300-420 Question 84
Single answerA network engineer has configured Bidirectional Forwarding Detection (BFD) on a pair of routers to enable fast link failure detection in a high-availability network. However, the BFD session is not establishing successfully. Upon investigation, the engineer identifies that the configurations on both routers are as follows:
Router A:
- BFD interval: 50ms
- BFD multiplier: 3
Router B:
- BFD interval: 500ms
- BFD multiplier: 5
What could be the primary reason why the BFD session is failing to establish?
- A
The BFD interval values do not match between Router A and Router B.
- B
The BFD multiplier values do not match between Router A and Router B.
- C
BFD requires the interval and multiplier values to be synchronized for the session to establish.
- D
The BFD session is failing because the configured interval on Router B exceeds the threshold for fast failure detection.
Show answer and explanation
Correct answer: A
Explanation
BFD sessions rely on the configured interval values being within a reasonable range across both routers. In this scenario, Router A is configured with a very low interval of 50ms, while Router B is configured with a much higher interval of 500ms. This discrepancy prevents the BFD session from establishing, as Router A's interval is too short to align with Router B's settings. Multiplier values do not need to match for the session to establish, as they are used locally on each router.
- A. Correct.
The BFD interval values do not need to match exactly, but they must be within an acceptable range for the session to establish. In this case, Router B’s interval of 500ms is too high compared to Router A’s interval of 50ms.
- B. Incorrect.
The BFD multiplier values do not need to match between routers for a BFD session to establish. They are independently configured and used to calculate the detection time.
- C. Incorrect.
BFD does not require the interval and multiplier values to be exactly synchronized between the routers. The interval values must simply be within an acceptable range.
- D. Incorrect.
Although the interval on Router B is high, BFD does not impose a hard threshold for what constitutes 'fast failure detection.' The issue lies in the mismatch between the intervals of Router A and Router B.