N10-009 Question 145
Single answerRoute selectionA network administrator is troubleshooting why traffic from a branch office to the 10.50.20.0/24 server subnet is leaving through an MPLS circuit instead of a newly installed VPN tunnel. The branch router has the following routes in its routing table for 10.50.20.0/24:
- OSPF route via 172.16.1.1
- Static route via 192.168.100.1 with an administrative distance of 5
- RIP route via 10.10.10.1
All next-hop addresses are reachable, and the destination network is identical in all three entries. Which route will the router select to forward traffic to 10.50.20.0/24?
- A
The OSPF route via 172.16.1.1, because OSPF converges faster than RIP and is preferred over dynamic routes with slower convergence
- B
The static route via 192.168.100.1, because the router prefers the route with the lowest administrative distance when the prefix length is the same
- C
The RIP route via 10.10.10.1, because RIP uses hop count and will be selected if the destination network is learned from multiple protocols
- D
The router will load balance across all three routes, because they point to the same destination network
Show answer and explanation
Correct answer: B
Explanation
This question tests practical route selection. Routers generally choose routes in this order: longest prefix match first, then administrative distance when the same destination network is learned from different routing sources, and then metric when comparing routes within the same routing protocol. Since all three entries are for 10.50.20.0/24, prefix length does not differentiate them. The static route has an administrative distance of 5, which is lower than OSPF's commonly used 110 and RIP's commonly used 120, so the static route is selected. This aligns with standard routing behavior documented by major vendors such as Cisco and is consistent with Network+ objectives covering route selection, administrative distance, and metric evaluation.
- A. Incorrect.
Incorrect. Although OSPF normally has a lower administrative distance than RIP, it does not beat a static route that has been configured with an even lower administrative distance. Route selection first considers the most specific prefix, and when the prefix length is identical, routers compare administrative distance across different route sources.
- B. Correct.
Correct. All three routes are for the same destination prefix, so the router next compares route source preference. A static route with an administrative distance of 5 is preferred over OSPF (commonly 110) and RIP (commonly 120). Because all next hops are reachable, the static route via 192.168.100.1 will be installed and used for forwarding.
- C. Incorrect.
Incorrect. RIP's hop count is used as a metric only when comparing RIP routes to other RIP routes. It is not used to beat routes learned from protocols with lower administrative distance. A common mistake is confusing metric comparison within a protocol with administrative distance comparison between protocols.
- D. Incorrect.
Incorrect. Equal-cost load balancing applies when multiple installed routes are equally preferred, typically with the same prefix length and compatible routing characteristics such as equal administrative distance and metric. In this scenario, the routes come from different sources with different administrative distances, so only the most preferred route is installed.