Clustering answers two different requirements that are easy to conflate, and it is worth separating them before the mechanism.
Availability and capacity
Availability means one member can fail and traffic continues. That requires enough capacity in the survivors to carry the load, which is the part people forget: two members each running at 70% do not survive losing one.
Capacity means the members share load so the cluster carries more than any one could. That requires traffic to be distributed, and it requires that distribution to keep a connection's packets going to the same member — because a firewall is stateful, and a packet arriving at a member that has never seen the connection is dropped as out of state.
Most clustering complexity across all vendors comes from that second requirement.
What ElasticXL changes
Check Point describes ElasticXL as a high-performance, flexible cluster solution for large-scale environments, and the operational shift is that the cluster is treated as one entity.
The consequence that matters day to day is what happens when you add a member. In a traditional cluster, a new member is a device to configure, add to the cluster object, license, and bring into sync — several steps, each with its own way to go wrong. Where the cluster is a single logical entity, adding capacity is closer to joining than to building.
The same applies to management: policy is installed to the cluster, and the members are an implementation detail rather than a list to maintain.
I am describing this at the level Check Point's own course material states it. The specific commands and the exact join procedure are the sort of detail that changes between releases, and the honest position is to take them from the administration guide for the version in front of you rather than from memory.
What stays true regardless of the mechanism
Some properties come from clustering itself, not from any implementation, and these are what troubleshooting rests on.
State must be shared. Members synchronise connection state so a failover does not drop established sessions. The sync network carries this, and it is the component whose failure produces the most confusing symptoms: the cluster looks healthy and connections break on failover.
Interfaces must agree. Members need matching interface configuration. A mismatch produces a member that cannot participate correctly, and the error usually names the interface, which is more helpful than most cluster errors.
Failover is not free. However fast, there is a moment where traffic is redirected. Long-lived connections may survive on synchronised state; anything mid-handshake may not.
A cluster is not a backup. It protects against a member failing. It does not protect against a bad policy, which installs to every member simultaneously.
Verifying it
The checks worth running before believing a cluster works:
Status — every member up, one active or all sharing depending on mode, and no member in a state nobody can explain.
Sync health — that state is actually being replicated, not merely that a sync interface exists.
A real failover, once, in a window. Force it and watch what happens to established connections. This is the test that finds the difference between a cluster that is configured and one that works, and it is worth the maintenance window precisely because the answer is sometimes uncomfortable.
What a CCSE candidate should be able to state cold
Clustering serves availability and capacity, and availability requires the survivors to have enough headroom to carry the load. Stateful inspection means a connection's packets must reach the member holding its state, which is where clustering complexity comes from. ElasticXL treats the cluster as a single entity, so adding capacity is closer to joining than to building, and policy is installed to the cluster rather than to members. Regardless of implementation: state is synchronised over a sync network whose failure breaks failover while the cluster looks healthy, interfaces must match, failover is not free for connections in flight, and a cluster is no protection against a bad policy that installs everywhere at once.