300-410 Question 4
Single answerAn enterprise network is running OSPF as its routing protocol. The network administrator notices that a specific router in Area 0 is advertising incorrect route information, which is impacting connectivity. To address this, the administrator wants to filter the incorrect routes at the OSPF ASBR level. Which method should the administrator use?
- A
Configure a distribute-list on the ASBR to filter routes in OSPF.
- B
Use a route-map applied to the OSPF process to filter routes.
- C
Apply an access-list directly to the OSPF process on the ASBR.
- D
Use a prefix-list applied as an inbound policy on the ASBR.
Show answer and explanation
Correct answer: B
Explanation
To filter routes in OSPF at the ASBR level, a route-map should be used. Route-maps are versatile and allow the administrator to apply granular policies for route filtering, making them the preferred method in this scenario. Distribute-lists and access-lists are not applicable for OSPF route filtering, and prefix-lists alone are insufficient without being part of a route-map.
- A. Incorrect.
A distribute-list cannot directly filter routes in OSPF because distribute-lists are used for filtering routes in protocols like EIGRP and RIP, not OSPF.
- B. Correct.
Using a route-map applied to the OSPF process is the correct method to filter routes at the ASBR level. Route-maps provide the flexibility to match and filter routes based on specific criteria.
- C. Incorrect.
Access-lists cannot be applied directly to the OSPF process for route filtering. They can be used in conjunction with route-maps but not as a standalone method.
- D. Incorrect.
Prefix-lists cannot be applied directly as inbound policies on the ASBR in OSPF. They are typically used to permit or deny specific prefixes but must be combined with other mechanisms like route-maps.