350-401 Question 463
Single answerA network engineer is troubleshooting an issue where an eBGP session between Router A and Router B is not establishing. Router A and Router B are directly connected, and the following configurations are applied:
Router A: router bgp 65001 neighbor 10.1.1.2 remote-as 65002 neighbor 10.1.1.2 update-source Loopback0
Router B: router bgp 65002 neighbor 10.1.1.1 remote-as 65001
What is the most likely cause of the issue?
- A
The update-source Loopback0 configuration on Router A is incorrect for a directly connected eBGP session.
- B
Router B is missing the 'update-source Loopback0' configuration.
- C
Router A and Router B do not have matching AS numbers in their configurations.
- D
BGP requires a static route or IGP to reach the neighbor's loopback address.
Show answer and explanation
Correct answer: A
Explanation
The issue is caused by a mismatch in how Router A and Router B are attempting to establish the BGP session. Router A is configured to use its loopback address as the update source, but Router B is not configured to expect this. In eBGP sessions, both routers must agree on the source of the connection. To resolve the issue, either remove the 'update-source Loopback0' command from Router A or configure Router B to use a loopback address and add the appropriate 'update-source Loopback0' command.
- A. Correct.
The 'update-source Loopback0' command is used to establish BGP sessions using the loopback address instead of the directly connected interface. However, since Router B is not configured to use a loopback as the update source, this mismatch causes the session to fail.
- B. Incorrect.
Router B does not need the 'update-source Loopback0' configuration because it is using its directly connected interface for the BGP session. Adding this configuration would not resolve the issue.
- C. Incorrect.
The AS numbers are correctly configured on both routers (Router A is in AS 65001, and Router B is in AS 65002). This is not the cause of the problem.
- D. Incorrect.
While it is true that a static route or an IGP is required to reach a loopback address, this is not relevant in this scenario because both routers are directly connected. The problem lies in the configuration mismatch of the update-source.