F5-CTS-LTM BIG-IP Local Traffic Manager Practice Question
When using an iRule to select a pool, what happens if the iRule contains a logic error that results in no pool being selected?
⚠ Common exam trap
Candidates often believe that an iRule logic error will cause the connection to drop immediately, forgetting that the BIG-IP will gracefully fall back to the Virtual Server's default pool.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The system selects the default pool defined on the VS.
If an iRule does not issue a 'pool' command, the BIG-IP defaults to the pool defined on the Virtual Server itself. If no pool is defined on the VS, the connection is reset. Understanding this fallback behavior is vital for debugging complex iRule logic. If an administrator fails to account for all conditions in an iRule, the traffic may behave unpredictably, falling back to default settings rather than failing outright.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The connection is immediately dropped.
Why it's wrong here
The connection is not immediately dropped if a default pool exists on the virtual server. The BIG-IP will attempt to route traffic to that default pool. Only if the virtual server itself lacks a default pool assignment will the connection be dropped or rejected by the BIG-IP system.
- ✓
The system selects the default pool defined on the VS.
Why this is correct
If the iRule executes without issuing a 'pool' command, the BIG-IP falls back to the default pool configured on the virtual server object. This design allows for graceful degradation or conditional routing where the iRule acts as an override rather than a mandatory requirement for every single connection request processed.
- ✗
The system routes traffic to the first pool in the list.
Why it's wrong here
The BIG-IP does not arbitrarily select the first pool in the configuration list. It relies on the specific default pool assignment of the virtual server. If that is missing, the request fails because the BIG-IP has no valid destination to route the traffic to, leading to a connection reset.
- ✗
The iRule execution stops and returns an error to the client.
Why it's wrong here
iRule execution completes successfully as long as no fatal script errors occur. If the logic fails to trigger a 'pool' command, it simply proceeds to the next phase of the connection processing. The client does not receive an error unless the system is unable to resolve the traffic destination.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official F5 exam blueprint
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