mediumMultiple Choice
BFD Timer Calculations for CCNP ENARSI
A network engineer runs the following command on Router R1:
R1# show bfd neighbors detail
IPv4 Sessions NeighborAddr LD/RD Int State Holdown(mult) Intf
10.1.1.2 1/3 Gi0/0 Up 3000(3) Gi0/0
Session state is UP and not using echo function. OurAddr: 10.1.1.1 Handle: 1 Local Diag: 0, Demand mode: 0, Poll bit: 0 MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3 Received MinRxInt: 1000000, Received Multiplier: 3 Holddown (hits): 3000(0) Rx Count: 100, Tx Count: 100
Based on this output, what is the BFD session's detection time?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The detection time is 3000 ms.
The detection time is the holddown timer, which is the negotiated transmit interval multiplied by the multiplier. The negotiated transmit interval is 1000 ms (maximum of local MinTxInt and received MinRxInt), and the multiplier is 3, so the detection time is 3000 ms.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The detection time is 3000 ms.
Why this is correct
Detection time equals the negotiated transmit interval multiplied by the multiplier. Both peers advertise 1000000 microseconds (1000 ms) with a multiplier of 3, giving 3000 ms, which matches the Holdown field shown in the neighbour table.
- ✗
The detection time is 1000 ms.
Why it's wrong here
Detection time is the negotiated transmit interval multiplied by the multiplier: 1000 ms × 3 = 3000 ms, matching the Holdown value. Using the interval alone is tempting because 1000 ms appears in MinTxInt, but that figure is the transmit rate, not the detection window.
- ✗
The detection time is 1500 ms.
Why it's wrong here
The multiplier of 3 applies to the 1000 ms negotiated interval, giving 3000 ms, not 1500 ms. Halving or otherwise adjusting the interval is tempting when echo function is disabled, but echo is already off here and does not alter the multiplication.
- ✗
The detection time is 9000 ms.
Why it's wrong here
Detection time is the multiplier applied to the negotiated transmit interval, not the holddown value: 3 × 1000000 µs = 3000 ms, so 9000 ms misreads the Holdown(mult) field. The 3000 ms figure tempts because it appears verbatim in the output, but that column is the holddown timer, not the detection calculation.
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