300-440 Question 129
Select 3A network engineer is troubleshooting a BGP issue in a topology where internal BGP (iBGP) peers are established between routers R1, R2, and R3. The network also uses VRF-Lite to segment traffic. R1 is advertising a route to R2, but R2 is not propagating the route to R3. What could be causing this issue?
- A
The next-hop address of the route is not reachable from R2.
- B
R2 does not have the 'next-hop-self' configuration for the iBGP session with R3.
- C
The VRF instance on R2 is misconfigured or missing.
- D
R2 and R3 do not have a full mesh iBGP relationship.
- E
The maximum number of BGP prefixes has been exceeded on R3.
Show answer and explanation
Correct answers: A, C, D
Explanation
In this scenario, the primary issues that could prevent R2 from propagating the route to R3 are related to BGP operational requirements and VRF-Lite configuration. The next-hop reachability is a critical aspect of BGP operation, as BGP requires the next-hop to be reachable for routes to be considered valid. A misconfigured VRF instance could isolate the routing table, preventing route propagation. Additionally, iBGP mandates a full mesh topology unless route reflectors are used. If R2 and R3 do not have an iBGP session, R2 cannot advertise the route to R3. Other options, such as the lack of 'next-hop-self' or prefix limits, do not directly relate to this specific issue.
- A. Correct.
If the next-hop address of the route advertised by R1 is not reachable from R2, the route will not be installed in the BGP table, and therefore R2 will not propagate it to R3.
- B. Incorrect.
The 'next-hop-self' configuration is not mandatory in this scenario. It is used to modify the next-hop attribute, but the lack of this configuration alone would not prevent route propagation.
- C. Correct.
A misconfigured or missing VRF instance on R2 can cause the route to fail to propagate because VRF-Lite isolates routing tables and ensures traffic segmentation.
- D. Correct.
iBGP requires a full mesh relationship unless route reflectors are used. If R2 and R3 do not have a full mesh iBGP session, R2 cannot propagate the route to R3.
- E. Incorrect.
Exceeding the maximum number of BGP prefixes on R3 would cause route rejection on R3, not an issue with route propagation from R2.