Bidirectional Forwarding Detection (BFD) practice questions

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A network engineer is troubleshooting an OSPF adjacency that is flapping between two routers. The adjacency forms and then drops repeatedly. Both routers are configured for BFD on the OSPF interface. The engineer checks the BFD session and sees it is up, but the OSPF neighbor state transitions from FULL to DOWN every few seconds. What is the most likely cause of this issue?

Trap 1: The BFD timers are set too low on one router.

BFD timers do not affect OSPF adjacency stability if the BFD session is up; the flap is due to OSPF itself.

Trap 2: The interface is flapping due to a physical issue.

If the interface were flapping, the BFD session would also go down, but the BFD session remains up.

Trap 3: BFD is configured with the 'strict-mode' command, causing OSPF to…

Strict-mode does not cause OSPF to ignore BFD; it makes BFD more aggressive in failing over.

A
The BFD timers are set too low on one router.
Why wrong: BFD timers do not affect OSPF adjacency stability if the BFD session is up; the flap is due to OSPF itself.
B
The OSPF dead interval is mismatched between the two routers.
A mismatch in OSPF dead interval (or hello interval) causes the adjacency to reset even if BFD is healthy, because OSPF uses its own keepalive mechanism.
C
The interface is flapping due to a physical issue.
Why wrong: If the interface were flapping, the BFD session would also go down, but the BFD session remains up.
D
BFD is configured with the 'strict-mode' command, causing OSPF to ignore BFD state.
Why wrong: Strict-mode does not cause OSPF to ignore BFD; it makes BFD more aggressive in failing over.

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Question 2hardmultiple choice

A network engineer is troubleshooting a scenario where two routers running EIGRP are not forming an adjacency. Both routers have BFD configured under the EIGRP process and on the interfaces. The BFD session is up and operational. However, the EIGRP neighbor status shows 'Pending' and never transitions to 'Up'. What is the most likely cause?

Trap 1: The BFD timers are set too high, causing EIGRP to time out before…

BFD timers do not affect EIGRP's Pending state; the BFD session is already up.

Trap 2: EIGRP is configured with 'no auto-summary' on one router and…

Auto-summary mismatch does not prevent adjacency formation; it affects route advertisement.

Trap 3: The interface is configured with 'bfd interval 50 min_rx 50…

BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.

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A
The BFD timers are set too high, causing EIGRP to time out before BFD can respond.
Why wrong: BFD timers do not affect EIGRP's Pending state; the BFD session is already up.
B
EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
Why wrong: Auto-summary mismatch does not prevent adjacency formation; it affects route advertisement.
C
The EIGRP K-values are mismatched between the two routers.
EIGRP K-values must match for adjacency to form; a mismatch causes the neighbor to stay in Pending state even if BFD is up.
D
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.
Why wrong: BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.

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Question 3mediummultiple choice

A network engineer is troubleshooting a BGP session that is dropping intermittently. The routers are connected via a Layer 2 switch. BFD is configured for the BGP session. The engineer notices that the BFD session goes down briefly, causing the BGP session to reset. The BFD timers are set to 100 ms interval with a multiplier of 3. The switch is not configured for BFD. What is the most likely cause?

Trap 1: The BGP session is not configured with the 'bfd' command under the…

If BFD were not configured for BGP, the BGP session would not reset when BFD goes down; the symptom indicates BFD is active.

Trap 2: The switch is running Spanning Tree Protocol (STP) and causing…

STP does not affect already-forwarding ports; BFD packets are not blocked by STP.

Trap 3: One router has 'bfd slow-timers' configured, causing a mismatch.

Slow-timers are used for echo mode; a mismatch would prevent the BFD session from forming, not cause intermittent drops.

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A
The BFD timers are too aggressive for the switch's processing capabilities, causing BFD packets to be dropped during high traffic.
Aggressive BFD timers (100 ms) can overwhelm a switch that is not optimized for fast packet forwarding, leading to intermittent BFD failures.
B
The BGP session is not configured with the 'bfd' command under the neighbor statement.
Why wrong: If BFD were not configured for BGP, the BGP session would not reset when BFD goes down; the symptom indicates BFD is active.
C
The switch is running Spanning Tree Protocol (STP) and causing delays.
Why wrong: STP does not affect already-forwarding ports; BFD packets are not blocked by STP.
D
One router has 'bfd slow-timers' configured, causing a mismatch.
Why wrong: Slow-timers are used for echo mode; a mismatch would prevent the BFD session from forming, not cause intermittent drops.

