Using a loopback interface for OSPF in a ClusterXL running on Gaia OS is not supported for Check Point versions R81 and lower.
This problem was fixed. The fix is included in:
Check Point recommends to always upgrade to the most recent version.
Once software version is upgraded to R81.10, the following steps are required for loopback interface to show in the output of "show ospf database area <area> detailed":
1. Configure loopback interface address for all cluster members so that they are in same subnet. This can be done on the gateway via cli or WebUI
2. Create/add loopback interfaces and configure them as cluster in the gateway's topology via SmartConsole. Make sure the interface name is same as that of the interface on the gateways.
3. Publish the change in (2) and Install policy to the Gateways.
4. Confirm that the loopback interface is showing as up in the output of "cphaprob -a if".
5. Confirm the loopback interface's cluster IP as router-id on both cluster members.
This solution has been verified for the specific scenario, described by the combination of Product, Version and Symptoms. It may not work in other scenarios.