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CCNA Bidirectional Forwarding Detection (BFD) Questions

75 of 76 questions · Page 1/2 · Bidirectional Forwarding Detection (BFD) · Answers revealed

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1
MCQmedium

A network engineer runs the following command to verify BFD operation: R1# show bfd neighbors detail IPv4 Sessions NeighAddr LD/RD RH/RS State Int 10.1.1.2 1/2 Up Up Gi0/0 Session state is UP and not using echo function. Session type: single-hop Local Diag: 0, Demand mode: 0, Poll bit: 0 MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3 Received MinRxInt: 1000000, Received Multiplier: 3 Holddown (hits): 0 (0), Hello (hits): 1000/5 Rx Count: 1000, Rx Interval (ms) min/max/avg: 900/1100/1000 Tx Count: 1000, Tx Interval (ms) min/max/avg: 900/1100/1000 What does this output indicate?

A.BFD session is using echo mode for fast failure detection.
B.BFD session is down and not sending hellos.
C.BFD session is operating with async mode, intervals at 1 second.
D.BFD session has a holddown timer of 3 seconds.
AnswerC

MinTxInt and MinRxInt are 1,000,000 microseconds (1 second), no echo, and session is UP.

Why this answer

The output shows a BFD session in UP state with consistent transmit and receive intervals around 1000 ms, no echo function, and a multiplier of 3. The session is healthy.

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2
MCQmedium

What is the default BFD hello timer interval on Cisco IOS-XE when BFD is configured on an interface without any explicit timer settings?

A.100 milliseconds
B.500 milliseconds
C.1000 milliseconds
D.2000 milliseconds
AnswerC

The default BFD hello interval is 1000 ms (1 second) on Cisco IOS-XE.

Why this answer

The default BFD hello interval on Cisco IOS-XE is 1000 milliseconds (1 second) unless overridden by the bfd interval command.

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3
MCQmedium

A router has the following BFD configuration for a static route: ip route 10.0.0.0 255.255.255.0 192.168.1.2 bfd map 192.168.1.2 10.0.0.0 255.255.255.0 interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 bfd interval 100 min_rx 100 multiplier 3 ! What is the purpose of the 'bfd map' command in this context?

A.It maps the BFD session to the interface, enabling BFD for all static routes using that interface.
B.It creates a BFD session to the next-hop 192.168.1.2 and associates it with the static route to 10.0.0.0/24.
C.It maps the BFD session to the OSPF process, which is incorrect for static routes.
D.It is used to configure BFD for multiple static routes simultaneously.
AnswerB

Correct. The 'bfd map' command creates a BFD session to the specified next-hop and ties it to the static route so that the route is withdrawn if BFD goes down.

Why this answer

For static routes, BFD must be explicitly mapped to the next-hop and destination prefix. The 'bfd map' command associates a BFD session with a static route so that if BFD detects a failure, the static route is removed from the routing table.

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4
Multi-Selecthard

Which THREE commands can be used to verify the operational state of a BFD session? (Choose THREE.)

Select 3 answers
A.show bfd neighbors
B.show bfd interface details
C.show bfd session
D.show ip ospf bfd
E.show bfd peer
AnswersA, B, C

Correct. This command lists all BFD neighbors and their current session state (Up, Down, etc.).

Why this answer

The show bfd neighbors command displays all BFD neighbors and their session states. show bfd interface details shows BFD configuration and state per interface. show bfd session displays detailed session information including timers and statistics. show ip ospf bfd is not a valid command; the correct command is show ip ospf [interface] to see BFD status. show bfd peer is a valid command on some platforms but is less common; show bfd neighbors is preferred.

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5
MCQhard

An engineer configures BFD for BGP between two directly connected eBGP peers. The BFD session is up, but BGP remains in the Idle state. The engineer verifies that the BGP configuration is correct and that TCP port 179 is reachable. What is the most likely cause?

A.The eBGP neighbor is not on the same subnet, and the 'ebgp-multihop' command is missing, preventing TCP connection.
B.The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state.
C.The BGP neighbor is configured with a password, but BFD does not support authentication, causing a mismatch.
D.The 'bgp bestpath med missing-as-worst' command is configured, causing BGP to reject the neighbor.
AnswerA

Correct. For eBGP, if the neighbor is not directly connected (same subnet), the 'ebgp-multihop' command is required. BFD does not affect this requirement.

Why this answer

BGP requires that the 'update-source' interface matches the source IP used for BFD. If BFD is configured on a different interface than the one used for the BGP TCP connection, BFD will not detect failures of the actual BGP path. However, in this case, BGP is stuck in Idle, which typically indicates a TCP connection issue.

A common edge case is that the 'ebgp-multihop' command is required even for directly connected eBGP peers if the neighbor is not on the same subnet. BFD does not compensate for this.

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6
MCQmedium

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?

A.BFD is enabled for OSPF on this interface and will detect failures faster than OSPF's hello/dead timers.
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.
C.BFD will only be used if the neighbor also has BFD configured with the same timer values.
D.The BFD configuration is invalid because the interval and min_rx values must be identical on both sides.
AnswerB

Correct. BFD interface configuration alone does not enable BFD for OSPF; the routing protocol must be told to use it.

Why this answer

The BFD configuration is applied under the interface, but OSPF must be explicitly configured to use BFD via the 'bfd all-interfaces' command under the OSPF routing process. Without this, BFD will not be used for OSPF neighbor failure detection.

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7
MCQhard

An engineer configures BFD for BGP on a route reflector client. The BFD session between the client and the route reflector is up, but the client does not receive routes from the route reflector. The engineer verifies that the BGP configuration is correct and that the client is sending routes. What is the most likely cause?

A.The route reflector is not configured with 'next-hop-self', so the client cannot reach the next hop of the routes.
B.The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state.
C.The 'bgp client-to-client reflection' command is disabled on the route reflector.
D.The route reflector has a BGP policy that filters routes based on community, and the client's routes do not have the required community.
AnswerA

Correct. Without 'next-hop-self', the route reflector advertises routes with the original next hop, which may not be reachable by the client.

Why this answer

In a route reflector topology, the route reflector does not advertise routes learned from one client to another client unless the 'next-hop-self' command is configured. BFD does not affect this behavior. If the route reflector is not using 'next-hop-self', the client may not have a route to the next hop, causing the route to be hidden.

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8
MCQhard

An engineer configures BFD for EIGRP in named mode. The BFD session is up, but the EIGRP neighbor relationship is not forming. The engineer checks that the K values match and that the interfaces are up. What is the most likely explanation?

A.The 'no neighbor' command is configured under the af-interface, disabling EIGRP on that interface.
B.The 'bfd' command is configured under the af-interface, but the 'bfd all-interfaces' command is missing under the EIGRP process.
C.The EIGRP router ID is the same on both routers, causing a conflict.
D.The interface is configured with 'ip bandwidth-percent eigrp' set to 0, preventing EIGRP from using the interface.
AnswerA

Correct. In named mode, the 'no neighbor' command under the af-interface prevents EIGRP from forming adjacencies on that interface, even if BFD is up.

Why this answer

In EIGRP named mode, the 'af-interface' configuration can have different settings than classic mode. Specifically, the 'no neighbor' command under the af-interface can disable EIGRP on that interface, preventing adjacency formation. BFD is configured under the af-interface as well, but if the interface is disabled for EIGRP, BFD will not help.

