300-410 Question 13
Single answerA network engineer is troubleshooting a route-map applied to a BGP neighbor in an enterprise network. The route-map is intended to filter routes based on specific tags, but it is not functioning as expected. Upon reviewing the configuration, the engineer discovers the following route-map and prefix-list configuration:
route-map FILTER_ROUTES permit 10
match tag 100
set local-preference 200
!
route-map FILTER_ROUTES permit 20
match ip address prefix-list FILTERED_ROUTES
!
ip prefix-list FILTERED_ROUTES seq 5 permit 192.168.1.0/24
What is the most likely reason the route-map is not filtering routes as expected?
- A
The route-map does not include a deny statement to explicitly block unwanted routes.
- B
The 'match tag' statement in sequence 10 is being ignored because no routes have the tag 100.
- C
The prefix-list FILTERED_ROUTES is incorrectly configured and is not matching any routes.
- D
The route-map needs to include a 'set tag' action to modify the tag of the routes being filtered.
Show answer and explanation
Correct answer: B
Explanation
The issue lies in the 'match tag' statement in sequence 10 of the route-map. For the route-map to successfully filter routes based on tags, the routes in question must already have the specified tag (100). If no routes in the routing table carry this tag, the route-map sequence will not match any routes, and the intended filtering will not occur. To resolve this, the engineer should verify that routes are being tagged correctly before applying the route-map.
- A. Incorrect.
While it is true that the route-map does not include a deny statement, this is not required for filtering. The route-map can still permit or modify routes without an explicit deny, as it will implicitly deny unmatched routes.
- B. Correct.
The 'match tag' statement in sequence 10 requires routes to have a tag of 100 to be matched. If no routes in the routing table have this tag, this part of the route-map will not be applied, causing the filtering to fail.
- C. Incorrect.
The prefix-list FILTERED_ROUTES is correctly configured to permit 192.168.1.0/24, so it is not the source of the issue in this scenario.
- D. Incorrect.
The 'set tag' statement is used to modify the tag of routes, not to match them. Since the objective is to match routes based on their existing tags, this is not relevant to the problem.