The default keepalive interval for DXL Clients' connection to a DXL Broker is ~30 minutes. This means that if a Broker goes offline without being able to send a shutdown notification to the fabric, the connected DXL Clients may take up to 30 minutes to become aware that the Broker is unavailable.
To directly answer your question: This 30 minute interval is within the expected behavior of a Client.
It is possible to modify the keepalive interval, but it is not recommended unless it is known for certain that there will be very few DXL Clients on the DXL fabric. Shortening the interval can greatly increase the amount of network traffic caused by the DXL Clients due to scaling issues (normal fabrics tend to have several thousand Clients).
I hope that helps clear things up!