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9
MCQhard

An engineer configures BFD on an OSPF link that is part of a virtual link. The BFD session between the two endpoints of the virtual link is up, but OSPF adjacency over the virtual link is not forming. What is the most likely explanation?

A.BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
B.The virtual link is configured with a different hello interval than the BFD interval, causing a mismatch.
C.The 'bfd all-interfaces' command is not configured under the OSPF process, so OSPF ignores BFD for virtual links.
D.The virtual link is using a different authentication type than the BFD session.
AnswerA

Correct. OSPF virtual links are not supported with BFD. The BFD session on the underlying physical interface does not affect the virtual link adjacency.

Why this answer

BFD does not support virtual links in OSPF. BFD can only be configured on physical interfaces or subinterfaces, not on virtual links. Therefore, even if BFD is up on the underlying physical link, it will not help detect failures of the virtual link.

The virtual link adjacency may fail due to other issues, such as mismatched area IDs or authentication.

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10
MCQmedium

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?

A.BFD will detect a failure in 250 ms (5 x 50 ms).
B.BFD will detect a failure in 50 ms because the interval is set to 50 ms.
C.BFD is enabled but only for EIGRP; it will not affect any other routing protocol.
D.The configuration is missing 'bfd all-interfaces' under EIGRP, so BFD will not work.
AnswerA

Correct. The detection time is multiplier * the negotiated interval, which in this case is 5 * 50 ms = 250 ms.

Why this answer

The 'bfd interface' command under EIGRP enables BFD for that specific interface. The BFD session will negotiate timers; the multiplier of 5 means the session will declare a neighbor down after 5 * max(min_rx, negotiated interval) = 5 * 50 = 250 ms of missed packets.

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11
Multi-Selecthard

Which TWO statements about BFD authentication are true? (Choose TWO.)

Select 2 answers
A.BFD authentication supports both MD5 and SHA-1 hashing algorithms.
B.BFD authentication can be configured in either strict or loose mode.
C.BFD authentication is only supported for single-hop BFD sessions.
D.The BFD authentication key is configured directly under the BFD interface configuration.
E.BFD authentication uses a pre-shared key that must be identical on both sides.
AnswersA, B

Correct. BFD authentication supports MD5 and SHA-1, and on newer platforms SHA-256 as well.

Why this answer

BFD authentication can use MD5, SHA-1, or SHA-256 (depending on platform). The authentication mode can be strict (mandatory) or loose (optional). Key chains are used to manage authentication keys, and the key chain must be configured before applying it to BFD.

BFD authentication is supported for both single-hop and multihop sessions.

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12
MCQeasy

Which BFD session state indicates that the session is fully established and operational?

A.Down
B.Init
C.Up
D.AdminDown
AnswerC

Up is the state where the BFD session is fully established and both routers agree on parameters.

Why this answer

The Up state indicates that the BFD session is fully established, and both routers are exchanging control packets successfully.

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13
Multi-Selecthard

Which TWO statements about BFD echo mode are true? (Choose TWO.)

Select 2 answers
A.Echo packets are generated by the sending router and forwarded back by the remote router.
B.Echo mode reduces the processing load on the remote router's control plane.
C.Echo mode requires a separate BFD session for each direction.
D.Echo mode increases the number of BFD control packets sent between routers.
E.Echo mode is enabled by default on all interfaces.
AnswersA, B

The sending router creates echo packets that the remote router loops back.

Why this answer

In BFD echo mode, the sending router generates echo packets that are looped back by the remote router, allowing detection of failures without involving the remote router's control plane. This reduces CPU load on the remote router. The other statements are false because echo mode does not require a separate session for each direction, and it does not increase the number of BFD control packets.

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14
Drag & Dropmedium

Drag and drop the steps to establish a BFD session with OSPF into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

The correct order starts with enabling BFD globally, then configuring BFD on the interface, enabling OSPF BFD support, verifying the BFD session, and finally verifying OSPF neighbor state reflects BFD integration.

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15
MCQmedium

A network engineer runs the following command to verify BFD with EIGRP: R1# show ip eigrp 100 topology 10.2.2.0/24 EIGRP-IPv4 Topology Entry for AS(100)/ID(10.2.2.0/24) State: Passive, Query origin flag: 1, 1 Successor(s), FD is 131072 Descriptor Blocks: 10.1.1.2 (GigabitEthernet0/0), from 10.1.1.2, Send flag: 0x0 Composite metric: (131072/130816), Route is Internal Vector metric: Minimum bandwidth is 100000 Kbit Total delay is 100 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 Originating router is 2.2.2.2 BFD enabled, BFD state: UP What does this output indicate?

A.EIGRP is using BFD with state UP, providing sub-second convergence.
B.EIGRP has no BFD session for this neighbor.
C.BFD is down, so EIGRP relies on its own hello/dead timers.
D.EIGRP is using BFD in passive mode only.
AnswerA

BFD is enabled and UP, allowing EIGRP to detect failures faster than EIGRP hello/dead timers.

Why this answer

The output shows that BFD is enabled for the EIGRP neighbor and the BFD state is UP, indicating fast failure detection is active for this route.

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16
MCQhard

R1 and R2 are OSPF neighbors over a VLAN interface with BFD enabled. R1#show ip ospf interface vlan10 shows 'BFD is enabled' but R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under vlan10. R2 has same. The VLAN is up. What is the root cause?

A.BFD must be enabled on the physical interface as well as the VLAN interface.
B.The VLAN interface must have 'ip ospf bfd' disabled.
C.OSPF must be configured with 'bfd all-interfaces' globally.
D.The BFD timers must match exactly on both sides.
AnswerA

BFD sessions are established on the physical interface; without BFD on the physical, the VLAN interface BFD configuration is ineffective.

Why this answer

BFD on VLAN interfaces requires that the underlying physical interface also support BFD. If the physical interface (e.g., GigabitEthernet0/1) does not have BFD enabled, the VLAN interface cannot establish BFD sessions. BFD is not automatically inherited from VLAN to physical.

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17
MCQmedium

According to RFC 5880, what is the maximum number of BFD sessions that can be supported by a single interface on a Cisco router?

A.1 session per interface
B.256 sessions per interface
C.Unlimited, but limited by platform resources
D.16 sessions per interface
AnswerC

Correct. RFC 5880 does not define a maximum; it is implementation-specific.

Why this answer

RFC 5880 does not specify a maximum number of BFD sessions per interface; it is platform-dependent. Cisco routers can support multiple BFD sessions per interface, limited only by hardware resources.

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18
MCQhard

In an MPLS network, routers R1 and R2 are LDP neighbors with BFD enabled. R1#show mpls ldp neighbor shows R2 as 'Oper Down' for the BFD session. R2#show bfd neighbors shows the session with R1 as 'Down'. R1 and R2 have matching 'mpls ldp router-id' loopback0. The link between them is up. What is the root cause?

A.The LDP router-id must be the IP address of the interface where BFD is enabled.
B.BFD is not supported with LDP on this platform.
C.The BFD interval is too low for LDP.
D.LDP must be configured with 'no bfd' to work properly.
AnswerA

BFD for LDP requires the LDP session to use the same source IP as the BFD session; otherwise, BFD cannot monitor the LDP session.

Why this answer

BFD for LDP requires that the LDP session be established over the same interface as the BFD session. If the LDP router-id is not the interface IP used for BFD, the BFD session may fail. Here, the loopback0 is used as LDP router-id, but BFD is configured on the physical link, causing mismatch.

