N10-009 Question 144
Single answerRoute selectionA branch office router has two paths to reach the 10.50.20.0/24 network. One path is a static route pointing to the ISP-managed MPLS circuit. The other path is learned through OSPF over a site-to-site VPN backup link. Users report that traffic is taking the VPN path even though the MPLS circuit is up and passing traffic. The network administrator wants the router to prefer the MPLS path during normal operation and use the VPN path only if the MPLS route is unavailable. Which action should the administrator take?
- A
Increase the OSPF cost on the VPN link so the static route is preferred
- B
Replace the static route with an OSPF route so both paths use the same routing protocol
- C
Configure the static route with a higher administrative distance than OSPF
- D
Ensure the static route has a lower administrative distance than the OSPF-learned route
Show answer and explanation
Correct answer: D
Explanation
This question tests practical route selection behavior. When multiple routes exist to the same destination prefix, routers first prefer the most specific route. If the prefix length is the same, the router compares administrative distance (AD) to decide which routing source is more trustworthy. Only after that does it evaluate the metric within the selected routing protocol. In most implementations, a standard static route has a lower AD than an OSPF route, so it is normally preferred. If OSPF is winning instead, the administrator should verify that the static route is present and configured with a lower AD than the OSPF route. Best practice for a primary/backup design is to keep the primary route with the lower AD and use a higher-AD route only as backup, often called a floating static route when the static route is the backup. This behavior is consistent with common routing documentation from major vendors such as Cisco and Juniper regarding route selection order: longest prefix match, administrative distance/preference, then protocol metric.
- A. Incorrect.
Incorrect. OSPF cost affects path selection only among OSPF routes. When a router is choosing between routes learned from different sources, it first compares administrative distance. A static route normally has a lower administrative distance than an OSPF route, so changing OSPF cost would not solve the issue if the static route is not being preferred due to route trust/preference settings.
- B. Incorrect.
Incorrect. Converting the static route to OSPF removes the intended primary/backup design and introduces unnecessary dependency on dynamic routing for both paths. It also does not directly address the requirement to prefer the MPLS path and keep the VPN as a backup. In real environments, a static route is commonly used as a preferred route or as part of a floating static design.
- C. Incorrect.
Incorrect. Setting the static route to a higher administrative distance than OSPF would make the static route less preferred, which is the opposite of the goal. This is sometimes done intentionally for a floating static route used as backup, but not when the static route should be primary.
- D. Correct.
Correct. Route selection between different routing sources is based first on longest prefix match, then on administrative distance, and then on the protocol metric within the same routing protocol. To make the MPLS static route preferred over the OSPF route, the static route must have a lower administrative distance than the OSPF-learned route. This aligns with common enterprise practice for primary path selection with dynamic-routing backup.