A network engineer is troubleshooting an issue where a host server (10.20.10.45/24) cannot… — CompTIA Network+

A network engineer is troubleshooting an issue where a host server ( $10.20.10.45/24$ ) cannot reach a database server ( $172.16.80.12/24$ ) across a multi-router topology containing active Access Control Lists (ACLs). Following the standard CompTIA network troubleshooting methodology, in what sequence should the engineer execute the following diagnostic and remediation steps?

1Identify the problem by gathering host IP configurations, interviewing affected users, and running initial ping tests to the local default gateway ( $10.20.10.1$ ) and remote IP destination.
2Establish a theory of probable cause by analyzing traceroute paths and evaluating router configuration logs for default route entries and ACL implicit deny drops.
3Test the theory to determine the cause by reviewing interface traffic stats and running `show ip access-lists` to inspect rule ordering and packet match counters on the intermediate router.
4Establish a plan of action and implement the solution by reordering the ACL rules to explicitly permit TCP traffic to $172.16.80.12$ prior to the implicit deny statement.
5Verify full system functionality by initiating database queries from the host server and implement preventive measures by updating network change logs.

Answer

The correct troubleshooting sequence follows the standard CompTIA methodology: 1. Identify the problem by gathering symptoms and testing basic gateway reachability; 2. Establish a theory of probable cause by analyzing routing and ACL logs; 3. Test the theory by running diagnostic CLI commands such as inspecting access lists; 4. Establish a plan of action and implement the fix by modifying the ACL rules; 5. Verify full system functionality and document the outcome.

The correct sequence adheres strictly to the CompTIA troubleshooting framework: Identify the problem (gathering symptoms and pinging gateway), Establish a theory of probable cause (analyzing traceroute and logs), Test the theory (inspecting ACL rule match counters via CLI), Establish a plan of action and implement the solution (reordering ACL permit/deny statements), and Verify full system functionality while documenting the outcome.

Step-by-Step Solution

Gather symptoms and run baseline reachability checks (ping local gateway and target host).

Establishes scope and confirms the boundaries of the reachability failure.

CompTIA troubleshooting methodology dictates identifying the problem as the first step.

Formulate a theory of probable cause based on traceroute output and network topology inspection.

Narrows potential causes to gateway unreachable, missing route, or ACL implicit deny.

Establishing a theory occurs after gathering preliminary diagnostic information.

Test the theory using targeted diagnostic commands (`show ip access-lists`).

Confirms the exact point of failure (e.g., traffic matching an unwanted ACL deny statement).

Theory testing must confirm the root cause before any configuration changes are made.

Develop a plan of action and modify the ACL configuration on the router.

Remediates the root cause by allowing required traffic through the gateway filter.

Implementation of fixes occurs only after the root cause is confirmed during theory testing.

Verify end-to-end connectivity and update documentation.

Ensures complete resolution and prevents regression.

Verification and documentation constitute the final steps of the structured troubleshooting framework.

Key Concept

CompTIA Structured Network Troubleshooting Methodology applied to Gateway, Routing, and ACL failures

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