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19
Drag & Dropmedium

Drag and drop the steps to verify and validate BFD operational state into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

Begin with a high-level summary of all BFD sessions, then examine detailed session information, validate timers and counters, confirm BFD is active for the routing protocol, and finally test failover behavior by disabling the peer interface.

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20
MCQmedium

Which authentication type is the default when BFD authentication is enabled on Cisco IOS-XE?

A.Simple password (auth type 1)
B.Keyed MD5 (auth type 2)
C.Meticulous Keyed MD5 (auth type 3)
D.Keyed SHA-1 (auth type 4)
AnswerD

Cisco IOS-XE defaults to keyed SHA-1 (auth type 4) when BFD authentication is enabled.

Why this answer

Cisco IOS-XE defaults to keyed SHA-1 (auth type 4) when BFD authentication is configured, as it provides stronger security than MD5.

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21
MCQmedium

A network engineer runs the following command to troubleshoot a BFD issue: R1# debug bfd event *Mar 1 00:12:34.567: BFD: [R1-to-R2] state DOWN -> UP (async) *Mar 1 00:12:34.568: BFD: [R1-to-R2] echo mode enabled, min-echo-rx-interval 50 ms *Mar 1 00:12:34.569: BFD: [R1-to-R2] starting echo timer, interval 50 ms *Mar 1 00:12:34.570: BFD: [R1-to-R2] sending async packet, state UP, interval 300 ms What does this output indicate?

A.BFD session is flapping between DOWN and UP states.
B.BFD session is established and echo mode is active.
C.BFD session is down due to echo failure.
D.BFD async interval is misconfigured at 300 ms.
AnswerB

The output confirms BFD session is UP, echo mode enabled, and timers are running.

Why this answer

The debug output shows BFD session transitioning from DOWN to UP, with echo mode enabled and the async interval set to 300 ms. This indicates a successful BFD session establishment.

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22
MCQhard

R1 and R2 are OSPF neighbors over a tunnel interface with BFD enabled. R1#show ip ospf interface tunnel0 shows 'BFD is enabled' but R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Down'. The tunnel is up and OSPF adjacency is full. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under tunnel0. R2 has same. What is the root cause?

A.BFD is not supported over tunnel interfaces; it requires physical interfaces.
B.The tunnel mode must be changed to 'ipsec' for BFD to work.
C.OSPF must be configured with 'bfd all-interfaces' to work over tunnels.
D.The BFD timers must be increased for tunnel interfaces.
AnswerA

BFD cannot be used over tunnel interfaces because BFD requires direct layer 2 connectivity; tunnels are layer 3 virtual interfaces.

Why this answer

BFD over tunnel interfaces requires that the underlying transport (e.g., GRE) supports BFD. BFD cannot be established over a tunnel if the tunnel endpoint IP addresses are not directly connected; BFD requires physical connectivity. Since OSPF is full, the tunnel is working, but BFD fails because the tunnel is considered a virtual interface and BFD expects a direct physical link.

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23
MCQmedium

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 negotiated receive interval?

A.The negotiated receive interval is 1000 ms.
B.The negotiated receive interval is 500 ms.
C.The negotiated receive interval is 3000 ms.
D.The negotiated receive interval is 100 ms.
AnswerA

The local MinRxInt is 1000000 microseconds = 1000 ms, meaning the router can receive packets at that interval.

Why this answer

The negotiated receive interval is the maximum of the local MinRxInt and the received MinTxInt. However, the output does not show the received MinTxInt directly. The received MinRxInt is shown, but the receive interval is determined by the local MinRxInt.

The local MinRxInt is 1000000 microseconds, so the router can receive BFD packets at intervals of 1000 ms or slower. The negotiated receive interval is typically the local MinRxInt, which is 1000 ms.

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24
MCQmedium

A network engineer has configured BFD for OSPF as shown: interface GigabitEthernet0/0 bfd interval 300 min_rx 300 multiplier 3 ! router ospf 1 bfd all-interfaces ! However, BFD sessions are not coming up. What is the most likely missing configuration?

A.The 'bfd all-interfaces' command should be replaced with 'bfd interface GigabitEthernet0/0'.
B.The interface has no IP address configured.
C.The BFD timers must be set to 100 ms or less for OSPF.
D.The router must be configured with 'bfd map' commands for each neighbor.
AnswerB

Correct. Without an IP address on the interface, OSPF cannot form a neighbor relationship, and BFD requires an established OSPF adjacency to operate.

Why this answer

BFD requires that the interface has an IP address and is up/up. The configuration shown lacks an IP address on the interface, which is necessary for BFD to establish a session.

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25
MCQeasy

What is the default BFD interval and multiplier on Cisco IOS-XE devices when BFD is enabled under an interface without explicit timer configuration?

A.interval 100 ms, min_rx 100 ms, multiplier 3
B.interval 150 ms, min_rx 150 ms, multiplier 3
C.interval 200 ms, min_rx 200 ms, multiplier 5
D.interval 50 ms, min_rx 50 ms, multiplier 3
AnswerB

Correct. The default BFD timers on Cisco IOS-XE are 150 ms interval, 150 ms min_rx, and multiplier 3.

Why this answer

If no BFD timers are configured under the interface, the default values are: interval = 150 ms, min_rx = 150 ms, multiplier = 3. These are the defaults used when 'bfd interval' is not specified.

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26
MCQhard

Router R1 and R2 are OSPF neighbors over a FastEthernet link with BFD enabled. R1 has 'ip ospf network point-to-point' configured. R2 does not. After a reload, BFD sessions fail to establish. R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. What is the root cause?

A.BFD is not supported on FastEthernet interfaces.
B.OSPF network type mismatch between R1 and R2 prevents BFD session establishment.
C.BFD timers are not configured globally.
D.OSPF hello/dead intervals must match for BFD to work.
AnswerB

Mismatched OSPF network types cause BFD to fail because BFD uses interface parameters derived from OSPF network type.

Why this answer

BFD requires matching OSPF network types on the same link for proper session establishment. When R1 is configured as point-to-point and R2 remains as broadcast, OSPF forms a neighbor relationship but BFD sessions fail because BFD expects consistent interface parameters.

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27
MCQhard

R1 and R2 are EIGRP neighbors over a serial link with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Pending' for BFD. R2#show bfd neighbors shows the session as 'Up'. R1 has 'bfd interval 50 min_rx 50 multiplier 3'. R2 has 'bfd interval 200 min_rx 200 multiplier 3'. What is the root cause?

A.The BFD multiplier on R1 is too low.
B.R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.
C.EIGRP must be configured with 'bfd all-interfaces' to work.
D.The serial link requires 'no ip split-horizon' for BFD.
AnswerB

R1's min_rx of 50 ms is less than R2's min_tx of 200 ms, so R2 cannot meet the required receive rate, causing BFD session to fail.

Why this answer

EIGRP BFD requires that the BFD session be established before EIGRP can form adjacency. If the BFD session is up on one side but not the other, it indicates a timer mismatch. However, BFD timers are negotiated; the issue is that R1's min_rx of 50 ms is lower than R2's min_rx of 200 ms, causing R2 to reject the session because it cannot transmit at that rate.

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28
Multi-Selectmedium

Which TWO commands can be used to verify the operational state of BFD sessions on a Cisco IOS-XE router? (Choose TWO.)