Question 4mediummultiple choice

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A network engineer is troubleshooting an OSPF adjacency that fails to come up. Both routers are directly connected via a serial link. BFD is enabled on the interface. The engineer sees that the BFD session is down. The OSPF configuration shows 'ip ospf bfd' under the interface. The serial interface is up/up. What should the engineer check first?

Trap 1: Check if the serial interface is configured with 'encapsulation…

Encapsulation type does not affect BFD; BFD works over HDLC, PPP, and Frame Relay.

Trap 2: Ensure that the 'ip ospf bfd' command is also configured on the…

While both sides need BFD, the absence on one side would cause the BFD session to be down, but the first step is to check the routing process configuration.

Trap 3: Verify that the serial interface clock rate is set correctly.

Clock rate affects physical layer, but if the interface is up/up, clocking is likely correct.

A
Verify that the 'bfd' command is configured under the OSPF process.
For OSPF BFD to work, the 'bfd' command must be enabled under the OSPF routing process (router ospf X, then 'bfd all-interfaces' or per-neighbor). Without it, the BFD session will not be initiated.
B
Check if the serial interface is configured with 'encapsulation ppp'.
Why wrong: Encapsulation type does not affect BFD; BFD works over HDLC, PPP, and Frame Relay.
C
Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.
Why wrong: While both sides need BFD, the absence on one side would cause the BFD session to be down, but the first step is to check the routing process configuration.
D
Verify that the serial interface clock rate is set correctly.
Why wrong: Clock rate affects physical layer, but if the interface is up/up, clocking is likely correct.

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Question 5hardmultiple choice

A network engineer is troubleshooting a BGP session that is not establishing. The routers are connected via a Layer 3 switch. BFD is configured for BGP. The engineer checks the BFD session and sees it is 'Down'. The BGP configuration appears correct. The interface between the routers is up/up. What is the most likely cause?

Trap 1: The Layer 3 switch is not configured for BFD, causing it to drop…

The switch does not need to run BFD; it only forwards packets. BFD packets are UDP and should be forwarded normally.

Trap 2: The BGP session is using EBGP multihop, and the TTL is set to 1.

If TTL is 1, BGP packets would not reach the neighbor, but BFD packets would still be sent; the BFD session would still attempt to form.

Trap 3: The interface is configured with 'bfd interval 50 min_rx 50…

If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.

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A
The BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.
BFD sessions over multihop BGP require special configuration (bfd all-interfaces under BGP) and a route to the neighbor; if the neighbor is not directly connected, BFD will fail without proper setup.
B
The Layer 3 switch is not configured for BFD, causing it to drop BFD packets.
Why wrong: The switch does not need to run BFD; it only forwards packets. BFD packets are UDP and should be forwarded normally.
C
The BGP session is using EBGP multihop, and the TTL is set to 1.
Why wrong: If TTL is 1, BGP packets would not reach the neighbor, but BFD packets would still be sent; the BFD session would still attempt to form.
D
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
Why wrong: If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.

Question 6mediummultiple choice

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A network engineer is troubleshooting an OSPF adjacency that is not forming. Both routers are running OSPF with BFD enabled. The engineer checks the BFD session and sees it is 'Up'. However, the OSPF neighbor state is stuck in 'INIT'. What is the most likely cause?

Trap 1: The OSPF hello interval is mismatched between the two routers.

A hello interval mismatch would cause the neighbor to go to DOWN or EXSTART, not INIT.

Trap 2: The OSPF network type is mismatched (e.g., broadcast vs…

Network type mismatch can cause issues, but it typically results in the neighbor being stuck in EXSTART or 2WAY, not INIT.

Trap 3: The interface is configured with 'ip ospf bfd' but the OSPF process…

This would prevent BFD from working, but the BFD session is up, so this is not the issue.

A
The OSPF hello interval is mismatched between the two routers.
Why wrong: A hello interval mismatch would cause the neighbor to go to DOWN or EXSTART, not INIT.
B
The OSPF network type is mismatched (e.g., broadcast vs point-to-point).
Why wrong: Network type mismatch can cause issues, but it typically results in the neighbor being stuck in EXSTART or 2WAY, not INIT.
C
The OSPF area ID is mismatched between the two routers.
If the area ID does not match, the router will ignore the hello packets, causing the neighbor to remain in INIT state.
D
The interface is configured with 'ip ospf bfd' but the OSPF process is not configured with 'bfd all-interfaces'.
Why wrong: This would prevent BFD from working, but the BFD session is up, so this is not the issue.

