's perspective, it can reach the VIPA by going out interface i3 to network N1, or out interface i4 to network N2. This scenario is supported by OSPF and by any router that supports OSPF.
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Document Information
Modified date:
15 June 2018
UID
swg21106020