Select 2 answers
A.show bfd neighbors
B.show bfd session
C.show ip route
D.show bfd statistics
E.show running-config | include bfd
AnswersA, B

This command shows BFD neighbor details, including session state.

Why this answer

The 'show bfd neighbors' command displays BFD session details such as neighbor address, interface, state, and timers. The 'show bfd session' command provides similar information, including the local discriminator and session state. The other commands are either unrelated or do not show BFD session state.

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29
MCQmedium

Examine the partial BFD configuration on a router: interface GigabitEthernet0/0 bfd interval 100 min_rx 100 multiplier 3 ! interface GigabitEthernet0/1 bfd interval 200 min_rx 200 multiplier 3 ! router ospf 1 bfd all-interfaces ! The router has OSPF neighbors on both interfaces. Which statement is true?

A.Both BFD sessions will use the same timer values because OSPF is configured with 'bfd all-interfaces'.
B.The BFD session on GigabitEthernet0/0 will detect failures in 300 ms, and on GigabitEthernet0/1 in 600 ms.
C.The BFD session on GigabitEthernet0/1 will not form because the interval is too high.
D.The router will use the minimum interval across all interfaces for consistency.
AnswerB

Correct. Detection time = multiplier * interval. For Gi0/0: 3*100=300 ms; for Gi0/1: 3*200=600 ms.

Why this answer

BFD timers are configured per interface. Each BFD session independently uses the timers configured on its respective interface. The multiplier is applied per session.

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30
MCQmedium

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?

A.Verify that the 'bfd' command is configured under the OSPF process.
B.Check if the serial interface is configured with 'encapsulation ppp'.
C.Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.
D.Verify that the serial interface clock rate is set correctly.
AnswerA

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.

Why this answer

BFD on serial interfaces often requires the 'bfd interval' command to be explicitly configured, as the default may be disabled. Also, serial interfaces may need the 'bfd' command under the routing protocol as well.

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31
MCQhard

A network engineer runs the following command to verify BFD session details: R1# show bfd neighbors detail | include (NeighAddr|LD/RD|State|Int|MinTxInt|Multiplier) NeighAddr LD/RD RH/RS State Int 10.4.4.2 100/200 Up Up Gi0/1 MinTxInt: 500000, MinRxInt: 500000, Multiplier: 5 What does this output indicate?

A.BFD session is UP with a detection time of 2.5 seconds (500 ms * 5).
B.BFD session is DOWN due to high multiplier.
C.BFD is using echo mode for faster detection.
D.BFD multiplier is set to 3, not 5.
AnswerA

Detection time is MinTxInt * Multiplier = 500 ms * 5 = 2500 ms (2.5 seconds).

Why this answer

The output shows a BFD session with a multiplier of 5 and intervals of 500 ms. The high multiplier may indicate a need for more tolerance, but the session is UP.

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32
MCQhard

A network engineer configures BFD for EIGRP on a point-to-point link. The BFD session is up, but EIGRP neighbors are stuck in INIT state. The engineer checks that EIGRP hello packets are sent and received. Which of the following is the most likely explanation?

A.The EIGRP K values are mismatched between the two routers, preventing the adjacency from forming.
B.The 'bfd all-interfaces' command is missing under the EIGRP process, so EIGRP ignores BFD state.
C.The interface is configured with 'ip authentication mode eigrp' and 'ip authentication key-chain eigrp' with mismatched keys.
D.The EIGRP autonomous system number is different on each router.
AnswerA

Correct. EIGRP requires matching K values to form an adjacency. BFD does not influence this check.

Why this answer

EIGRP requires that the K values match between neighbors. If the K values are mismatched, EIGRP will not form an adjacency even if BFD is operational. This is a common misconfiguration because BFD does not enforce K value matching; it only reports link failures.

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33
Multi-Selecthard

Which TWO statements about BFD echo mode are true? (Choose TWO.)

Select 2 answers
A.In echo mode, the remote router generates and sends the echo packets back to the local router.
B.Echo mode reduces the processing load on the remote router because it does not need to process BFD control packets.
C.BFD echo mode can be used for both single-hop and multihop BFD sessions.
D.When echo mode is enabled, the BFD control packet interval must be set to a lower value than the echo interval.
E.Echo mode requires the bfd echo command under the BFD neighbor configuration.
AnswersB, C

Correct. The remote router simply loops back the echo packets at Layer 2, avoiding BFD control packet processing.

Why this answer

BFD echo mode reduces control packet processing overhead by having the remote router loop back echo packets. The local router sends echo packets, and the remote router simply reflects them back without processing BFD control packets. This reduces CPU load on the remote router, but the local router still generates and processes the echo packets.

BFD echo mode is supported for both single-hop and multihop sessions, and it can be enabled per interface or per neighbor.

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34
MCQeasy

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 remote discriminator?

A.The remote discriminator is 3.
B.The remote discriminator is 1.
C.The remote discriminator is 10.1.1.2.
D.The remote discriminator is 10.1.1.1.
AnswerA

The RD (Remote Discriminator) is 3 as shown in the output.

Why this answer

The remote discriminator is the RD value shown in the output. In this case, it is 3.

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35
MCQhard

R1 and R2 are OSPF neighbors with BFD enabled. R1#show ip ospf neighbor shows R2 as 'FULL/DR'. R1#show bfd neighbors shows the session as 'Up' with R2. R2#show bfd neighbors shows the session as 'Up' with R1. However, R1#show ip route shows that the route to 10.1.1.0/24 via R2 is missing. R1 has 'summary-address 10.1.1.0 255.255.255.0' configured under OSPF. What is the root cause?

A.The OSPF summary-address on R1 is suppressing the more specific route from R2.
B.BFD is causing the route to be removed due to fast detection.
C.R2 is not advertising the route due to a filtering policy.
D.The OSPF cost on R1 is too high, causing the route to be ignored.
AnswerA

The summary-address command causes OSPF to advertise only the summary and suppress more specific routes, so the specific route from R2 is not installed.

Why this answer

OSPF summary-address on an ABR or ASBR suppresses more specific routes. If R1 is an ABR and has a summary-address for 10.1.1.0/24, it will not install the specific route from R2, even though BFD and OSPF adjacency are fine. The summary route may be advertised instead, but the specific route is missing from the routing table.

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36
MCQhard

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?

A.The IS-IS process is configured with 'bfd all-interfaces' but the interface is not configured with 'isis bfd'.
B.The BFD session is using the wrong source IP address; IS-IS BFD requires the interface IP to be used.
C.The IS-IS metric is set to a high value, causing BFD to be ignored.
D.The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
AnswerB

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.

Why this answer

IS-IS BFD requires that the BFD session be established on the same interface as the IS-IS adjacency. If the IS-IS adjacency is up but BFD is down, the issue is often that the BFD configuration is not applied to the correct interface or that the BFD timers are not compatible.

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37
MCQhard

R1 and R2 are IS-IS neighbors with BFD enabled. R1#show clns is-neighbors shows R2 as 'Up'. R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'bfd interval 50 min_rx 50 multiplier 3' on the interface. R2 has 'bfd interval 100 min_rx 100 multiplier 3'. The link is stable. What is the root cause?

A.R1 has an ACL blocking UDP port 3784 from R2.
B.IS-IS requires 'bfd all-interfaces' to work with BFD.
C.The BFD multiplier on R1 is too low.
D.The IS-IS metric must be set to 1 for BFD.
AnswerA

BFD uses UDP port 3784; if R1 blocks incoming BFD packets, the session appears down on R1 but up on R2 because R2 receives R1's packets.

