300-420 Question 192
Select 3A company is deploying an MPLS Layer 3 VPN to interconnect its geographically distributed branch offices. The service provider has implemented the MPLS infrastructure, and the company is responsible for configuring the CE routers. During the configuration process, the network engineer notices that routes from one branch office are not being advertised to other branches. Which of the following could be the cause of this issue?
- A
The VRF configuration on the CE router is missing.
- B
The BGP sessions between the CE and PE routers are not established.
- C
The MPLS forwarding table on the PE router is empty.
- D
The same RD (Route Distinguisher) is used for all VRFs in the MPLS network.
- E
The MP-BGP configuration on the PE router does not include the correct route targets.
Show answer and explanation
Correct answers: A, B, E
Explanation
In an MPLS Layer 3 VPN, routes are advertised using MP-BGP between PE routers, and the CE routers must establish BGP sessions with the PE routers to exchange routes. VRF configuration on the CE ensures that traffic is correctly mapped, while route targets in MP-BGP control which routes are shared between VRFs. Any misconfiguration in these areas can disrupt route advertisement between branch offices.
- A. Correct.
The VRF configuration on the CE router is crucial because it maps incoming and outgoing traffic to the appropriate VRF. Without this, routes cannot be assigned to the correct VPN.
- B. Correct.
BGP sessions between the CE and PE routers are required for route exchange. If these sessions are not established, no routes will be advertised.
- C. Incorrect.
The MPLS forwarding table on the PE router is related to MPLS label switching but does not directly affect route advertisement between branches.
- D. Incorrect.
The Route Distinguisher (RD) is used to differentiate overlapping IP addresses in different VRFs. Using the same RD for all VRFs does not prevent route advertisement but can cause conflicts in overlapping networks.
- E. Correct.
The MP-BGP configuration must include correct route targets to ensure routes are imported and exported between VRFs. Incorrect or missing route targets can prevent route advertisement.