When multiple buses are connected using a service integration bus link, you can connect individual topic space destinations to create a publish/subscribe configuration. Messages published in a topic space in one bus are accessible to subscribers on a topic space in another bus.
You can connect topic space destinations in neighboring buses by configuring a topic space mapping when you define the service integration bus link routing properties. Each entry in the mapping allows a local topic space destination to be mapped to a topic space in the foreign bus. A foreign bus is a representation of another service integration bus, or a WebSphere MQ queue manager, with which an existing service integration bus can exchange messages. As a result of establishing the topic space mappings, any subscribers to topics on the topic space in the local bus can receive messages published on those topics in the topic space in the other foreign bus. Unlike point-to-point queue destinations, publish/subscribe applications create producers and subscriptions to topic spaces in their local bus rather than identifying topic spaces that exist in foreign buses. You must configure the applications so that they have topic space mappings to route messages appropriately.
When connecting topic spaces in more than two buses, there is no restriction in how multiple buses can be connected. However, there is a restriction on how their topic spaces are connected using topic space mappings. For guidance on how to create topic space mappings, refer to Configuring topic space mappings between service integration buses.
The correct example shows that messages published in Bus 1 are automatically flowed through Bus 1 to Bus 3, if Bus 3 has a suitable subscription for those messages. You create a mapping from the topic space in Bus 1 to the topic space in Bus 2, and another mapping from the topic space in Bus 3 to the topic space in Bus 1.