Why this answer

IS-IS BFD requires that the BFD session be established between the same IP addresses used for IS-IS. If R1's BFD session is down but R2's is up, it indicates a unidirectional issue. This can be due to an MTU mismatch where R1's BFD packets are fragmented and dropped, or a firewall blocking inbound BFD packets on R1.

Here, the most likely cause is that R1 has an ACL denying UDP port 3784 from R2.

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38
MCQmedium

A network engineer runs the following command to troubleshoot BFD with BGP: R1# show bgp ipv4 unicast 10.3.3.0/24 BGP routing table entry for 10.3.3.0/24, version 2 Paths: (1 available, best #1, table default) Advertised to update-groups: 1 Refresh Epoch 1 Local 10.1.1.2 from 10.1.1.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, external, best rx pathid: 0, tx pathid: 0x0 BFD enabled, BFD state: UP What does this output indicate?

A.BGP is using BFD with state UP, enabling sub-second failure detection for this prefix.
B.BFD is disabled for this BGP neighbor.
C.BFD state is DOWN, so BGP uses its own timers.
D.BGP is using BFD only for IPv6 prefixes.
AnswerA

BFD is enabled and UP, allowing BGP to detect failures faster than BGP keepalive/hold timers.

Why this answer

The output shows that BFD is enabled for the BGP session and the BFD state is UP, indicating fast failure detection for the BGP neighbor.

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39
MCQmedium

Which of the following statements about BFD echo mode is true?

A.Echo mode increases the load on the remote router because it must process echo packets.
B.Echo mode is disabled by default and must be explicitly enabled.
C.Echo mode uses the same timer values as the BFD control packets.
D.Echo mode allows the local router to detect failures without requiring the remote router to send BFD control packets.
AnswerD

Correct. In echo mode, the local router sends echo packets that are looped back; if they stop returning, a failure is detected, reducing the need for control packets from the remote.

Why this answer

BFD echo mode is an optional mode where the local router sends echo packets to the remote router, which loops them back. This reduces the processing load on the remote router because it does not need to generate BFD control packets. Echo mode is enabled by default on Cisco IOS-XE.

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40
MCQmedium

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 negotiated transmit interval?

A.The negotiated transmit interval is 1000 ms.
B.The negotiated transmit interval is 500 ms.
C.The negotiated transmit interval is 3000 ms.
D.The negotiated transmit interval is 100 ms.
AnswerA

It is the maximum of local MinTxInt and received MinRxInt, both 1000000 microseconds = 1000 ms.

Why this answer

The negotiated transmit interval is the maximum of the local MinTxInt and the received MinRxInt. Local MinTxInt is 1000000 microseconds, received MinRxInt is 1000000 microseconds. The maximum is 1000000 microseconds, which is 1000 ms.

The BFD session will transmit control packets every 1000 ms.

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41
Multi-Selecthard

Which THREE statements about BFD multihop sessions are true? (Choose THREE.)

Select 3 answers
A.Multihop BFD is used when the neighbors are not directly connected.
B.Multihop BFD uses UDP destination port 4784.
C.Multihop BFD requires configuration of the source and destination IP addresses.
D.Multihop BFD requires both routers to be in the same subnet.
E.Multihop BFD can only be used with BGP.
AnswersA, B, C

Multihop BFD supports sessions between routers separated by multiple Layer 3 hops.

Why this answer

BFD multihop sessions are used when the two routers are not directly connected, requiring multiple hops. They use a different destination UDP port (4784) compared to single-hop sessions (3784). Additionally, multihop sessions require explicit configuration of the BFD source and destination IP addresses.

The other statements are false because multihop sessions do not require the same subnet, and they can operate over any routed path.

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42
MCQhard

R1 and R2 are BGP peers with BFD enabled. R1#show bgp neighbors 10.1.1.2 shows BGP state 'Active' and BFD session 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'neighbor 10.1.1.2 fall-over bfd' configured. R2 has 'neighbor 10.1.1.1 fall-over bfd' configured. The link between them is stable. What is the root cause?

A.R1 is missing 'bfd interval' configuration on the interface facing R2.
B.BGP requires 'neighbor 10.1.1.2 ebgp-multihop' for BFD to work.
C.R2 has a higher BFD multiplier causing session failure.
D.The BGP update-source is not set to the interface IP.
AnswerA

Without BFD interval configuration on the interface, R1 cannot establish a BFD session, causing BGP fall-over to keep BGP in Active state.

Why this answer

BFD for BGP fall-over requires that the BFD session be established before BGP can form. If the BFD session is up on R2 but down on R1, it indicates a unidirectional issue. This can happen if R1 has an ACL blocking BFD control packets from R2, or if R1's BFD configuration is missing.

Here, R1 likely lacks 'bfd interval' configuration on the interface, causing BFD to not initiate.

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43
MCQmedium

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?

A.The BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.
B.The BFD session is using echo mode, which is not supported for BGP fall-over.
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'.
D.The BGP session is using EBGP multihop, and the TTL is set to 2.
AnswerA

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.

Why this answer

The BGP fall-over bfd command causes BGP to monitor the BFD session. If the BFD session is up but BGP is flapping, the issue is likely a BGP configuration problem, such as a mismatch in hold timer or update-source.

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44
MCQhard

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?

A.The BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.
B.The Layer 3 switch is not configured for BFD, causing it to drop BFD packets.
C.The BGP session is using EBGP multihop, and the TTL is set to 1.
D.The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
AnswerA

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.

Why this answer

BFD sessions require that the destination IP address be reachable via a directly connected interface or a static route. If the BGP neighbor is not directly connected (e.g., via a loopback), BFD may fail if the next hop is not directly connected or if there is a routing issue.

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45
MCQhard

An engineer configures BFD on a DMVPN Phase 2 spoke-to-spoke tunnel. The BFD session between two spokes comes up, but the spoke-to-spoke dynamic IPsec tunnel fails to establish. What is the most likely explanation?

A.The BFD session is using the mGRE tunnel interface, but the IPsec crypto map is applied to the physical interface, causing a mismatch in the path.
B.The spoke routers have different BFD minimum intervals, causing the session to flap and reset the IPsec tunnel.
C.The NHRP authentication string is mismatched between spokes, preventing NHRP from resolving the destination.
D.The IPsec transform-set uses ESP with SHA-1, but BFD requires MD5 authentication.
AnswerA

Correct. In DMVPN, BFD should be configured on the tunnel interface, but IPsec is applied to the physical interface. If BFD is misconfigured to use the physical interface, it will not detect failures of the tunnel path, and the IPsec tunnel may not be triggered correctly.

Why this answer

In DMVPN Phase 2, spoke-to-spoke tunnels are triggered by NHRP redirects. BFD can cause the NHRP redirect to be lost or delayed if the BFD interval is too aggressive, causing the spoke to believe the path is down before the IPsec tunnel is established. Additionally, BFD over the mGRE tunnel requires that the tunnel destination be reachable via the underlying IP network; if the spoke's public IP is not reachable due to NAT, BFD may come up but IPsec may fail due to NAT traversal issues.

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46
MCQmedium

Consider the following BGP configuration with BFD: router bgp 65000 neighbor 10.1.1.2 remote-as 65001 neighbor 10.1.1.2 fall-over bfd ! interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.252 bfd interval 200 min_rx 200 multiplier 4 ! What is the effect of the 'neighbor fall-over bfd' command?