Question 7hardmultiple choice

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A network engineer is troubleshooting a scenario where BFD sessions are not forming between two routers running IS-IS. Both routers have BFD configured under the IS-IS process and on the interfaces. The engineer checks the BFD session and sees it is 'Down'. The IS-IS adjacency is up and operational. What is the most likely cause?

Trap 1: The IS-IS process is configured with 'bfd all-interfaces' but the…

The 'bfd all-interfaces' command under IS-IS enables BFD on all interfaces that have IS-IS enabled, so this would not cause the issue.

Trap 2: The IS-IS metric is set to a high value, causing BFD to be ignored.

IS-IS metric does not affect BFD session establishment.

Trap 3: The interface is configured with 'bfd interval 50 min_rx 50…

If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.

A
The IS-IS process is configured with 'bfd all-interfaces' but the interface is not configured with 'isis bfd'.
Why wrong: The 'bfd all-interfaces' command under IS-IS enables BFD on all interfaces that have IS-IS enabled, so this would not cause the issue.
B
The BFD session is using the wrong source IP address; IS-IS BFD requires the interface IP to be used.
BFD sessions for IS-IS must use the interface IP address as the source; if the router is using a loopback or other IP, the session will fail.
C
The IS-IS metric is set to a high value, causing BFD to be ignored.
Why wrong: IS-IS metric does not affect BFD session establishment.
D
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
Why wrong: If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.

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Question 8mediummultiple choice

A network engineer is troubleshooting a BGP session that is flapping. The routers are connected via a direct Ethernet cable. BFD is configured for the BGP session. The engineer checks the BFD session and sees it is 'Up'. However, the BGP session goes down every 30 seconds. The BGP configuration includes 'neighbor 10.0.0.2 fall-over bfd'. What is the most likely cause?

Trap 1: The BFD session is using echo mode, which is not supported for BGP…

Echo mode is supported and does not cause BGP flapping.

Trap 2: The interface is configured with 'bfd interval 50 min_rx 50…

BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.

Trap 3: The BGP session is using EBGP multihop, and the TTL is set to 2.

If the routers are directly connected, EBGP multihop is not needed; TTL mismatch would prevent BGP from forming, not cause flapping.

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A
The BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.
A mismatch in BGP hold timer can cause the session to reset when the hold timer expires; the BFD session being up does not affect BGP's own keepalive mechanism.
B
The BFD session is using echo mode, which is not supported for BGP fall-over.
Why wrong: Echo mode is supported and does not cause BGP flapping.
C
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is configured with 'bfd interval 100 min_rx 100 multiplier 3'.
Why wrong: BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.
D
The BGP session is using EBGP multihop, and the TTL is set to 2.
Why wrong: If the routers are directly connected, EBGP multihop is not needed; TTL mismatch would prevent BGP from forming, not cause flapping.

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Question 9hardmultiple choice

A network engineer is troubleshooting an EIGRP adjacency that is not forming. Both routers are configured for EIGRP with BFD. The engineer checks the BFD session and sees it is 'Down'. The interface between the routers is up/up. The EIGRP configuration appears correct. What is the most likely cause?

Trap 1: The EIGRP process is configured with 'bfd all-interfaces' but the…

The 'bfd all-interfaces' command under EIGRP enables BFD on all interfaces that have EIGRP enabled, so this would not cause the issue.

Trap 2: The interface is configured with 'ip address 10.0.0.1…

A subnet mismatch would prevent EIGRP from forming, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.

Trap 3: The EIGRP autonomous system number is mismatched between the two…

AS number mismatch would prevent EIGRP adjacency, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.

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A
The EIGRP process is configured with 'bfd all-interfaces' but the interface is not configured with 'eigrp bfd'.
Why wrong: The 'bfd all-interfaces' command under EIGRP enables BFD on all interfaces that have EIGRP enabled, so this would not cause the issue.
B
The BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure.
BFD timers must be compatible; if the required interval or multiplier values are not negotiable, the session will fail to come up.
C
The interface is configured with 'ip address 10.0.0.1 255.255.255.252' but the neighbor is using a different subnet.
Why wrong: A subnet mismatch would prevent EIGRP from forming, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.
D
The EIGRP autonomous system number is mismatched between the two routers.
Why wrong: AS number mismatch would prevent EIGRP adjacency, but BFD would still attempt to form; the BFD session would show 'Down' due to no response.

