300-410 Bidirectional Forwarding Detection (BFD) Practice Test 4 — 15 Questions

Question 1

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?

Accepted Answer

BGP is using BFD with state UP, enabling sub-second failure detection for this prefix.. 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.

Answer

BFD is disabled for this BGP neighbor.

Answer

BFD state is DOWN, so BGP uses its own timers.

Answer

BGP is using BFD only for IPv6 prefixes.

Question 2

A network engineer runs the following command to verify BFD session details:

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?

Accepted Answer

BFD session is UP with a detection time of 2.5 seconds (500 ms * 5).. 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.

Answer

BFD session is DOWN due to high multiplier.

Answer

BFD is using echo mode for faster detection.

Answer

BFD multiplier is set to 3, not 5.

Question 3

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?

Accepted Answer

BFD session went DOWN because of echo timeout, indicating possible path issue.. 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.

Answer

BFD session is UP and stable.

Answer

BFD async packets are failing, causing session down.

Answer

BFD session is flapping due to misconfigured multiplier.

Question 4

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?

Accepted Answer

BFD is enabled for LDP, allowing faster LDP session failure detection.. 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.

Answer

BFD is disabled for this LDP session.

Answer

LDP is using BFD only for IPv6.

Answer

BFD state is DOWN, so LDP uses its own hello timers.

Question 5

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?

Accepted Answer

Static route is installed with BFD tracking, and BFD session is UP.. 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.

Answer

Static route is not using BFD.

Answer

BFD state is DOWN, so the static route is removed.

Answer

Static route is using BFD only for IPv6.

Question 6

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

Accepted Answer

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

Answer

100 milliseconds

Answer

500 milliseconds

Answer

2000 milliseconds

Question 7

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

Accepted Answer

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

Answer

BFD Echo packets

Answer

BFD Poll packets

Answer

BFD Hello packets

Question 8

How does BFD prevent loops in a network where multiple BFD sessions might exist between the same pair of routers?

Accepted Answer

By matching the Your Discriminator field against the local discriminator. BFD uses a mandatory Your Discriminator field in Control packets; a router will only accept a BFD Control packet if the Your Discriminator matches its own local discriminator, preventing misdirected packets from creating loops.

Answer

By using TTL values to limit packet propagation

Answer

By requiring authentication in all BFD packets

Answer

By using sequence numbers in BFD Control packets

Question 9

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

Accepted Answer

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

Answer

Simple password (auth type 1)

Answer

Keyed MD5 (auth type 2)

Answer

Meticulous Keyed MD5 (auth type 3)

Question 10

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

Accepted Answer

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

Answer

1

Answer

2

Answer

4

Question 11

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

Accepted Answer

255. 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.

Answer

1

Answer

64

Answer

128

Question 12

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

Accepted Answer

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

Answer

Down

Answer

Init

Answer

AdminDown

Question 13

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

Accepted Answer

Echo mode is enabled by default. 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.

Answer

Echo mode is disabled by default

Answer

Echo mode is only available for multihop sessions

Answer

Echo mode requires explicit configuration with the bfd echo command

Question 14

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

Accepted Answer

2^32 (4,294,967,296). The discriminator fields are 32-bit values, allowing up to 2^32 (approximately 4.29 billion) unique session identifiers.

Answer

2^16 (65536)

Answer

2^24 (16,777,216)

Answer

Unlimited

Question 15

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

Accepted Answer

Enable BFD globally using the bfd command. 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.