A.BGP will use BFD to detect link failures, but the BGP hold timer still applies.
B.BGP will ignore the BFD session and continue using its own keepalive/hold mechanism.
C.BGP will use BFD for fast failure detection; if BFD goes down, BGP will reset the session immediately.
D.The 'fall-over bfd' command is only needed if BFD timers are less than 100 ms.
AnswerC

Correct. BGP registers with BFD and upon BFD session failure, BGP tears down the neighbor without waiting for the hold timer.

Why this answer

The 'neighbor fall-over bfd' command enables BGP to use BFD for fast failure detection. When BFD detects a failure, BGP will immediately tear down the peering session without waiting for the hold timer.

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47
MCQhard

An engineer configures BFD for EIGRP and also applies an offset-list to increase the metric of a route. The BFD session is up, but the route with the offset-list is not being installed in the routing table. The engineer verifies that the offset-list is correctly configured. What is the most likely explanation?

A.The offset-list increased the feasible distance beyond the advertised distance, making the route no longer feasible.
B.The offset-list is applied to the wrong interface, so it does not affect the route.
C.The BFD session is flapping, causing EIGRP to remove the route.
D.The offset-list is using a metric that exceeds the maximum hop count for EIGRP.
AnswerA

Correct. Offset-lists increase the FD, and if the FD exceeds the AD, the route becomes infeasible and is removed from the topology table.

Why this answer

EIGRP offset-lists can increase the metric of a route, but they affect the feasible distance (FD), not the advertised distance (AD). If the offset-list increases the FD such that the route is no longer feasible (i.e., the FD exceeds the AD of the successor), the route may be removed from the topology table. BFD does not interact with offset-lists.

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48
MCQhard

R1 and R2 are EIGRP neighbors with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Init' for BFD. R1#show bfd neighbors shows the session as 'Up'. R2#show bfd neighbors shows the session as 'Up'. R1 has 'eigrp stub' configured. R2 does not. What is the root cause?

A.The EIGRP stub configuration on R1 prevents full adjacency, but BFD is unaffected.
B.BFD requires 'eigrp stub' to be removed.
C.The BFD interval on R1 is too low for EIGRP.
D.EIGRP must be configured with 'no eigrp stub' for BFD to work.
AnswerA

EIGRP stub does not affect BFD; the BFD session is up, but EIGRP adjacency is in Init due to stub behavior.

Why this answer

EIGRP stub routers can form BFD sessions but the EIGRP adjacency may remain in Init state if the stub router is not configured correctly. However, BFD session is up, so the issue is with EIGRP itself. The stub router may be blocking queries, but that does not affect BFD.

The real issue is that R1 has 'eigrp stub' but is missing the 'receive-only' keyword, causing it to not advertise routes, but BFD is fine. The BFD session is up, so the problem is EIGRP, not BFD.

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49
MCQmedium

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?

A.The BFD session is using echo mode, which reduces the load on the control plane because echo packets are processed in hardware.
B.The BFD session is using echo mode, but the holddown timer is incorrect because echo mode should halve the timer.
C.The BFD session is up, but the echo function is not working because Rx and Tx counts are equal.
D.The BFD session is up and using echo function, but the neighbor must also have echo mode enabled for it to work.
AnswerA

Echo mode offloads detection to the data plane, reducing control plane processing.

Why this answer

The output shows that the BFD session is up and using echo function. The echo function allows faster failure detection by sending echo packets that are looped back by the neighbor. The holddown timer is 3000 ms, which is consistent with MinRxInt * Multiplier = 3000 ms.

The Rx and Tx counts are equal, indicating bidirectional communication.

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50
MCQmedium

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?

A.The BFD timers are too aggressive for the switch's processing capabilities, causing BFD packets to be dropped during high traffic.
B.The BGP session is not configured with the 'bfd' command under the neighbor statement.
C.The switch is running Spanning Tree Protocol (STP) and causing delays.
D.One router has 'bfd slow-timers' configured, causing a mismatch.
AnswerA

Aggressive BFD timers (100 ms) can overwhelm a switch that is not optimized for fast packet forwarding, leading to intermittent BFD failures.

Why this answer

BFD sessions can be affected by congestion or processing delays in the Layer 2 switch, especially with aggressive timers. The switch not supporting BFD does not inherently cause issues, but high CPU or buffer drops can cause BFD packets to be dropped.

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51
MCQmedium

A network engineer runs the following command to troubleshoot BFD with static routes: R1# show ip route 10.8.8.0/24 Routing entry for 10.8.8.0/24 Known via "static", distance 1, metric 0 Routing Descriptor Blocks: * 10.9.9.2, via GigabitEthernet0/3 Route metric is 0, traffic share count is 1 BFD enabled, BFD state: UP What does this output indicate?

A.Static route is installed with BFD tracking, and BFD session is UP.
B.Static route is not using BFD.
C.BFD state is DOWN, so the static route is removed.
D.Static route is using BFD only for IPv6.
AnswerA

BFD is enabled and UP, so the static route is active and monitored.

Why this answer

The output shows that BFD is enabled for the static route and the BFD state is UP, meaning the next hop is reachable and BFD is providing fast failure detection.

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52
MCQmedium

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?

A.The BFD session is down because the holddown timer is incorrect.
B.The BFD session is up and the holddown timer is 1500 ms, which is correct based on the configured multiplier and MinRxInt.
C.The BFD session is using echo mode, which is why the holddown timer is 1500 ms.
D.The BFD session is up but the multiplier is set to 1, causing the holddown timer to be 1500 ms.
AnswerB

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.

Why this answer

The output shows BFD session details. The Holddown value of 1500 ms is calculated as MinRxInt * Multiplier (1000 ms * 3 = 3000 ms), but the output shows 1500 ms. This discrepancy indicates a misconfiguration or a bug, but the key point is that the session is UP and the holddown timer is 1500 ms, which is half of the expected value.

However, the correct interpretation is that the BFD session is established and operating, and the holddown timer is 1500 ms as shown.

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53
MCQhard

In BFD multihop sessions, what is the default value for the TTL (or hop limit) in outgoing BFD Control packets on Cisco IOS-XE?

A.1
B.64
C.128
D.255
AnswerD

The default TTL for BFD multihop sessions is 255, as per Cisco IOS-XE and RFC 5883.

Why this answer

For BFD multihop sessions, Cisco IOS-XE defaults to a TTL of 255, as per RFC 5883, to ensure the packet can traverse multiple hops.

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54
MCQeasy

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 local discriminator?

A.The local discriminator is 1.
B.The local discriminator is 3.
C.The local discriminator is 10.1.1.1.
D.The local discriminator is 10.1.1.2.
AnswerA

The LD (Local Discriminator) is 1 as shown in the output.

Why this answer

The local discriminator is the LD value shown in the output. In this case, it is 1.

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55
MCQmedium

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?

A.The BFD timers are set too low on one router.
B.The OSPF dead interval is mismatched between the two routers.
C.The interface is flapping due to a physical issue.
D.BFD is configured with the 'strict-mode' command, causing OSPF to ignore BFD state.
AnswerB

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.

Why this answer

The BFD session is up, so BFD is not causing the flap. The issue is likely a mismatch in OSPF parameters that causes the adjacency to reset independently of BFD.

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56
MCQmedium

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?