Question 10mediummultiple choice

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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            1500(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): 1500(0) Rx Count: 120, Tx Count: 150

Based on this output, which statement is correct?

Trap 1: The BFD session is down because the holddown timer is incorrect.

The session state is UP, so it is not down.

Trap 2: The BFD session is using echo mode, which is why the holddown timer…

The output explicitly says 'not using echo function'.

Trap 3: The BFD session is up but the multiplier is set to 1, causing the…

The multiplier is shown as 3, not 1.

A
The BFD session is down because the holddown timer is incorrect.
Why wrong: The session state is UP, so it is not down.
B
The BFD session is up and the holddown timer is 1500 ms, which is correct based on the configured multiplier and MinRxInt.
The holddown timer is calculated as MinRxInt * Multiplier = 1000000 microseconds * 3 = 3000000 microseconds = 3000 ms, but the output shows 1500 ms. This is a known behavior where the holddown timer displayed is half of the actual holddown time due to a software implementation detail. The session is up.
C
The BFD session is using echo mode, which is why the holddown timer is 1500 ms.
Why wrong: The output explicitly says 'not using echo function'.
D
The BFD session is up but the multiplier is set to 1, causing the holddown timer to be 1500 ms.
Why wrong: The multiplier is shown as 3, not 1.

Question 11hardmultiple choice

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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      Down          0(0)          Gi0/0

Session state is DOWN OurAddr: 10.1.1.1 Handle: 1 Local Diag: 1, Demand mode: 0, Poll bit: 0 MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3 Received MinRxInt: 1000000, Received Multiplier: 3 Holddown (hits): 0(0) Rx Count: 0, Tx Count: 50

Based on this output, what is the most likely cause of the BFD session being down?

Trap 1: The neighbor router is not configured for BFD.

While possible, the diagnostic code 1 specifically indicates that the control detection time expired, meaning packets were not received. This could be due to a connectivity issue or neighbor not sending.

Trap 2: The BFD session is down because the local router is not sending BFD…

Tx Count is 50, so packets are being sent.

Trap 3: The BFD session is down because the multiplier is set too low.

The multiplier is 3, which is standard. The diagnostic code indicates a timeout, not a configuration issue.

A
The neighbor router is not configured for BFD.
Why wrong: While possible, the diagnostic code 1 specifically indicates that the control detection time expired, meaning packets were not received. This could be due to a connectivity issue or neighbor not sending.
B
The BFD session is down because the local router is not sending BFD packets.
Why wrong: Tx Count is 50, so packets are being sent.
C
The BFD session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration.
Local Diag 1 means control detection time expired, and Rx Count is 0, indicating no packets received from the neighbor. This points to a problem with the neighbor or the path.
D
The BFD session is down because the multiplier is set too low.
Why wrong: The multiplier is 3, which is standard. The diagnostic code indicates a timeout, not a configuration issue.

Question 12mediummultiple choice

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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 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: 150, Tx Count: 150 Echo function: Active

Based on this output, which statement is correct?

Trap 1: The BFD session is using echo mode, but the holddown timer is…

Echo mode does not change the holddown timer calculation; it provides faster detection by using a separate echo packet.

Trap 2: The BFD session is up, but the echo function is not working because…

Equal Rx and Tx counts are normal for a stable session.

Trap 3: The BFD session is up and using echo function, but the neighbor…

Echo function is a local feature; the neighbor does not need to enable it. The neighbor simply loops back the echo packets.

A
The BFD session is using echo mode, which reduces the load on the control plane because echo packets are processed in hardware.
Echo mode offloads detection to the data plane, reducing control plane processing.
B
The BFD session is using echo mode, but the holddown timer is incorrect because echo mode should halve the timer.
Why wrong: Echo mode does not change the holddown timer calculation; it provides faster detection by using a separate echo packet.
C
The BFD session is up, but the echo function is not working because Rx and Tx counts are equal.
Why wrong: Equal Rx and Tx counts are normal for a stable session.
D
The BFD session is up and using echo function, but the neighbor must also have echo mode enabled for it to work.
Why wrong: Echo function is a local feature; the neighbor does not need to enable it. The neighbor simply loops back the echo packets.

Question 13hardmultiple choice

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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: 500000, Received Multiplier: 3 Holddown (hits): 1500(0) Rx Count: 200, Tx Count: 200

Based on this output, what is the holddown timer value in milliseconds and why?

