Describe a method for troubleshooting routing issues using ping, traceroute, and route-table/view commands to isolate topology problems.

Describe a method for troubleshooting routing issues using ping, traceroute, and route-table/view commands to isolate topology problems.

• A. Test reachability with ping, identify path and failure point with traceroute, examine routing table and neighbor adjacencies, and review logs; verify ACLs and NAT as needed.
• B. Test only physical layer cables, ignoring routing configurations.
• C. Replace devices first; only then test.
• D. Disable all routing protocols to see if traffic flows.

Answer: (A)

Begin with a layered approach that traces the path from source to destination and checks what the network believes about that path. Start by using ping to confirm end-to-end reachability to the destination or to the next hop. If ping fails, you quickly know there’s a basic connectivity issue that needs fixing before deeper analysis. If ping succeeds, you’re ready to map the actual path with traceroute and look for where the path is failing or deviating. The point where traceroute stops or shows an unexpected hop helps pinpoint topology problems, such as a misconfigured link, a routing decision at a particular device, or a device that is dropping or rate-limiting traffic. Next, turn to the routing state itself. Inspect the routing table or the device’s view of routes to see what networks are considered reachable, what the next hops are, and which paths are installed as the best ones. This reveals whether the expected routes exist, whether there are static vs. dynamic routes, and if any anomalies like missing routes or incorrect next hops are present. Checking neighbor adjacencies for dynamic protocols is also important—if peers aren’t forming sessions or aren’t exchanging routes, the network won’t converge properly even if the physical path looks okay. Logs add another layer of visibility, capturing events such as route updates, protocol flaps, or policy decisions that could affect reachability. Don’t overlook policy and translation effects. Verifying access-control lists and NAT ensures that traffic isn’t being blocked or rewritten in ways that would produce symptoms similar to topology problems, even when the routing looks correct. By combining reachability tests, path mapping, routing-state verification, and policy checks, you can isolate topology problems methodically rather than guessing or making wholesale changes.

Begin with a layered approach that traces the path from source to destination and checks what the network believes about that path. Start by using ping to confirm end-to-end reachability to the destination or to the next hop. If ping fails, you quickly know there’s a basic connectivity issue that needs fixing before deeper analysis. If ping succeeds, you’re ready to map the actual path with traceroute and look for where the path is failing or deviating. The point where traceroute stops or shows an unexpected hop helps pinpoint topology problems, such as a misconfigured link, a routing decision at a particular device, or a device that is dropping or rate-limiting traffic.

Next, turn to the routing state itself. Inspect the routing table or the device’s view of routes to see what networks are considered reachable, what the next hops are, and which paths are installed as the best ones. This reveals whether the expected routes exist, whether there are static vs. dynamic routes, and if any anomalies like missing routes or incorrect next hops are present. Checking neighbor adjacencies for dynamic protocols is also important—if peers aren’t forming sessions or aren’t exchanging routes, the network won’t converge properly even if the physical path looks okay. Logs add another layer of visibility, capturing events such as route updates, protocol flaps, or policy decisions that could affect reachability.

Don’t overlook policy and translation effects. Verifying access-control lists and NAT ensures that traffic isn’t being blocked or rewritten in ways that would produce symptoms similar to topology problems, even when the routing looks correct. By combining reachability tests, path mapping, routing-state verification, and policy checks, you can isolate topology problems methodically rather than guessing or making wholesale changes.