400-007 · Question #243
Two routers R1 and R2 are directly connected through an Ethernet link. Both routers are running OSPF over the Ethernet link and OSPF has been registered with BFD. R1 has been set up to transmit BFD…
The correct answer is B. After the initial timer exchange, R1 sets its transmission rate to the R2 Required Min RX interval. BFD negotiates the actual transmit interval as the greater of the local Desired Min TX and the remote Required Min RX, so R1 must slow its transmission to 100 ms to match R2's receive capability.
Question
Two routers R1 and R2 are directly connected through an Ethernet link. Both routers are running OSPF over the Ethernet link and OSPF has been registered with BFD. R1 has been set up to transmit BFD at a 50 ms interval, but R2 can receive only at a 100 ms rate due to platform limitations. What does this mean?
Options
- AAfter the initial timer exchange, R2 sets its transmission rate to the R1 Desired Min TX interval
- BAfter the initial timer exchange, R1 sets its transmission rate to the R2 Required Min RX interval
- CTimers renegotiate indefinitely, so the timer exchange phase never converges
- DR2 sets the P-bit on all BFD control packets until R2 sends a packet with the F-bit set
How the community answered
(29 responses)- A7% (2)
- B66% (19)
- C7% (2)
- D21% (6)
Why each option
BFD negotiates the actual transmit interval as the greater of the local Desired Min TX and the remote Required Min RX, so R1 must slow its transmission to 100 ms to match R2's receive capability.
R2's own transmission rate is governed by R1's Required Min RX interval, not R1's Desired Min TX interval - this answer incorrectly reverses the TX and RX roles.
Per RFC 5880, each BFD session negotiates its transmit interval as the maximum of the local system's bfdDesiredMinTxInterval and the remote system's bfdRequiredMinRxInterval. Because R2's Required Min RX is 100 ms and R1's Desired Min TX is 50 ms, R1 adopts 100 ms as its actual transmit rate to accommodate R2's platform constraint. This negotiation occurs during the initial control packet exchange and results in a stable, converged timer.
BFD timer negotiation converges after the initial control packet exchange and does not renegotiate indefinitely.
The Poll (P-bit) and Final (F-bit) mechanism is used to signal parameter changes after a session is already established, not during initial session timer negotiation.
Concept tested: BFD timer negotiation and interval convergence
Source: https://www.rfc-editor.org/rfc/rfc5880
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