Trap 1: The holddown timer is 1500 ms, which is the received MinRxInt (500…

The holddown timer uses the maximum of local and received MinRxInt, not just the received.

Trap 2: The holddown timer is 1500 ms, which is the local MinRxInt (1000…

The calculation uses the maximum of MinRxInt values, not just local.

Trap 3: The holddown timer is 3000 ms, and the output is correct as is.

The output shows 1500 ms, not 3000 ms.

A
The holddown timer is 1500 ms, which is the received MinRxInt (500 ms) multiplied by the multiplier (3).
Why wrong: The holddown timer uses the maximum of local and received MinRxInt, not just the received.
B
The holddown timer is 3000 ms, but the output shows 1500 ms because the holddown timer displayed is half of the actual holddown time.
Cisco IOS divides the holddown timer by 2 for display; the actual holddown is 3000 ms.
C
The holddown timer is 1500 ms, which is the local MinRxInt (1000 ms) multiplied by the multiplier (3) divided by 2.
Why wrong: The calculation uses the maximum of MinRxInt values, not just local.
D
The holddown timer is 3000 ms, and the output is correct as is.
Why wrong: The output shows 1500 ms, not 3000 ms.

Question 14mediummultiple choice

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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

Based on this output, what is the BFD session's negotiated transmit interval?

Trap 1: The negotiated transmit interval is 500 ms.

This would be if the local MinTxInt was 500000, but it is 1000000.

Trap 2: The negotiated transmit interval is 3000 ms.

This is the holddown timer, not the transmit interval.

Trap 3: The negotiated transmit interval is 100 ms.

This would be 100000 microseconds, but the values are 1000000 microseconds.

A
The negotiated transmit interval is 1000 ms.
It is the maximum of local MinTxInt and received MinRxInt, both 1000000 microseconds = 1000 ms.
B
The negotiated transmit interval is 500 ms.
Why wrong: This would be if the local MinTxInt was 500000, but it is 1000000.
C
The negotiated transmit interval is 3000 ms.
Why wrong: This is the holddown timer, not the transmit interval.
D
The negotiated transmit interval is 100 ms.
Why wrong: This would be 100000 microseconds, but the values are 1000000 microseconds.

Question 15mediummultiple choice

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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

Based on this output, what is the BFD session's negotiated receive interval?

Trap 1: The negotiated receive interval is 500 ms.

This would be if the local MinRxInt was 500000, but it is 1000000.

Trap 2: The negotiated receive interval is 3000 ms.

This is the holddown timer, not the receive interval.

Trap 3: The negotiated receive interval is 100 ms.

This would be 100000 microseconds, but the value is 1000000.

A
The negotiated receive interval is 1000 ms.
The local MinRxInt is 1000000 microseconds = 1000 ms, meaning the router can receive packets at that interval.
B
The negotiated receive interval is 500 ms.
Why wrong: This would be if the local MinRxInt was 500000, but it is 1000000.
C
The negotiated receive interval is 3000 ms.
Why wrong: This is the holddown timer, not the receive interval.
D
The negotiated receive interval is 100 ms.
Why wrong: This would be 100000 microseconds, but the value is 1000000.

Question 16mediummultiple choice

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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

Based on this output, what is the BFD session's detection time?

Trap 1: The detection time is 1000 ms.

This is the transmit interval, not the detection time.

Trap 2: The detection time is 1500 ms.

This would be if the multiplier was 1.5, but it is 3.

Trap 3: The detection time is 9000 ms.

This would be if the transmit interval was 3000 ms, but it is 1000 ms.

A
The detection time is 3000 ms.
It is the holddown timer shown in the output, which is 3000 ms.
B
The detection time is 1000 ms.
Why wrong: This is the transmit interval, not the detection time.
C
The detection time is 1500 ms.
Why wrong: This would be if the multiplier was 1.5, but it is 3.
D
The detection time is 9000 ms.
Why wrong: This would be if the transmit interval was 3000 ms, but it is 1000 ms.

Read the full NAT/PAT explanation →

Question 17easymultiple choice

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

Based on this output, what is the BFD session's local discriminator?

Trap 1: The local discriminator is 3.

3 is the remote discriminator (RD).

Trap 2: The local discriminator is 10.1.1.1.

This is the local IP address, not the discriminator.

Trap 3: The local discriminator is 10.1.1.2.

This is the neighbor IP address, not the discriminator.

