300-420 Question 127
Select 2An enterprise network is experiencing issues with unnecessary routes being advertised between OSPF areas, leading to suboptimal routing and increased overhead. As a network engineer, you are tasked with implementing route filtering to address this problem. Which of the following methods can be used to filter routes in OSPF?
- A
Apply a distribute-list to an OSPF interface.
- B
Use a route-map with prefix-list applied to OSPF redistribution.
- C
Implement area filters using the 'filter-list' command between OSPF areas.
- D
Use a route-policy to block OSPF external Type-5 LSAs.
- E
Configure 'summary-address' to suppress specific routes.
Show answer and explanation
Correct answers: B, C
Explanation
In OSPF, route filtering can be achieved through specific mechanisms such as using route-maps with prefix-lists during redistribution or applying the 'filter-list' command to control inter-area route advertisement. These methods effectively limit the propagation of unwanted routes, helping to optimize routing and reduce overhead. Other options, such as distribute-lists and route-policies, are not applicable or supported for OSPF, and 'summary-address' focuses on summarization rather than direct filtering.
- A. Incorrect.
Distribute-lists are not supported for filtering routes within native OSPF configurations. They are primarily used in EIGRP or BGP, not OSPF.
- B. Correct.
Route-maps can be used with prefix-lists during OSPF redistribution to control which routes are injected into OSPF from another routing protocol.
- C. Correct.
The 'filter-list' command can be used to filter routes between OSPF areas, effectively preventing certain routes from being advertised across area boundaries.
- D. Incorrect.
Route-policy is a term typically associated with BGP, not OSPF. OSPF external Type-5 LSAs cannot be blocked using route-policy.
- E. Incorrect.
The 'summary-address' command is used to aggregate and summarize routes advertised into OSPF, but it does not filter specific routes directly.