N10-009 Question 151
Single answerSubinterfaces, Administrative distance, Prefix length, MetricA network administrator is troubleshooting traffic from the 10.20.30.0/24 VLAN, which reaches other internal networks through a router-on-a-stick configuration. The router has subinterface G0/1.30 configured for VLAN 30 with IP address 10.20.30.1/24. Users in VLAN 30 can reach the local gateway, but traffic to 10.50.10.25 is taking a slower backup WAN path instead of the primary MPLS path. The administrator checks the routing table and sees these entries:
- O 10.50.10.0/24 via 172.16.1.2, metric 20
- S 10.50.10.0/25 via 192.168.100.2, administrative distance 1
- O 0.0.0.0/0 via 172.16.1.2, metric 10
Which route will the router use to forward traffic from a host in VLAN 30 to 10.50.10.25?
- A
The OSPF 10.50.10.0/24 route, because OSPF metrics are lower than static route administrative distance
- B
The static 10.50.10.0/25 route, because the longest prefix match is chosen before considering administrative distance
- C
The default route, because it has the lowest metric among all listed routes
- D
The subinterface G0/1.30 directly, because the source VLAN determines the outbound route
Show answer and explanation
Correct answer: B
Explanation
This scenario tests practical route selection using subinterfaces, prefix length, administrative distance, and metric. The subinterface matters because it provides inter-VLAN routing for VLAN 30 in a router-on-a-stick design, but it does not override routing logic. For destination 10.50.10.25, the router compares matching routes and first applies longest prefix match. The route 10.50.10.0/25 is more specific than 10.50.10.0/24, so it wins immediately. Only if two routes had the same prefix length would the router compare administrative distance, and if those were equal for the same protocol, metric would then matter. This behavior is consistent with standard IP routing best practices and vendor documentation such as Cisco routing table and route selection references, which describe longest prefix match as the first major decision point in forwarding.
- A. Incorrect.
Incorrect. This reflects a common misconception between route selection stages. Routers first select the most specific matching prefix. Since 10.50.10.25 falls within 10.50.10.0/25, that route is preferred over 10.50.10.0/24 regardless of OSPF metric. Administrative distance is only compared when routes have the same prefix length.
- B. Correct.
Correct. The destination 10.50.10.25 matches both 10.50.10.0/24 and 10.50.10.0/25, but the router uses the longest prefix match first. A /25 is more specific than a /24, so the static route to 192.168.100.2 is selected. Administrative distance and metric are not evaluated until after prefix length has narrowed the candidates.
- C. Incorrect.
Incorrect. The default route is only used when no more specific route matches the destination. Because both 10.50.10.0/24 and 10.50.10.0/25 explicitly match 10.50.10.25, the default route is not considered for forwarding this packet.
- D. Incorrect.
Incorrect. The subinterface G0/1.30 is the Layer 3 gateway for VLAN 30 and receives traffic from that VLAN, but route selection is based on the destination IP address, not the source VLAN alone. The router will forward the packet according to the best matching route in the routing table.