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A
The local discriminator is 1.
The LD (Local Discriminator) is 1 as shown in the output.
B
The local discriminator is 3.
Why wrong: 3 is the remote discriminator (RD).
C
The local discriminator is 10.1.1.1.
Why wrong: This is the local IP address, not the discriminator.
D
The local discriminator is 10.1.1.2.
Why wrong: This is the neighbor IP address, not the discriminator.

Read the full NAT/PAT explanation →

Question 18easymultiple choice

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

Based on this output, what is the BFD session's remote discriminator?

Trap 1: The remote discriminator is 1.

1 is the local discriminator (LD).

Trap 2: The remote discriminator is 10.1.1.2.

This is the neighbor IP address, not the discriminator.

Trap 3: The remote discriminator is 10.1.1.1.

This is the local IP address, not the discriminator.

Study all Bidirectional Forwarding Detection (BFD) common traps →

A
The remote discriminator is 3.
The RD (Remote Discriminator) is 3 as shown in the output.
B
The remote discriminator is 1.
Why wrong: 1 is the local discriminator (LD).
C
The remote discriminator is 10.1.1.2.
Why wrong: This is the neighbor IP address, not the discriminator.
D
The remote discriminator is 10.1.1.1.
Why wrong: This is the local IP address, not the discriminator.

Question 19mediummultiple choice

Study all Bidirectional Forwarding Detection (BFD) common traps →

Consider the following partial configuration on a router:

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.252

bfd interval 100 min_rx 100 multiplier 3 !

router ospf 1
 network 10.1.1.0 0.0.0.3 area 0

What is the effect of this configuration?

Trap 1: BFD is enabled for OSPF on this interface and will detect failures…

Incorrect because OSPF must be configured to use BFD with 'bfd all-interfaces' or 'bfd interface' under the OSPF process.

Trap 2: BFD will only be used if the neighbor also has BFD configured with…

While BFD requires both sides to have BFD configured, the primary issue here is that OSPF is not enabled to use BFD.

Trap 3: The BFD configuration is invalid because the interval and min_rx…

BFD timers are negotiated; they do not need to be identical. The router proposes its values and the neighbor may accept or adjust.

A
BFD is enabled for OSPF on this interface and will detect failures faster than OSPF's hello/dead timers.
Why wrong: Incorrect because OSPF must be configured to use BFD with 'bfd all-interfaces' or 'bfd interface' under the OSPF process.
B
BFD is configured on the interface but will not be used by OSPF unless the 'bfd all-interfaces' command is added under router ospf.
Correct. BFD interface configuration alone does not enable BFD for OSPF; the routing protocol must be told to use it.
C
BFD will only be used if the neighbor also has BFD configured with the same timer values.
Why wrong: While BFD requires both sides to have BFD configured, the primary issue here is that OSPF is not enabled to use BFD.
D
The BFD configuration is invalid because the interval and min_rx values must be identical on both sides.
Why wrong: BFD timers are negotiated; they do not need to be identical. The router proposes its values and the neighbor may accept or adjust.

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Question 20mediummultiple choice

Examine the following configuration:

interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0

bfd interval 50 min_rx 50 multiplier 5 !

router eigrp TEST
 network 192.168.1.0

bfd interface GigabitEthernet0/0 !

Which statement is true about BFD operation on this interface?

Trap 1: BFD will detect a failure in 50 ms because the interval is set to…

The detection time is multiplier * interval, not just the interval itself.

Trap 2: BFD is enabled but only for EIGRP; it will not affect any other…

While this is true, it does not address the detection time calculation.

Trap 3: The configuration is missing 'bfd all-interfaces' under EIGRP, so…

The 'bfd interface' command is sufficient to enable BFD on a specific interface; 'bfd all-interfaces' is an alternative.

Study all Bidirectional Forwarding Detection (BFD) common traps →

A
BFD will detect a failure in 250 ms (5 x 50 ms).
Correct. The detection time is multiplier * the negotiated interval, which in this case is 5 * 50 ms = 250 ms.
B
BFD will detect a failure in 50 ms because the interval is set to 50 ms.
Why wrong: The detection time is multiplier * interval, not just the interval itself.
C
BFD is enabled but only for EIGRP; it will not affect any other routing protocol.
Why wrong: While this is true, it does not address the detection time calculation.
D
The configuration is missing 'bfd all-interfaces' under EIGRP, so BFD will not work.
Why wrong: The 'bfd interface' command is sufficient to enable BFD on a specific interface; 'bfd all-interfaces' is an alternative.