300-420 Question 30
Select 3A network engineer is configuring route filtering on a branch router to ensure that only specific prefixes are advertised to a neighboring router using OSPF. They decide to use a distribute-list. Which of the following statements about this configuration are correct?
- A
A distribute-list can filter routes in OSPF based on an access control list (ACL).
- B
Distribute-lists in OSPF can only filter routes being advertised, not routes being received.
- C
A distribute-list applied to OSPF can prevent specific routes from being installed in the routing table.
- D
Route filtering through a distribute-list in OSPF does not affect routes already in the link-state database (LSDB).
- E
Distribute-lists in OSPF can filter external routes redistributed from other protocols.
Show answer and explanation
Correct answers: A, D, E
Explanation
Distribute-lists are a basic tool for route filtering in OSPF and work by filtering routes based on ACLs or prefix lists. They primarily influence route advertisement or installation in the OSPF routing table but do not modify the link-state database (LSDB). Additionally, they can filter external routes that are redistributed into OSPF from other protocols. This makes them a flexible but foundational technique for route control in OSPF environments.
- A. Correct.
Correct: A distribute-list can use an ACL to filter routes in OSPF. This is a core function of distribute-lists for route filtering.
- B. Incorrect.
Incorrect: Distribute-lists in OSPF can filter both routes being advertised and routes being received, depending on how they are applied.
- C. Incorrect.
Incorrect: OSPF distribute-lists do not directly prevent routes from being installed in the routing table. They limit route advertisement but do not affect the LSDB or directly block routes from being installed.
- D. Correct.
Correct: OSPF distribute-lists do not impact the LSDB. They only influence route advertisement or installation, depending on configuration.
- E. Correct.
Correct: Distribute-lists can filter external routes redistributed into OSPF from protocols like BGP, EIGRP, or static routes.