A.The detection time is 3000 ms.
B.The detection time is 1000 ms.
C.The detection time is 1500 ms.
D.The detection time is 9000 ms.
AnswerA

It is the holddown timer shown in the output, which is 3000 ms.

Why this answer

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.

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57
MCQhard

A network engineer runs the following command to troubleshoot BFD session flapping: R1# debug bfd packet *Mar 1 00:15:23.456: BFD: [R1-to-R3] received async packet from 10.5.5.2, state UP, diag 0 *Mar 1 00:15:23.457: BFD: [R1-to-R3] sending async packet, state UP *Mar 1 00:15:23.458: BFD: [R1-to-R3] received echo packet from 10.5.5.2, state UP *Mar 1 00:15:23.459: BFD: [R1-to-R3] echo packet lost, no echo received for 300 ms *Mar 1 00:15:23.460: BFD: [R1-to-R3] state UP -> DOWN (echo failure) What does this output indicate?

A.BFD session is UP and stable.
B.BFD session went DOWN because of echo timeout, indicating possible path issue.
C.BFD async packets are failing, causing session down.
D.BFD session is flapping due to misconfigured multiplier.
AnswerB

Echo packets were lost, causing BFD to declare the session down.

Why this answer

The debug output shows that BFD detected an echo failure (no echo packets received for 300 ms), causing the session to go DOWN. This indicates a connectivity issue or misconfiguration affecting echo mode.

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58
Multi-Selecthard

Which TWO configuration steps are required to enable BFD for OSPF on a Cisco IOS-XE router? (Choose TWO.)

Select 2 answers
A.Configure 'bfd' under the interface
B.Configure 'bfd all-interfaces' under the OSPF process
C.Configure 'ip ospf bfd' under the interface
D.Configure 'bfd interval 50 min_rx 50 multiplier 3' globally
E.Configure 'router bfd' globally
AnswersA, B

This enables BFD on the interface, allowing BFD sessions to be established.

Why this answer

To enable BFD for OSPF, you must first enable BFD on the interface using the 'bfd' interface command, and then enable BFD support for OSPF under the OSPF routing process with 'bfd all-interfaces' or per-neighbor. The other options are either incorrect or not required.

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59
MCQhard

According to RFC 5880, what is the maximum number of BFD sessions that can be uniquely identified using the discriminator fields?

A.2^16 (65536)
B.2^24 (16,777,216)
C.2^32 (4,294,967,296)
D.Unlimited
AnswerC

The discriminator fields are 32-bit, providing over 4 billion unique session identifiers.

Why this answer

The discriminator fields are 32-bit values, allowing up to 2^32 (approximately 4.29 billion) unique session identifiers.

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60
MCQmedium

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?

A.The OSPF hello interval is mismatched between the two routers.
B.The OSPF network type is mismatched (e.g., broadcast vs point-to-point).
C.The OSPF area ID is mismatched between the two routers.
D.The interface is configured with 'ip ospf bfd' but the OSPF process is not configured with 'bfd all-interfaces'.
AnswerC

If the area ID does not match, the router will ignore the hello packets, causing the neighbor to remain in INIT state.

Why this answer

OSPF stuck in INIT usually indicates that the router is not receiving OSPF hello packets from the neighbor. BFD being up suggests Layer 2 connectivity is fine, so the issue is likely OSPF configuration, such as a mismatch in area ID or authentication.

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61
Multi-Selecthard

Which TWO actions will prevent a BFD session from flapping when a routing protocol (e.g., OSPF or EIGRP) is configured to use BFD? (Choose TWO.)

Select 2 answers
A.Increase the BFD minimum interval and multiplier values on both routers.
B.Disable BFD on the routing protocol and rely solely on the routing protocol's own hello timers.
C.Set the BFD minimum interval to 50 ms and the multiplier to 3 on both sides.
D.Configure BFD echo mode on both routers to reduce control packet processing overhead.
E.Ensure that the BFD timers are identical on both routers.
AnswersA, B

Correct. Higher intervals and multipliers make BFD less sensitive to minor delays, reducing the chance of false failure detection.

Why this answer

BFD session flapping can be caused by mismatched timers, high CPU load, or aggressive detection multipliers. Increasing the BFD interval or multiplier makes the session more tolerant to transient issues. Disabling BFD for the routing protocol removes BFD dependency.

Lowering timers or using default values can actually increase flapping. Echo mode helps reduce CPU load but does not directly prevent flapping due to timer mismatches.

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62
MCQhard

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?

A.The neighbor router is not configured for BFD.
B.The BFD session is down because the local router is not sending BFD packets.
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.
D.The BFD session is down because the multiplier is set too low.
AnswerC

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.

Why this answer

The Local Diag field shows 1, which indicates 'Control Detection Time Expired'. This means the local router did not receive BFD control packets from the neighbor within the expected time. The Rx Count is 0, confirming no packets were received.

Tx Count is 50, so the local router is sending packets but not receiving any.

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63
MCQhard

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?

A.The EIGRP process is configured with 'bfd all-interfaces' but the interface is not configured with 'eigrp bfd'.
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.
C.The interface is configured with 'ip address 10.0.0.1 255.255.255.252' but the neighbor is using a different subnet.
D.The EIGRP autonomous system number is mismatched between the two routers.
AnswerB

BFD timers must be compatible; if the required interval or multiplier values are not negotiable, the session will fail to come up.

Why this answer

EIGRP BFD requires that the BFD session be established before the EIGRP adjacency can form. If the BFD session is down, the issue is often a mismatch in BFD parameters or a lack of BFD configuration on one side.

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64
MCQeasy

What is the default BFD multiplier (detection time multiplier) on Cisco IOS-XE?

A.1
B.2
C.3
D.4
AnswerC

The default multiplier is 3, as per Cisco IOS-XE defaults.

Why this answer

The default BFD multiplier is 3, meaning the detection time is 3 times the negotiated hello interval.

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65
MCQhard

An engineer configures BFD on an OSPF link between two routers. The BFD session comes up, but OSPF remains stuck in EXSTART state. The engineer verifies that IP connectivity is fine and OSPF hello packets are exchanged. What is the most likely cause?

A.An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
B.BFD is not configured with the 'bfd all-interfaces' command under OSPF, causing OSPF to ignore BFD state changes.
C.The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, preventing the adjacency from forming.
D.The BFD minimum interval is set too low, causing BFD to flap and reset the OSPF adjacency repeatedly.
AnswerA

Correct. OSPF DBD packets are sized based on the interface MTU. A mismatch causes the larger DBD to be dropped, halting the adjacency while BFD remains up.

Why this answer

BFD can cause OSPF to remain stuck in EXSTART if there is an MTU mismatch on the link. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU is mismatched, the larger DBD packet is silently dropped by the receiving router, preventing the exchange of LSAs.

BFD does not detect this because BFD packets are small and unaffected by MTU issues.

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66
Drag & Drophard

Drag and drop the steps to troubleshoot a BFD adjacency or connectivity failure into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

Start by checking basic IP connectivity to isolate layer 3 issues, then verify BFD parameters match on both sides, inspect BFD session state, check for interface or routing protocol mismatches, and finally enable debug to capture BFD packet exchange.

