N10-009 Question 313
Single answerRoute selection: Routing table, Default routesA network administrator is troubleshooting why a workstation on 192.168.10.0/24 cannot reach an external SaaS application at 198.51.100.25. The workstation has the following IPv4 routing table:
Destination Netmask Gateway Interface 0.0.0.0 0.0.0.0 192.168.10.1 192.168.10.50 192.168.10.0 255.255.255.0 On-link 192.168.10.50 192.168.10.50 255.255.255.255 On-link 192.168.10.50 192.168.10.255 255.255.255.255 On-link 192.168.10.50 198.51.100.0 255.255.255.0 192.168.10.254 192.168.10.50
The administrator learns that 192.168.10.254 is a decommissioned test router that is no longer forwarding traffic. Which route will the workstation use when sending traffic to 198.51.100.25, and why?
- A
The default route to 192.168.10.1, because default routes are preferred for any off-subnet destination
- B
The route to 198.51.100.0/24 via 192.168.10.254, because the most specific matching route is selected before the default route
- C
The on-link route to 192.168.10.0/24, because the next hop 192.168.10.254 is on the local subnet
- D
No route will be selected, because the presence of a failed gateway automatically removes the matching route from the table
Show answer and explanation
Correct answer: B
Explanation
This question tests route selection using the routing table and default routes. A host chooses the route with the longest matching prefix, often called longest-prefix match. In this case, 198.51.100.25 matches both the default route (0.0.0.0/0) and the specific route 198.51.100.0/24, but /24 is more specific than /0, so it wins. The host will therefore forward the packet to 192.168.10.254. If that next hop is down, connectivity fails, but the route selection logic does not fall back to the default route just because the preferred gateway is nonfunctional. This behavior aligns with standard IP routing practice described in networking documentation and vendor routing references: route lookup is based first on prefix specificity, with the default route used only when no better match exists.
- A. Incorrect.
Incorrect. A default route (0.0.0.0/0) is only used when no more specific route matches the destination. Because 198.51.100.25 matches the 198.51.100.0/24 route, the workstation will try that route first rather than the default gateway.
- B. Correct.
Correct. Routing decisions are based on longest-prefix match. The destination 198.51.100.25 falls within 198.51.100.0/24, which is more specific than the default route 0.0.0.0/0. The workstation will therefore send traffic to next hop 192.168.10.254, even if that router is no longer functioning.
- C. Incorrect.
Incorrect. The on-link route for 192.168.10.0/24 applies only to destinations in the local 192.168.10.0 network. Although the next hop 192.168.10.254 is reachable at Layer 2 on the local subnet, the actual destination 198.51.100.25 is not on-link, so the host still uses the matching network route that points to the next hop.
- D. Incorrect.
Incorrect. Hosts do not automatically remove a static or learned route simply because the next-hop gateway is unreachable or failed. Unless routing information is updated by the operating system, an administrator, or a dynamic routing process, the route remains in the table and continues to be selected if it is the best match.