WebSphere Application Server Version 6.x can interoperate with your other e-business systems, including other versions of WebSphere Application Server. Interoperability provides a communication mechanism for WebSphere Application Server nodes that are at different versions, running on separate machines
Interoperability support enhances migration scenarios with more configuration options. Interoperating is often more convenient or practical during the migration of a configuration from an earlier WebSphere Application Server version to a later one. Some machines can have the earlier product version and other machines can have the later version. An environment of machines and application components at different software version levels can involve both interoperability and coexistence.
WebSphere Application Server Version 6.1 is generally interoperable with WebSphere Application Server Version 5.x and Version 6.0.x. However, there are specific requirements to address for each version. In general, you should apply the latest fix level to support interoperability. If this is not possible, then the following interim fixes can be used to support your environment.