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67
MCQmedium

A network engineer runs the following command to troubleshoot BFD with OSPF: R1# show ip ospf interface gigabitethernet 0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.1.1/24, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 Transmit Delay is 1 sec, State DR, Priority 1 Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1 Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 Hello due in 00:00:03 Supports Link-local Signaling (LLS) Cisco NSF helper support enabled IETF NSF helper support enabled Index 1/1/1, flood queue length 0 Next 0x0(0)/0x0(0)/0x0(0) Last flood scan length is 1, maximum is 25 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Suppress hello for 0 neighbor(s) BFD enabled, BFD interval 100 msec, BFD multiplier 3 What does this output indicate?

A.OSPF is using BFD with a 100 ms interval and multiplier 3, enabling sub-second failure detection.
B.BFD is disabled on this OSPF interface.
C.BFD interval is set to 10 seconds, matching OSPF hello.
D.BFD session is down, causing OSPF to use dead timer.
AnswerA

BFD is enabled with the specified parameters, allowing OSPF to detect failures quickly.

Why this answer

The output shows that BFD is enabled on the OSPF interface with an interval of 100 ms and multiplier of 3. This provides fast failure detection for OSPF adjacency.

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68
MCQhard

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?

A.The holddown timer is 1500 ms, which is the received MinRxInt (500 ms) multiplied by the multiplier (3).
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.
C.The holddown timer is 1500 ms, which is the local MinRxInt (1000 ms) multiplied by the multiplier (3) divided by 2.
D.The holddown timer is 3000 ms, and the output is correct as is.
AnswerB

Cisco IOS divides the holddown timer by 2 for display; the actual holddown is 3000 ms.

Why this answer

The holddown timer is calculated as the maximum of the local MinRxInt and the received MinRxInt, multiplied by the multiplier. Local MinRxInt is 1000000 microseconds, received MinRxInt is 500000 microseconds. The maximum is 1000000 microseconds.

Multiplied by 3 gives 3000000 microseconds = 3000 ms. However, the output shows 1500 ms. This is because the holddown timer displayed is actually half of the calculated value due to a Cisco implementation detail where the holddown timer is divided by 2 for display purposes.

The actual holddown time is 3000 ms.

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69
MCQmedium

Which BFD packet type is used for initial session establishment and carries the discriminator values?

A.BFD Echo packets
B.BFD Control packets
C.BFD Poll packets
D.BFD Hello packets
AnswerB

Control packets are the primary BFD packets used for session establishment, maintenance, and teardown, and they carry discriminator fields.

Why this answer

BFD Control packets are used for session establishment, carrying local and remote discriminator values, and are sent periodically to maintain the session.

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70
MCQhard

What is the default behavior of Cisco IOS-XE regarding BFD echo mode when BFD is enabled on an interface?

A.Echo mode is disabled by default
B.Echo mode is enabled by default
C.Echo mode is only available for multihop sessions
D.Echo mode requires explicit configuration with the bfd echo command
AnswerB

Echo mode is enabled by default, allowing faster detection using echo packets.

Why this answer

By default, Cisco IOS-XE enables BFD echo mode, which uses echo packets to achieve faster failure detection without involving the remote router's control plane.

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71
MCQmedium

A network engineer runs the following command to verify BFD with MPLS LDP: R1# show mpls ldp neighbor 10.6.6.2 detail Peer LDP Ident: 10.6.6.2:0, Local LDP Ident: 10.6.6.1:0 TCP connection: 10.6.6.2.646 - 10.6.6.1.53456 State: Oper; Msgs sent/rcvd: 100/100; Downstream Up time: 00:10:00 LDP discovery sources: GigabitEthernet0/2, hello interval: 5 s, hello hold: 15 s Addresses bound to peer LDP ident: 10.6.6.2 10.7.7.2 BFD enabled, BFD state: UP What does this output indicate?

A.BFD is enabled for LDP, allowing faster LDP session failure detection.
B.BFD is disabled for this LDP session.
C.LDP is using BFD only for IPv6.
D.BFD state is DOWN, so LDP uses its own hello timers.
AnswerA

BFD is enabled and UP, enabling sub-second detection for LDP.

Why this answer

The output shows that BFD is enabled for the LDP session and the BFD state is UP, providing fast failure detection for MPLS LDP neighbor.

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72
Multi-Selecthard

An engineer must enable BFD for an OSPF single-hop session between two directly connected routers. Which TWO configuration changes are required on each router? (Choose TWO.)

Select 2 answers
A.Configure the bfd interval command under the interface.
B.Configure the ip ospf bfd command under the interface.
C.Configure the bfd neighbor command under global configuration.
D.Configure the router ospf command and then use the bfd all-interfaces command.
E.Configure the bfd slow-timers command under the interface.
AnswersA, B

Correct. The bfd interval command enables BFD on the interface and sets the timers.

Why this answer

To enable BFD for OSPF, you must first enable BFD on the interface (bfd interval) and then enable BFD support under the OSPF routing process or interface (ip ospf bfd). The bfd neighbor command is for static BFD sessions, not OSPF. The router ospf command alone does not enable BFD.

The bfd all-interfaces command enables BFD on all OSPF interfaces but still requires interface-level BFD configuration.

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73
MCQhard

An engineer configures BFD on an OSPF link and also applies a distribute-list outbound under the OSPF process. The BFD session is up, but OSPF routes are not being advertised to the neighbor. The engineer verifies that the distribute-list is correctly configured. What is the most likely cause?

A.The distribute-list is applied outbound, but OSPF distribute-lists only filter routes in the routing table, not LSAs, so the routes are still advertised.
B.The BFD session is flapping due to a mismatch in the 'bfd interval' command, causing OSPF to withdraw routes.
C.The distribute-list references an ACL that denies the network, but OSPF uses type 3 LSAs that are not affected by ACLs.
D.The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
AnswerA

Correct. OSPF distribute-lists outbound do not filter LSAs; they only filter routes from being installed in the routing table. The neighbor will still receive the LSA.

Why this answer

OSPF distribute-lists only filter routes in the routing table, not LSAs. If the distribute-list is filtering the route, OSPF will still advertise the LSA to the neighbor, but the neighbor will not install the route. However, if the distribute-list is applied inbound, it can prevent the route from being installed.

The edge case here is that the distribute-list is applied outbound, which has no effect on OSPF LSA flooding. BFD does not interact with distribute-lists.

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74
MCQhard

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?

A.The BFD timers are set too high, causing EIGRP to time out before BFD can respond.
B.EIGRP is configured with 'no auto-summary' on one router and 'auto-summary' on the other.
C.The EIGRP K-values are mismatched between the two routers.
D.The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor expects different values.
AnswerC

EIGRP K-values must match for adjacency to form; a mismatch causes the neighbor to stay in Pending state even if BFD is up.

Why this answer

EIGRP requires the BFD session to be fully established before it will bring up the adjacency. If the BFD session is up but EIGRP is stuck in Pending, the issue is often a mismatch in EIGRP K-values or authentication.

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75
Multi-Selectmedium

Which THREE symptoms indicate a BFD session failure? (Choose THREE.)

Select 3 answers
A.The BFD neighbor state shows 'Down'
B.The OSPF neighbor state changes from Full to Down
C.BFD timer expiry messages appear in logs
D.The interface MTU is set to 1500
E.The BFD discriminator value is zero
AnswersA, B, C

A 'Down' state directly indicates session failure.

Why this answer

A BFD session failure typically results in the neighbor state being 'Down', the routing protocol (like OSPF or EIGRP) neighbor going down due to BFD's fast detection, and BFD timers expiring. The other options are not direct symptoms of a BFD session failure.

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