300-410 Question 126
Select 3An enterprise network is running BGP with an external service provider. The network engineer notices that a specific prefix advertised by the service provider is not being installed in the routing table of the enterprise router. Upon investigation, the engineer identifies that the prefix is visible in the BGP table but not in the RIB. What could be the potential reasons for this behavior?
- A
The prefix has a higher administrative distance compared to an existing route in the RIB.
- B
The BGP next-hop for the prefix is not reachable.
- C
The prefix is filtered by an inbound BGP route policy.
- D
The maximum-prefix limit configured for the BGP session has been reached.
- E
The prefix is marked as 'dampened' due to route flap damping configuration.
Show answer and explanation
Correct answers: A, B, C
Explanation
In this scenario, the prefix is visible in the BGP table, so the BGP session is established, and the prefix is being advertised by the service provider. However, for the prefix to be installed in the RIB, specific conditions must be met. The prefix must have the lowest administrative distance among competing routes, its next-hop must be reachable, and it must not be filtered by any route policies. The maximum-prefix limit and route flap damping do not apply in this case as the prefix is already in the BGP table.
- A. Correct.
Correct. If the prefix has a higher administrative distance than an existing route already in the RIB (e.g., a static or OSPF route), BGP will not install the route in the routing table.
- B. Correct.
Correct. If the next-hop IP address for the prefix is not reachable, BGP will not install the route in the routing table.
- C. Correct.
Correct. Inbound route policies can filter or suppress specific prefixes, preventing them from being installed in the RIB.
- D. Incorrect.
Incorrect. The maximum-prefix limit affects the establishment of the BGP session itself, not the installation of individual prefixes in the RIB.
- E. Incorrect.
Incorrect. While route flap damping can suppress a prefix, it would not appear in the BGP table if it were dampened.